36 B4 Hazardous Substances -Preventive and Protective Measures

Hazardous substances -preventive and protective

measures

Hazardous substances -preventive and protective measures

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Table Of Contents

TElement B4: Hazardous substances - preventive and protective measures6T ...................................... 4

T1.0 Preventive and protective measures6T ........................................................................................................... 4

1.1 Control of Substances Hazardous to Health ............................................................................................. 5


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T6.0 Personal protective equipment 6T ................................................................................................................... 69

T6.1 Eye Protection6T .................................................................................................................................................... 74 6.2 HEAD PROTECTION .......................................................................................................................................... 77


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Element B4: Hazardous substances - preventive and protective measures

Learning outcomes

On completion of this element, candidates should be able to:

Explain the strategies used in the preventative and control of exposure to

hazardous substances

Explain the specific strategy to be adopted when considering the control of

exposure to carcinogenic substances

Describe the various types of Personal Protective Equipment (PPE) available for

use with hazardous substances, their effectiveness, and the relevant

specifications and standards to be met

Relevant Standards

International Labour Office, Safety in the Use of Chemicals at Work, an ILO Code

of Practice, ILO, 1993. ISBN: 9221080064

Section 6: Operational control measures (see controls in S.6.5 S6.9)

International Labour Office, Ambient Factors in the Workplace, an ILO Code of

Practice, ILO, 2001. ISBN 922111628

Minimum hours of tuition 6 hours.

1.0 Preventive and protective measures

MHSWR Principles of Prevention generally applicable to all workplace risks

COSHH Regs 2002 contain similar principles, but specific to chemical control of

course, where overlapping duties all requirements apply!

COSHH Reg 7(7) and Schedule 2A Principles of Good Practice (mentioned in Element

B3) applied where not possible to prevent exposure

- Have to be applied (along with compliance with WELs) in order to demonstrate

control is adequate

- COSHH Regs supported by a raft of ACoPs/Guidance main one is L5


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Our study of chemical health hazards in the previous study units has followed the

logical approach of good occupational hygiene practice:

Recognition of the health hazard, i.e. the effects of exposure to chemical agents;

Quantification of the extent of the hazard, i.e. measurement and analysis of

chemical agents;

Assessment of the risk to health, i.e. the application of hygiene standards.

We are now ready to move on to the final stage, which is the selection and

implementation of appropriate control measures.

In some respects this stage may be seen as the most important since it is by the

effective application of workplace controls that we actually improve the working

environment and reduce or eliminate the risk of occupational ill-health. Often the risk

assessments that we are required to carry out under COSHH and other regulations are

seen as an end in themselves. Do not forget that risk assessment is simply a tool to

enable us to achieve the standards of occupational safety and health in the workplace

that the law requires. It is therefore the development and maintenance of effective

control measures that create the safe and healthy workplace that we are aiming for,

rather than the assessment exercise itself.

1.1 Control of Substances Hazardous to Health

The Control of Substances Hazardous to Health Regulations 2002 (as amended)

provide a systematic framework and control strategy to prevent occupational ill-health

arising from the use of harmful substances at work. The Regulations require the

employer to plan, manage and monitor the use of chemicals, micro-organisms and

other hazardous substances by means of:

Assessment

Prevention or control

Maintenance of control measures

Monitoring

Health surveillance

Training

We shall discuss these elements in detail below, but first we must be clear about the

scope of the Regulations and what is meant by a hazardous substance.


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A substance hazardous to health is generally defined in the COSHH Regulations as

any substance or preparation (natural, artificial, solid, liquid, gas, vapour, micro-

organism) capable of causing adverse health effects or disease arising from work

activities. This wide definition can be categorised under the following five headings:

(a) Substances in Part I of the Approved Supply List (see CHIP, Unit B2) classified as

very toxic, toxic, harmful, corrosive or irritant.

(b) Substances with a maximum exposure limit or occupational exposure standard.

(c) Biological agents capable of causing infection, allergy, toxicity or other human

health hazards.

(d) Any dust at a substantial concentration in air.

(e) Any substance not included in the categories above but which create a comparable

health hazard.

However, lead and asbestos are specifically excluded from the COSHH Regulations

since they are covered specifically by the Control of Lead at Work Regulations

2002 and the Control of Asbestos at Work Regulations 2002.

A substance should be regarded as hazardous to health if it is hazardous in the form in

which it occurs in the work activity, whether or not its mode of causing injury to health

is known, and whether or not the active constituent has been identified.

A substance hazardous to health is not just a single chemical compound but also

includes mixtures of compounds, micro-organisms, allergens, etc.

In considering whether a substance is hazardous to health, the following additional

factors should be taken into account:

Different forms of the same substance may present different hazards, e.g. a

solid may present a negligible hazard but, as a dust of respirable size (less than

5 m) may be very hazardous.

Many substances contain impurities which could present a greater hazard than

the substance they contaminate, e.g. crystalline silica is often present in

minerals which would otherwise present little or no hazard.

Some substances have a fibrous form which may present a potentially serious

hazard to health if the fibres are of a certain size or shape (e.g. asbestos).


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Some substances may be known to cause ill-health, but the causative agent may

not have been identified, e.g. certain textile dusts causing byssinosis.

Combined or sequential exposures to various substances may have additive or

synergistic effects.

A substantial concentration of dust should be taken as a concentration of 10

mg/m3, 8-hour time-weighted average, of total inhalable dust, or 4 mg/m3, 8-

hour time-weighted average, of respirable dust where there is no indication of

the need for a lower value, e.g. in Guidance Note EH40.

Epidemiological data which indicate that a micro-organism or its products is the

cause of a hazard to health at work.

There are a number of sources of information available to give some indication of the

hazardous properties of chemical agents and substances:

Information on labels complying with the Chemicals (Hazard Information and

Packaging for Supply) (Amendment) Regulations 2005 , or from classifying the

substance by applying the criteria in those Regulations;

Information provided in data sheets as required by CHIP;

Guidance material published by the Health and Safety Executive or other

authoritative bodies;

Experience obtained and information gathered as a result of previous use of the

substance or similar substances;

Technical reference sources, such as textbooks, scientific and technical papers, trade

journals, etc.;

Professional institutions, trades associations, trades unions and specialist

consultancy services.

1.2 Assessment of Risk

Every employer is required to carry out an assessment of the risks to health created by

any activity likely to expose employees to a substance hazardous to health. The

purpose of the assessment is to enable a valid decision to be made about measures

necessary to control substances hazardous to health arising from any work.


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It also enables the employer to demonstrate, both to himself and to other persons,

that all factors pertinent to the work have been considered, and an informed and valid

judgment has been reached about the risks, the steps which need to be taken to

achieve and maintain adequate control, the need for monitoring exposure at the

workplace, and the need for health surveillance.

It is, of course, a requirement of the Regulations that any person who carries out any

work on behalf of the employer by way of an assessment should possess sufficient

knowledge, skill and experience to be able to perform the work effectively. This means

the employer must ensure that the person to whom any work is delegated is

competent. It may necessitate engaging outside specialists, but the same duty of

competency applies.

A suitable and sufficient assessment should include:

An assessment of the risk to health;

Steps which need to be taken to achieve adequate control of exposure;

Identification of other action necessary to ensure control.

Preliminary Survey

The first practical step in the assessment of ill-health hazards will be the preliminary

assessment or walk-through survey. As the name implies, it will entail walking

through all parts of the works organisation to see and list all substances used or

handled in the production processes: the offices, laboratories, cleaning, maintenance

and all other areas. It may be convenient to break down the organisation into three

categories: raw materials, production and finished goods. A visit to the company buyer

should elicit a list of all raw materials used and all suppliers should be able to supply

health and safety information.

Production processes may enhance a particular hazard, e.g. spray painting, welding,

sand blasting and degreasing processes, by releasing contaminants which are easily

absorbed into the body by inhalation, absorption or ingestion. These should be

carefully noted.

The sales department should be able to produce company health and safety literature

relating to use of company products, and the laboratory should provide similar

information about research and development products.


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In a well-run organisation, much of this will have been carried out in the past, but its

veracity depends on four important criteria:

Comprehensiveness of previous assessments;

Existence of adequate documentation;

Competency of staff carrying out the assessments;

Whether or not changes (in formulation, processing, equipment, etc.) have been

taken into consideration.

The preliminary assessment (survey) must be comprehensive, which means covering

every location within the workplace. The most logical plan is to proceed from goods

inward, through production, to goods outward, and to complete the survey by

looking at all ancillary areas such as offices, canteens, washrooms, boiler rooms, etc.

1.3 Planning

Proper planning demands a cooperative effort, since it requires:

Site plans

Plant and process details

Details of safety procedures and control measures in force Staff information:

numbers, job descriptions, training, etc.

Data on accidents and ill-health and health surveillance information

Much of this information will only be available through discussions with colleagues

within the organisation, ranging from supervisors and line management, through safety

representatives, to middle and senior management, e.g. works managers, chief

chemists, chemical and maintenance engineers, and supplemented by occupational

health physicians and personnel officers.

Once the basic preparation has been completed, the preliminary survey can be started,

preferably accompanied by a local supervisor familiar with the processes and work

activities in his area. By this means, the surveyor will be able to follow the production

processes from goods inwards to goods outwards, together with the associated

ancillary areas such as laboratory, maintenance, etc.


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Timing

The timing of a survey must depend upon the type and extent of the work activities.

Not all work activities occur simultaneously or consecutively; some work is cyclical or

occasional. For example, maintenance may be carried out according to a fixed routine

(often at weekends) or when plant breaks down and it is likely that a preliminary

survey performed in normal time on Mondays to Fridays would miss this work. It is

essential that such routine, out-of-hours work should be planned into the assessment,

reinforcing the need for teamwork in this respect.

1.4 Documentation

Provided that careful documentation of all findings revealed by the preliminary survey

are kept, they can easily be transformed into the full COSHH assessment by returning

later to any problems identified during the walk-through which require further

investigation.

At its simplest, it may necessitate writing to a supplier for more information about the

product; and at its most complex, it may involve a major monitoring exercise to

evaluate the propensity of air contaminants likely to cause ill-health, or to evaluate the

capture efficiency of local exhaust ventilation equipment.

We can summarise the purpose of a preliminary survey in Figure 1.1.

Figure 1.1 The preliminary survey

1.5 The Full Assessment

An adequate assessment cannot be achieved simply by walking through the premises.

It requires a structured and organised system of information-gathering and recording

so that assessment against the maximum exposure limits and occupational exposure

standards, and the information upon which they are based, can be made and recorded.

As in Figure 1.1, the assessment is not a once-and-for-all process; it needs to be

complemented by regular updating, with records amended to provide a sound basis for

providing information on which to base future assessments. This is also needed to alert

those responsible to changes in the processes or control measures.


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Before a full assessment can be conducted it will be necessary to have the following

information on the area to be assessed:

Both the substances used and the composition of any proprietary mixtures,

together with their quantities.

Physical nature of the substances (gas, solid, liquid, dust, fume, aerosol, mist,

spray, etc.).

Location and number of possible sources of evolution of the substances in an

area and their relative contribution to the different emissions.

Effect those substances are likely to have on the body.

Work patterns: some operators move around a production process, others stay

in a fixed position as the work passes by.

Effectiveness of control measures, which will mainly concern the testing and

recording of their effectiveness.

Results of any biological monitoring together with the collective results of health

surveillance.

Results of any air sampling which has been carried out in work areas.

Where valid standards exist, representing adequate control, comparison of the

findings of the assessment with those standards. If comparison with external

standards demonstrates that control is likely to be inadequate or become

inadequate then the assessment should go on to determine the steps, or, in the

case of existing work, the further steps which need to be taken to obtain and

sustain adequate control. In making this comparison the use of personal

protective equipment should only be considered as a method of control after all

other measures have been taken so far as is reasonably practicable.

Of course, the amount of detailed work involved in carrying out the assessment will

vary and depend on the extent to which:

The degree and nature of the risk and conclusions about the adequacy of proposed

or existing control measures are immediately obvious.

Knowledge has already been gained as a result of previous experience.

Existing records are valid, concerning the nature of the substances involved, the

numbers and categories of employees potentially exposed, their work activities, the

results of exposure experienced so far and the suitability of existing methods of

control.


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In some cases it will only be necessary to read the suppliers information sheets to

conclude that existing practices are sufficient to ensure adequate control of exposure.

In other circumstances, it may be necessary to read HSE Guidance Notes,

manufacturers standards, technical papers or trade literature, in order to be able to

estimate the effects of the likely exposure before deciding what control measures

should be applied.

The COSHH Regulations state that no work which is liable to expose anyone to

substances hazardous to health may be carried on unless an assessment has been

made. This means:

Evaluating the risks to health arising from work involving substances hazardous

to health; and then

Establishing what has to be done to meet the requirements of the whole of the

COSHH Regulations.

1.6 Eight Principles for Adequately Controlling Exposure

EightPrinciples of Good Practice to be applied in adequately controlling exposure:

Specific controls discussed later an overview here:


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1. Design and operate processes and activities to minimise emission, release and

spread of substances hazardous to health.

Best done at design stage.

Usually cheaper to do this rather than remove contaminant from workplace once it has

been dispersed.

Also means segregation, isolation, LEV, etc.

2. Take into account all relevant routes of exposure inhalation, skin and ingestion

when developing control measures.

As discussed in previous elements (B2 and B3) will be identified in risk assessment.

Some chemicals will present significant multiple exposure routes (e.g. inhalation and

skin absorption).

So, if inhalation is the most significant exposure route, need to control airborne

concentrations.

3. Control exposure by measures that are proportional to the health risk.

More severe health risks (i.e. more serious effects, more likely) require more stringent

controls.

4. Choose the most effective and reliable control options that minimise the escape and

spread of substances hazardous to health.

This relates to the Hierarchy of controls referred to in Regulation 7 - discussed later.

Take care not to see the hierarchy as too rigid could lead you to assume that certain

controls are always good and others always bad whatever the circumstances.

5. Where adequate control of exposure cannot be achieved by other means, provide, in

combination with other control measures, suitable personal protective equipment.

May need to supplement controls with PPE discussed in more detail later.

6. Check and review regularly all elements of control measures for their continuing

effectiveness.


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This means doing all the usual checks to make sure things still work properly as

intended, e.g. LEV checks, exposure monitoring, etc.

7. Inform and train all employees on the hazards and risks from substances with which

they work, and the use of control measures developed to minimise the risks.

People need to know

8. Ensure that the introduction of measures to control exposure does not increase the

overall risk to health and safety.

An important point e.g. proposed new methods of working to control a hazardous

chemical might introduce new (or increased) risk i.e. of musculoskeletal injury.

Take a holistic view links in with reg 3 of MHSWR 1999.

1.7 Monitoring

HSE Guidance Note EH42, Monitoring Strategies for Toxic Substances, which we have

already referred to, offers useful advice on the principles of sensible, cost-effective

monitoring of hazardous substances; the key is careful planning. To leap in and start

measuring airborne concentrations in an attempt to reassure employees that

something is being done is liable to create more problems than it will solve. The basic

philosophy should be: Do not measure unless you know what you are measuring and

what it is you will do with the results.

Returning for a moment to the preliminary assessment, first identify the substances,

the process and the workers, then devise a monitoring strategy.

We have already looked at monitoring strategies, particularly as regards airborne

hazards, in some detail. Here we give only a general outline as presented in the

COSHH Regulations.

Preliminary Survey

A preliminary monitoring survey should concentrate on those groups of staff most

likely to be significantly exposed to a hazardous substance. If their exposures are

measured and found to present insignificant health risks when compared with current

standards, then lesser-exposed staff need not be assessed in detail.


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On the other hand, preliminary monitoring may reveal the need for a much more

extensive study to determine the extent of the exposure within working groups.

Monitoring is required when any of the following circumstances apply, unless suitable

procedures do not exist, or cannot be devised, or it is immediately obvious that control

is adequate:

When failure or deterioration of the control measures could have a serious effect on

health,

either because of the toxicity of the substance or because of the extent of potential

exposure, or

both;

When measurement is necessary so as to be sure that a maximum exposure limit, or

occupational exposure standard, or any self-imposed working standard is not

exceeded; or

When necessary as an additional check on the effectiveness of any control measure

provided in accordance with the Regulations; and always in the case of substances or

processes specified in Schedule 4 (at present, vinyl chloride monomer and vapour, or

spray given off from electrolytic chromium-plating processes).

Personal and Environmental Monitoring

The thrust of COSHH is to control and limit personal exposure and, to this end,

personal monitoring is recommended. But, insofar as control measures are being

evaluated, their efficiency and effectiveness may be assessed by environmental

measurements as much as by personal dosimetry. An advantage of the environmental

approach is that it tests the actual effectiveness of the control strategy and is thereby

much simpler and more straightforward to operate.

If it can be demonstrated that the control keeps the workplace atmosphere well within

acceptable hygiene standards, then it may not be necessary to carry out personal

sampling to ascertain the extent of individual exposures. Against this must be set the

possibility that different workers may be exposed to different concentrations in

different workplaces; a major advantage of personal monitoring is that it can be

measured in the operators breathing zone and is more likely to represent what is

actually inhaled.


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Records

Occupational hygiene monitoring should be designed to produce a commentary on the

work processes and show areas of control which require further attention. Once the

airborne concentration has been measured, whether by personal or environmental

sampling, it will be necessary to report and record the findings.

The report should include:

A brief summary;

The detailed findings;

A discussion of the implications of the findings; and

A set of clear recommendations for future action.

Any data obtained should be recorded in a standard format to permit later comparison

and evaluation, as, for example, following any process or raw material changes.

To be regarded as suitable a record should provide sufficient information to determine:

When the monitoring was done and what the results were;

What monitoring procedures were adopted, including duration; and

The locations where samples were taken, the operations in progress at the time and,

in the case of personal samples, the names and jobs of individuals concerned.

The records may be kept in any format, but the information must be readily retrievable

and in an easily understandable form, and kept in such a way that the results can be

compared with any health surveillance records.

1.8 Health Surveillance

Objectives

The objectives of health surveillance where employees are exposed to substances

hazardous to health in the course of their work are:


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The protection of the health of individual employees by detection as soon as

possible of any adverse changes which may be attributed to exposure to

substances hazardous to health;

To assist in the evaluation of measures taken to control exposure;

The collection, maintenance and use of data for the detection and evaluation of

hazards to health;

To assess, in relation to specific work activities involving micro-organisms

hazardous to health, the immunological status of employees.

Procedures for Attaining Objectives

Health surveillance will always include keeping individual health records and a range of

procedures capable of achieving the above four objectives, i.e.

Biological monitoring: the measurement and assessment of workplace agents or

their metabolites, either in tissues, secreta or expired air, or any combination in

exposed workers.

Biological effect monitoring: the measurement and assessment of early biological

effects in exposed workers.

Medical surveillance: clinical examinations and measurements of physiological and

psychological effects of exposure to hazardous substances in the workplace, as

indicated by alterations in body function or constituents.

Enquiries about symptoms: inspection or examination by a suitably qualified

person (e.g. an occupational health nurse).

Inspection by a responsible person (e.g. chrome ulceration by a supervisor or

manager).

Review of records and occupational history during and after exposure: to

check correctness of the assessment or risks to health and to indicate if the

assessment needs reviewing.

Substances Adverse to Health

For a limited schedule of substances (see Table 1.1) and for others (e.g. suspected

carcinogens, man-made mineral fibres, rubber dust and fume, and leather dust) which


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may give rise to identifiable adverse health effects, the health surveillance measures

mentioned above must be carried out.

Substances for which

Medical

Processes

Surveillance is Appropriate

Vinyl chloride monomer (VCM) In manufacture, production, storage,

reclamation,

discharge, transport use or polymerisation

Nitro or amino derivatives of

phenol

In the manufacture of nitro or amino

derivatives and

and of benzene or its

homologues

making of explosives with the use of any of

these

substances

Potassium or sodium

chromate or In manufacture

dichromate

o-Tolidine and its salts

In manufacture, formation or use of these

substances

o-Dianisidine and its salts

3,3-Dichlorobenzidine and its

salts

Auramine In manufacture

Magenta

Carbon disulphide Processes in which these substances are used,

or given off

Disulphur dichloride as vapour, in the manufacture of indiarubber or

of articles

Benzene, including benzol or goods made wholly or partly of indiarubber

Carbon tetrachloride

Trichloroethylene

Pitch In the manufacture of blocks of fuel consisting

of coal,

coal dust, coke or slurry, with pitch as a

binding substance

Table 1.1


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Vaccination regimes

This will vary from different occupation for example clinical staff with patient contact

may be required to have a vaccination regime consisting of:-

Hepatitis B

TB

Rubella

Diphtheria

Polio

2.0 Control

The key word in the title of COSHH is Control. Although there is, as we have seen, an

emphasis on assessment, measurement and monitoring, it is the controlof exposure to

hazardous substances which provides the actual long-term benefits to employees

health.

The assessments and monitoring strategies described above will enable employers to

focus attention on those parts of their enterprise where control is most required.

Within the COSHH Regulations, the principal duty on the employer is to prevent

exposure of employees to substances hazardous to health. However, where this is not

reasonably practicable, the employer must consider other options to ensure adequate

control of exposure.

The hierarchy of control measures available includes the following:

Substitution with less toxic substances

Isolation or enclosure of process

Local extract ventilation

General ventilation

Personal protective equipment

Controlled exposure

Hygiene measures


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Although substitution with a harmless or less hazardous substance is the ideal option,

in practice engineering controls or personal protective equipment are the measures

most widely employed.

Engineering Controls

Where the use of substances hazardous to health is unavoidable it may be necessary to

employ engineering controls on plant, processes and handling systems to prevent

exposures above the workplace exposure limit (WEL).

Such controls generally involve provision of effective exhaust ventilation to prevent

release of hazardous substances into the operators breathing zone.

Examples of commonly used engineering controls include:

Glove boxes , which are total enclosures accessed through flexible gloves and

kept under negative pressure to prevent any release of contaminant.

Fume hoods , which are partial enclosures accessed through a vertical sliding

sash. Again the enclosure is kept under negative pressure so the air flow is

through the sash into the hood to prevent any release of contaminant.

Captor hoods , which are placed as near as possible to the hazard and capture

contaminants by an air flow into the hood before they reach the operator.

Receptor hoods , which are large structures designed to capture contaminants

which have been directed naturally into the hood by thermal draughts,

directional movement, or by local generation.

We will examine each of these engineering controls in more detail later in this study

unit.

Local Exhaust Ventilation (LEV)

LEV is the standard control measure for dealing with dusts, vapours and fumes which

are generated from a point source. The harmful contaminant is extracted at the point

of generation to prevent it entering the general atmosphere. The direction of the

ventilation flow should be away from the breathing zone of any operators. To be

effective, LEV must be properly designed and located close to the source of

contamination (see later in study unit).

Use of Control Measures


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Where the assessment indicates that control measures should be used in order to

protect the health of the workforce, employers must ensure that the controls specified

are used. Employers may require local managers to implement checks at regular

intervals to ensure that equipment and procedures are properly applied.

Employees also have a responsibility to ensure that the specified control measures are

used, including reporting any defects identified.

2.1 Maintenance of Control Measures

All control measures, whether items of hardware or systems of work or procedures,

should be properly maintained. Regular checks should be made to ensure they

continue to operate as intended so that prevention or adequate control of exposure is

sustained.

It may include visual checks on enclosures and exhaust ventilation by a responsible

person, looking for obvious defects such as damage, wear or malfunction. In addition

there should be preventative servicing, paying attention to points such as integrity of

ductwork, speed, lubrication and cleanliness of fans and motors, and tightness of belts.

Where LEV is provided it should be thoroughly examined and tested at least once in

every 14 months; the examination and test should include the following points:

Identification and location of LEV plant

Date of last thorough examination and test

Conditions at time of test

Information on performance showing:

Its intended operating performance

Whether it still achieves this performance

If not, repairs needed

Methods used to establish performance parameters (i.e. air flow measurements,

pressure measurements, air sampling, etc.)

Date of examination and test

Details of any repairs

Authorisation of responsible person


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Records of examinations and tests and of repair work carried out should be kept for at

least five years.

Personal Protective Equipment (PPE)

Personal protective equipment (PPE) is often thought of as the first line of defence

against substances hazardous to health. However, in the hierarchy of control measures

available PPE comes well down the list. It is unacceptable to require employees to be

encumbered by PPE when it is the process itself which should be enclosed, or the

hazardous substance which should be eliminated by substitution. Employers often take

the attitude that the hazard is inevitable and therefore it is the employee who must

endure the discomfort of wearing PPE.

The COSHH Regulations only allow the use of PPE as a method of achieving control

where the employer is able to show that prevention of exposure to the hazardous

substance concerned is not reasonably practicable, or there is no other means of

control.

In circumstances where respiratory protective equipment (RPE) is prescribed as a

method of control, the employer must ensure it is suitably maintained. This means

having it examined at suitable intervals and tested where appropriate.

The topics we have outlined so far are summarised in flowchart form in Figure 1.2.


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Figure 1.2

2.2 Disposal of Substances Hazardous to Health

Earlier we discussed the meaning of the term hazardous substance and the potential

for such material to cause injury to health in work activities. Control of these

substances should be seen as a continuing requirement, beginning when the substance

is introduced into the workplace and only ending when the material is safely disposed

of.

It is appropriate, therefore, to consider the disposal of substances hazardous to health

as the next stage after the legal requirements of COSHH have been met. Substances

which require special consideration for disposal are termed Special Waste. One of the

criteria in defining Special Waste is that the substance is dangerous to life.

Substances hazardous to health therefore often require special treatment when it

comes to disposal.


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Special Waste

In July 2005, the Hazardous Waste (England and Wales) Regulations 2005 were

implemented in England and Wales, replacing the Special Waste Regulations 1996.

They were introduced with the aim of simplifying the documentation associated with

the collection and disposal of hazardous waste, and creating a cradle to grave

approach to its disposal.

The term special waste has been replaced by hazardous waste. Hazardous wastes

are the most dangerous wastes and are identified with an asterisk in the List of Wastes

(England) Regulations 2005. As well as all of the previous special wastes, the list now

includes approximately 200 additional types of waste. For example, NiCad

(rechargeable) batteries or items containing such batteries are on the list.

The requirement for hazardous waste producers to provide a three-day pre-notification

off each consignment of special waste has been replaced by a more streamlined

registration requirement.

It is an offence for hazardous waste to be collected from a site that has not been

registered or exempted. All non-exempt sites that produce hazardous waste must be

registered with the Environment Agency, even if they are unlikely to have that waste

collected for some time. Some sites, such as offices, shops and schools, can be exempt

from the requirement to register as a hazardous waste producer, if they expect to

produce less than 200kg of hazardous waste a year (although they would have to

register if they went over that threshold).

The Regulations ban the mixing of different categories of hazardous waste, or the

mixing of hazardous waste with non-hazardous waste or other substances or materials,

unless this is authorised by permit or licence.

The Regulations will require a new consignment note to be used, in place of the old

Section 62. Producers and consignors must keep a register of consignment note copies

for three years and carriers must keep copies of consignment notes for 12 months.

The Regulations also introduce Waste Acceptance Criteria (WAC) requiring

characterisation of the waste input of hazardous waste landfill sites. It is likely that

landfill sites will require initial sampling and analysing of water, and perhaps analysis of

every load in some cases.


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The Regulations place the onus of responsibility for ensuring correct classification,

transportation and disposal firmly on the waste producer. As such, fixed penalty fines

of up to 300 will be issued to those companies who provide false information or fail to

notify their premises. In addition, if convicted of not complying with the Regulations in

a Magistrates' Court, the perpetrator could face a fine of up to 5,000 and/or two years

in prison. More serious offences may be tried in the Crown Court where there is no

limit on the level of fines which can be imposed.

Disposal of Special Waste

Toxic Substances

Toxic materials must be handled with extreme care to avoid any possibility of

inhalation, ingestion or skin absorption. Skin and eye contact must be prevented by

providing impervious gloves, protective clothing and eye protection. Inhalation of

vapour or dust must also be prevented and therefore good ventilation and/or

respiratory protection may be required.

Toxic waste should be stored separately and away from flammable or explosive

substances, including self-igniting or water reactive materials. Since it is likely to be

classified as Special Waste, special provisions will govern the disposal of toxic waste

including the need for consignment notes to ensure that it is disposed of at a licensed

site. The producer of the waste must keep on site for at least two years a register of

copies of all consignment notes relating to Special Waste produced at that site. The

disposer of the waste must also keep similar site records relating to disposal.

Infectious Substances

This category includes clinical waste, because of the high risk of infection associated

with it.

Any direct skin contact must be prevented. Protective gloves and clothing should be

worn and surgical masks may be required for work with infected or clinical material.

Non-clinical infectious waste such as viruses or pathogenic bacteria must be autoclaved

before disposal.

Clinical waste such as dressings, tissue and animal carcasses must be contained in

yellow clinical waste disposal bags which are securely sealed. Incineration is the

preferred method of disposal although discarded syringe needles, some types of


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pharmaceutical and chemical waste, and non-infected animal carcasses may be

deposited at landfill sites.

Corrosive Substances

Inhalation, ingestion or skin contact can cause serious damage to lungs, internal

organs, skin or eyes. In addition, such substances are often quite reactive and may

produce toxic or irritant fumes on reaction with other materials.

Operators must be protected from contact or inhalation of vapours. Heavy duty

protective clothing including gloves, eye protection and footwear will be required.

Respiratory protection may also be necessary. Acids, alkalis and laboratory chemicals

are specifically classed as Special Waste and their corrosive properties may require

storage in stainless steel, glass, or plastic containers.

Corrosive waste will be well outside the pH range limits for discharge to sewer or

watercourse and therefore even small quantities are prohibited from discharge by this

route without treatment to neutralise.

3.0 Control strategies

Earlier in the course we emphasised that the prevention of occupational ill-health relies

on the control of harmful chemical agents in the workplace. In other studies you will

have examined the hierarchy of control measures that are available to control

substances hazardous to health and we have briefly reviewed them earlier in this study

unit. Now we must consider these control options in more detail.

Methods of Control

If we look at the complete range of control options that are available for harmful

agents they can be divided into two broad groups:

Administrative controls , which concentrate on systems and procedures and

include:

Elimination

Substitution

Isolation or segregation


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Maintenance

Information, supervision and training

Hygiene

Engineering controls , which involve engineering hardware and include:

Process change

Enclosure

Ventilation

Personal protective equipment

It is also useful to consider the application of these different types of control in terms

of when and how they might be introduced and applied. It is much more effective and

economical to consider control measures at the design stage of a process and to

integrate them into the final plant or process design.

If this is not done and inadequate control is identified later in the lifetime of the plant,

it becomes much more difficult and expensive to bolt on health and safety control

measures. It is therefore important to be aware of what controls need to be considered

when plant or process is being designed, and what options are available when

problems arise on existing processes.

Design Safety Control Measures

When a new process is being designed, the flow sheet of materials can be scrutinised

at each stage to consider the following control features:

Careful selection of substances based on their hazardous properties, (physical,

chemical, toxicity, WEL, special hazards, emergency procedures).

Design of a totally enclosed process or system with a high standard of

containment.

Unwanted contaminants made safe as part of the process (by use of scrubbers,

absorbers or incinerators).


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Distance operators by use of automation or remote control.

Minimise maintenance through good design.

Operational Safety Control Measures

Once a plant or process is in operation the COSHH assessment methodology requires

us to demonstrate that the control of substances hazardous to health is sufficient to

prevent harm and occupational ill-health. If assessment demonstrates that existing

controls are inadequate then the level of risk must be reduced to an acceptable level

by applying remedial action.

The possible control measures that may be introduced can be classified under the

broad headings described in the next section of the study unit.

3.1 Control measures

Elimination

This represents the best and safest option since the hazard is removed from the

workplace completely. Elimination is the first control measure in the hierarchy and the

question: Can we do away with this hazard altogether? is one that should be asked at

the start of any risk assessment process.

In practice it is possible that if the chemical hazard can be eliminated it is likely to be

through substitution of a hazardous chemical with a harmless one, or by a process

change that no longer requires the hazardous chemical to be used. We shall consider

these control options in more detail below.

Substitution

If a chemical substance in use in the workplace is found to give rise to an unacceptable

level of risk then the use of alternative materials should be considered.


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When we studied the health effects classification of chemicals and the CHIP Regulations

we identified the broad range of harmful effects that give us concern, i.e. toxicity,

corrosiveness, irritation, and carcinogenicity.

We also considered the factors that affect the risk to the individual such as

concentration, solubility in body fluids, particle size, and susceptibility of individuals.

This information often provides the basis from which alternative materials that

represent a lower risk of occupational ill-health can be selected.

If we start by examining exactly what properties of the substance are important in its

workplace use, we can then consider alternatives that possess the similar functional

properties with a lower degree of hazard. For example, toxic pigments based on lead

have been substituted with less toxic pigments based on titanium dioxide without any

loss in essential properties.

Similarly silica has been replaced by alumina in the pottery industry, thus eliminating

the risk of silicosis. In addition, toxic organic solvents such as benzene have been

replaced by safer alternatives in paints and adhesives.

Even when the corrosive property of the chemical is important, as in cleaning

materials, it has been possible to replace highly corrosive mineral acids such as

sulphuric and hydrochloric, with weaker organic acids such as citric. Also in some cases

it may be possible to avoid a dangerous dilution operation by buying in chemicals at a

weaker strength rather than using cheaper bulk concentrates that put operators who

have to handle them at risk.

Finally, a change of form of the chemical substance may reduce the risk to the

workforce. A reduction in temperature or increase in pressure can condense gases to

liquids or cause liquids to solidify, with a corresponding reduction in potentially harmful

exposure. In this way the same chemical substance continues to be used but its form is

changed to reduce the harmful effects arising from exposure.

Changing the Process

It is likely that if the intention is to control the risk at source then some modifications

to the process itself will be necessary. It is often the case that significant reductions in

exposure can be achieved by minor modifications to the process, such as:


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Temperature reduction (referred to above) to reduce the generation of airborne

vapour

Prevent surface evaporation by covering (e.g. plastic balls or foam)

Wet methods to reduce dust generation

Use of pellets or flakes rather than powder

Automation or remote handling

Replace spraying with brush painting or dipping

Replace soluble compounds with insoluble compounds to reduce the risk of

absorption

Avoid the generation of fine dust or fume in the respirable particle size range

These are generally used measures but in practice each process or work activity needs

to be evaluated individually to determine the options for modifications. The aim is

always to secure the required reduction in risk without adversely affecting the viability

of the process.

Ventilation

As we mentioned earlier when we discussed the COSHH Regulations, although

elimination and substitution are our idealistic first choice of control measure, in practice

we often have to rely on some form of occupational ventilation to control the working

environment.

Local exhaust ventilation removes toxic dusts, gases or vapours at source before they

reach a level that could constitute a danger to health, whereas dilution ventilation

dilutes to a safe level contaminants that have been allowed to become airborne. Since

ventilation is such an important control measure we need to consider it in more detail

and re-examine its principles and method of operation.

At this point we shall therefore only acknowledge its position in the general hierarchy

of control measures and carry out a more comprehensive examination of occupational

ventilation systems later.

3.1 Control measures (Cont.)

Isolation or Segregation


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We noted earlier that these control strategies are administrative ones that attempt to

remove or protect the worker from proximity of exposure to the chemical contaminant.

They involve simple and sometimes effective methods, but do nothing to remove the

hazard itself.

A partial enclosure involving simple screening using physical barriers can be effective in

protecting against substances which are hazardous by skin contact or absorption. More

sophisticated isolation systems such as total enclosures under negative pressure, or

partial enclosures such as fume hoods fall into the category of engineering controls

(see later when we examine ventilation).

Segregation techniques involve administrative controls over either the number and/or

type of worker exposed, or the time period of exposure.

Reducing the numbers of employees exposed is a risk control method that is

particularly important for protecting those especially at risk from hazardous substances

on the basis of:

Age:

Young employees may be more vulnerable to particular agents

Older employees may be at risk through age-related conditions

Sex:

Pregnant women may need to be excluded from processes involving substances such

as lead due to the possibility of damage to the foetus.

The need to give special consideration to new and expectant mothers is covered in

the Management of Health and Safety at Work Regulations 1999 and in a special

guide from the HSE (HS(G)122).

Segregation can be achieved by separating from the process all but essential

personnel. Those that have to be involved in the process must be protected by other

means.

Reducing the time period of exposure is another method of segregation from the

chemical hazard. If exposure is limited to certain predetermined maximum levels, the

risk to health can be significantly reduced.


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Again there is no attempt to remove the hazard itself, but simply to control the effects

of exposure. This strategy forms the basis for the establishment of occupational

exposure limits which you will remember are based on time-averaged exposures over 8

hours (long term) or 15 minutes (short term).

Another strategy based on reduced time period of exposure is to restrict the operation

of certain hazardous processes to periods when the number of workers present is

small, such as at nights or weekends.

Maintenance and Housekeeping

The basis of this control strategy in relation to the prevention of harm from chemical

agents is to reduce the uncontrolled release of contaminants into the workplace or the

unnecessary accumulation of waste substances.

Proactive maintenance schedules and planned shut-down periods for more major work

should reduce the likelihood of breakdowns, leaks or spills. In addition, if a systematic

review of plant and processes is carried out it should be possible to identify in advance

the locations and occasions when unwanted emissions of harmful substances may

occur.

By devising suitable procedures, providing remedial materials and equipment, and

training staff, the combination of planned maintenance and coordinated housekeeping

should enable tight control of unwanted emissions, discharges and accumulations of

chemical contamination and waste.

Simple but effective techniques to achieve this include:

The use of vacuum methods rather than the sweeping of particulates

Timely removal of settled particulates from horizontal surfaces before they are

disturbed into the atmosphere

Immediate attention being paid to leaks and spills

Prompt disposal of solvent-contaminated rags and wipes

A related control strategy to this is the maintenance of good personal hygiene

throughout the workforce. This is important when the materials in question may be

rapidly absorbed by the skin or interact with it.


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Hygiene considerations may need to include:

Washing facilities, with attention paid to possible harm arising from aggressive

detergents used on the skin.

Provision of work clothes with changing and laundering facilities.

Designated areas for eating, drinking and smoking. These requirements are usually

incorporated into general workplace precautions requiring:

Maintenance of good standards of housekeeping.

Prohibition of smoking, eating and drinking where chemical substances are

stored or used.

High standards of personal hygiene including hand washing.

Minimum use of chemical agents in the workplace.

Information, Instruction and Training

The provision of information, instruction and training is an important supplement to the

other control strategies we have already referred to. In general, managers and

supervisors must be informed of the hazards in their area and the means by which

they are to be assessed and controlled. The workers who are at risk from exposure to

chemical agents must understand:

The risks arising from their work including additional factors that might increase

the risk, (e.g. smoking).

The precautions that should be taken including the reasons for them and their

correct application.

Monitoring procedures and their role in them.

Health surveillance arrangements and the significance of the results.

Emergency procedures.

It is likely that some information, such as emergency procedures, will be of importance

to all the workforce. In addition, specific information relating to the particular risks and

the preventative and protective measures associated with the individuals own

workplace will need to be provided on possibly an individual or small group basis. To


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ensure comprehension, information may need to be supplemented by instruction by

supervisors.

3.1 Control measures (Cont.)

Emergency and spillage procedures

Prepare plans and procedures to deal with accidents, incidents and emergencies

It should be a normal part of the risk assessment procedure that you consider the need

for special procedures to be used in the event of an accidental release or exposure to

the substance(s) in question. However, COSHH also requires that you prepare special

written emergency plans and procedures where the potential risks from such an

exposure go well beyond those associated with normal day-to-day work.

Examples given in the COSHH ACOP of events which may fall within this category are:

1. any serious process fire which could give rise to a serious risk to health

2. any serious spillage or flood of corrosive agent liable to make contact with an

employees skin

3. any failure to contain biological, carcinogenic or mutagenic agents

4. any acute process failure that could lead to a sudden release of chemicals, e.g. an

exothermic reaction that results in the release of toxic fumes.

5. Any threatened significant exposure over a WEL

Special emergency plans and procedures should be capable of:

mitigating the effects of the incident

restoring the situation to normal as soon as possible

limiting the extent of any risks to the health of any employees and anyone else

likely to be affected [e.g. people in the neighbourhood]

Further details of the required content of such a plan are given in the COSHH ACOP.

Personal Protective Equipment


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You will remember from your earlier studies of PPE that this control measure is the

least desirable for the operative, and is often inappropriate and ineffective. By using

PPE there is no attempt to reduce or eliminate the hazard and therefore it should only

be considered or used as a last resort.

Under certain circumstances, however, PPE may be deemed to be necessary:

Where the risk cannot be adequately controlled by any of the measures already

referred to.

As a short-term temporary measure where there is an immediate risk that needs

to be controlled until more acceptable control measures are introduced.

During maintenance operations where the existing control measures are

disabled, to allow access.

Where urgent action is required as part of emergency procedures during a

spillage or other loss of containment.

The key issues to consider in the selection of PPE are:

Evaluation of the type of hazard so that the appropriate sort of PPE can be chosen.

Determination of the appropriate hygiene standard for the hazard to enable selection

of the correct type of PPE which will give adequate protection.

Consideration of the needs of the user taking into account individual suitability,

comfort and ease of movement.

However, it is important not to lose sight of the fact that PPE is never the ideal solution

for protecting workers from the harmful effects of chemical agents. Remember that the

effectiveness of the control measure depends solely on the worker using it properly,

and there are a number of factors that can prevent it being used effectively:

Discomfort and restricted movement

Difficulty in fitting it and removing it

Visual obstruction

Time-consuming cleaning and maintenance

3.2 Control Measures Summarised

It might seem that there is a vast range of different control measures that can be used

to protect the worker from occupational ill-health arising from exposure to harmful


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chemicals. We have made reference to the hierarchy of controls in order to

demonstrate the systematic approach that should be made in examining each measure

in turn before a final decision is reached.

The hierarchy sets out the available controls in order of effectiveness, starting with the

aim of eliminating the hazard, significantly reducing its potential for harm, or securely

containing it, before more indirect controls such as controlled exposure or individual

protection are considered.

If the process of exposure to a chemical agent is considered as a three stage process:

emission of agent (vapour, liquid or solid)

transmission of agent (airborne contaminant, splash, spillage)

absorption of agent (inhalation, ingestion, skin contact)

Then we can see where the various control strategies apply:

control emission at the point of release (source)

prevent or control transmission of the agent to the individual (transmission

path)

protect the individual to minimise exposure and absorption (receiver).

The control measures that we have considered can be categorised in terms of these

three general control objectives, and this is shown in Table 3.1.

Source Transmission Path Receiver

Elimination Dilution ventilation Automation

Substitution Housekeeping Reduced exposure

Enclosure Maintenance Enclose worker

Process change Education/training

LEV PPE

Table 3.1: Summary of Control Measures

We can see that the preferred measures which are also the more effective onesare

those that control emission at source.

Less effectiveare those that attempt to prevent control or transmission of the agent

since we have now allowed the agent to escape into the workplace.


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Least effectiveare control measures that concentrate only on the individual and

consequently allow the chemical agent to completely permeate the workplace.

We have set out the range of strategies and control measures that are available and

the philosophy behind their selection. However, as we noted earlier, engineering

controls based on ventilation systems are some of the most common control measures

that you are likely to encounter with chemical agents and we shall now consider these

in detail.

4.0 Occupational Ventilation

We have reviewed the hierarchy of control measures for chemical agents and

considered the ways in which each of these control strategies or techniques might be

utilised in the workplace.

In the case of ventilation we acknowledged its special importance in controlling the

working environment and thus the need to study the operation and application of

ventilation techniques in more detail.

We shall begin our study of occupational ventilation by examining the physical

properties of chemical agents that are relevant to the design and operation of

ventilation systems and also some key ventilation terms, before moving on to review

the more important types of ventilation system, including the ways in which we

monitor and maintain their performance.

Physical Properties of Chemical Agents

Liquids

Liquids can be imagined as a large collection of small particles of material moving in

constant turmoil in the confines of the vessel in which they are contained.

The energy of motion of each particle varies considerably and there are always a small

number of particles energetic enough to leave the liquid and form a vapour over the

liquid surface.

The amount of motion within the liquid is dependent upon its temperature. Just above

its melting point, only a few particles are able to become vapour. With a rise in

temperature, the motion increases and more particles become part of the vapour state,

before returning to the liquid.


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When the whole liquid has enough energy for the vapour particles to become

sufficiently energised that they move freely from the liquid and remain in the vapour

state, then the liquid is at its boiling temperature.

The process of moving into the vapour state is called vaporisation. The process of

returning to the liquid state is called condensation.

4.1 Vapour Pressure

The motion of the liquid particles becoming vapour over the liquid produces its own

pressure against the atmospheric pressure. The number of vapour particles sustained

in the atmosphere over a liquid gives the liquid a vapour pressure. The number of

vapour particles in equilibrium with the liquid depends upon the temperature of the

liquid.

The curve given in Figure 6.4 illustrates how the vapour pressure varies with

temperature.

When the vapour pressure equals the atmospheric pressure the liquid is totally free to

produce vapour. This corresponds to the boiling temperature. You will note:


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Variation in atmospheric pressure will alter the boiling temperature. This is why

water boils at room temperature by application of a vacuum and why the boiling

temperature of water increases in a pressurised steam boiler.

A liquid has the ability to vaporise below its normal atmospheric boiling

temperature.

The ability of a liquid to vaporise will also depend upon the mass and size of its

molecules. In general at a given temperature, small low-mass molecules evaporate at

a higher rate than larger high-mass molecules.

Hence, low relative molecular mass solvents will have a lower boiling temperature than

higher relative molecular mass solvents and will vaporise more easily to form vapour

concentrations of potential toxic or corrosive harm, or to produce a vapour/air mixture

within the flammable limits.

Evaporation

Evaporation refers to the process whereby the vapour formed over a liquid does not

return to the liquid. The liquid volume will then decrease. Evaporation occurs at all

temperatures, the vapour being removed mechanically from over the liquid surface,

e.g. by an air flow.

The rate of evaporation of a liquid depends upon two main factors: firstly the

temperature of the liquid, i.e. the higher the temperature the greater the vaporisation;

and secondly the area of the free surface, i.e. the greater the area the greater the

evaporation rate.

Relative Vapour Density

Relative vapour density (RVD) is a useful concept which compares the density of a gas

or vapour with that of air under the same conditions.

Any RVD(air) value less than 1.0 indicates that the gas/vapour is less dense than air

and will tend to rise in air until it becomes dispersed at high levels in the atmosphere

or layers under a roof or structure which prevents it rising.

Any RVD(air) value greater than1.0 indicates that the gas/vapour will tend to fall to the

lowest floor or ground level and layer over the surface. The vapours of organic solvents

encountered occupationally will all have an RVD(air) greater than 1.0. Gases will have

values that are greater or less than 1.0.


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A selection of examples is given in Table 4.1.

Substance RVD(air)

Solvent Vapours

Ethanol 1.6

Ethoxyethane (ether) 2.6

1,1,1, trichloroethane 4.6

Trichloroethylene 4.5

Cyclohexane 2.9

Gases

Carbon dioxide 1.5

Carbon monoxide 1

Hydrogen 0.07

Methane 0.55

Hydrogen sulphide 1.2

Table 4.1

4.2 Layering of Gaseous Substances

Gaseous substances diffuse slowly into the atmosphere around them. The slowness of

the diffusion process is clearly demonstrated by the brown plume of oxides of nitrogen

issuing from the exhaust stacks of most nitric acid plants. The gradual widening gas

layer can be clearly seen extending for miles before diffusion into the air occurs.

In coal mines, a thin layer of methane in the roof of the workings can cause fires or

explosions a long way from the ignition source as the flame tracks through the

methane layer. The layering problem of gaseous products with RVD(air) over 1.0 is

demonstrated in sewers where hydrogen sulphide can layer over the floor to a depth

which could envelop a sewer worker descending from a manhole. It is therefore vitally

important that atmospheric testing of sewers begins at the lowest level. A cursory test

taken in the descent shaft can lead to potential gassing accidents (and has indeed done

so).

The vapours from liquid petroleum gases (LPG) with relative molecular masses over 40

will also layer over the lowest ground level to produce a potential fire, or simple

anoxic/narcotic hazard in sewers, cellars or service tunnels.


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Solids

Solids is a term used to define particulate solid matter. For the purposes of

occupational hygiene, hazardous solids are usually dusts, i.e. small size particles which

are capable of being carried in an air flow. The most important physical characteristics

of dusts in ventilation theory are their mass/size ratio. Where particle sizes are very

small they are capable of being supported in still air by collision with the random

motion of the air molecules.

This is called Brownian movement. (A crude analogy would be a group of footballers

keeping the ball airborne by continuous heading.) Solids in this size range are easily

transported in an air flow.

When the particle size increases the air molecules are not able to support the particles

and they fall under gravity. To transport solids in this size range a dynamic air flow

force has to be applied to overcome gravitational forces.

Air flow velocity capable of providing the necessary forces has been computed for

various particulate materials. As a general rule air flow velocities of about 20 m s1 are

able to support most dusts encountered occupationally.

Microscopic equilibrium between gas and

liquid. Note that the rate of evaporation of

the liquid is equal to the rate of

condensation of the gas.

Microscopic equilibrium between gas and

solid. Note that the rate of evaporation of

the solid is equal to the rate of

condensation of the gas.

4.3 Ventilation Terminology

Pressure

For air to flow there must always be a pressure difference (pd) between the inlet and

outlet of the system. In occupational ventilation the pressure difference is generally


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created by the use of a fan. Thermal draughts, i.e. air flow caused by differences in air

density, can be used to assist the fan action.

The pressure in air flow, i.e. the total pressure (Pt), is made up of two pressure

components, static pressure (Ps) and velocity pressure (Pv), where:

Pt = Ps + Pv

Pv is always a positive value but Ps can be either negative or positive; on the suction

side of a fan it is negative, on the discharge side positive. The sign of the Pt will

depend upon the relative value of Ps and Pv.

Static pressure is exerted in all directions in a fluid. The velocity pressure is

proportional to the kinetic energy of the fluid and is operated parallel to the direction of

flow. Within a given system there can be an interchange between the contribution

made by static and velocity pressure.

This relationship is shown dramatically when a venturi or orifice is inserted in a flow

system. The increase in flow velocity of the fluid through the constriction causes a

rapid fall in static pressure to satisfy the need for increased kinetic energy.

Air Volume Flow

The volume flow of air through a local exhaust ventilation system depends upon the

average air velocity (v) across the ducting and the cross-sectional area (csa) of the

ducting. The volume flow rate (Q) = v csa.

Where dilution ventilation is used, the air change rate is determined by first calculating

the total throughput of air per hour and then dividing this by the volume of the

workplace.

Inlet/Exhaust Air Flow Patterns

The inlet/exhaust air flow pattern of a local exhaust ventilation system is a very

importantpractical piece of knowledge which you must thoroughly understand. It is

best appreciated by studying Figure 6.5.


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You should note that for an inlet and exhaust section of equal dimension the exhaust

air flow maintains an influence on the surrounding air for a considerable distance from

the exit point.

The inlet air flow has little effect upon the surrounding air. Calculations have shown

that at about a distance of one diameter from the inlet section the air velocity is about

one tenth of the face velocity of the inlet.

On the other hand, the outlet velocity falls to only one tenth of the exit face velocity at

about 30 diameters. Local exhaust ventilation systems therefore have an inherent

inefficiency within their operating mechanism.

Face Velocity

Face velocity refers to the velocity at the opening of the hood of a local exhaust

ventilation system.

Capture Velocity

Capture velocity is the minimum velocity required to remove a pollutant from its source

and draw it safely into the hood of the exhaust system.

Transport Velocity

Transport velocity is the minimum velocity required in ventilation systems to prevent

solid airborne contaminants falling out of the air flow and depositing in the ducting.

4.4 Ventilation Systems

Dilution Ventilation

Dilution ventilation operates by simply diluting the contaminant concentration to an

acceptable level. This is achieved by efficiently changing the whole workplace air over a

given period of time, i.e. air changes per hour.

The workplace air is extracted by the use of fans set in the walls or roof.

The system removes gaseous contaminants (sometimes fumes) and has two main

purposes:


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To reduce the concentration of a contaminant to below the occupational exposure

limit.

To keep the concentration of a flammable substance to below its lower explosive

limit.

Where both a harmful and flammable substance is encountered, e.g. propanone

(acetone), then control of the first objective will invariably control the second.

Dilution ventilation has fairly limited use as an effective control strategy in occupational

hygiene. It can, however, be used with reasonable success provided the contaminants

conform, where applicable, to the following:

The DEL of the harmful substance is high.

The vapour pressure of liquid is low, i.e. it has a low evaporation rate.

The rate of formation of a gaseous product is slow.

Operators are not in close contact with the contamination generation point.

A hazardous substance is carried swiftly away from the operator, e.g. by hot

gases.

When contaminants are to be removed from a workplace using dilution ventilation, two

important criteria have to be considered:

The first is the rate of contaminant generation and hence the number of air

changes per hour required. Relevant factors involved in contaminant generation

of vapour from liquid include the vapour pressure and potential to evaporate at

the operating temperature of the system; the surface area of the liquid surface

in contact with the workplace air; the potential increased surface area, e.g.

contact adhesives generate vapour at a much greater rate after they have been

spread over a surface, or complex metal parts with a covering of solvent after

they have been removed from a degreasing bath.

The other criterion is the position of the extraction fans. The important factor

which controls the positioning of the extraction fan unit is the RVD(air) of the

contaminant. As indicated earlier, the RVD(air) of common solvents is greater

than one, therefore they will tend to layer over the lowest floor area in the

workplace. For such conditions fans should be positioned in the walls at a low

level. Where there is a fire hazard, e.g. in the use of ethoxyethene (ether), then

the motors of the fans should be flame proofed to at least a ZONE 1

classification, i.e. an area where an explosive mixture is likely to occur during

normal working.


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Where the RVD(air) is less than one then the contaminant will rise; for this situation

the fan must be positioned high on the workplace walls or in the roof. Simple propeller

type fans are used for dilution ventilation systems as illustrated in Figure 6.6.

b4image015.jpg

A major problem in setting up an efficient dilution ventilation system is the formation

of dead areas. These are areas in the workplace which, owing to the air flow pattern

produced by the extraction fan and the inlet of make up air, remain dormant and so

the air is not changed. Dead areas can be detected by the use of smoke tracer tubes. A

high density of smoke will remain in the unventilated areas.

A secondary problem with dead areas is that they can move from one position in the

workplace to another. Such moves can be produced by changing the inlet for the

make-up air, i.e. in cold weather the inlet may be spread over the workplace via the

cracks between windows and doors.

In hot weather indiscriminate opening of doors and windows will produce a quite

different flow pattern. Moving the position of machinery or workbenches can also cause

the same problem. To help reduce the problem, controlled air make-up inlets can be

constructed.

Where large quantities of air are being used to carry out the dilution process then

consideration must be given to recycling heat losses from the workplace. This can be


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achieved by using heat exchange systems whereby make-up air is heated by the

exhausted air.

4.5 Local Exhaust Ventilation

Local exhaust ventilation operates by removing a contaminant at the point of

generation and ducting it away in an air flow to a safe place.

In general, a local exhaust ventilation system is made up of five main parts, as shown

in Figure 6.7 :

The hood or exhaust inlet

Ducting

Filter or purifying system

Fan and motor

Exhaust outlet


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4.6 Types of Local Exhaust Ventilation Systems

There are basically two types of local exhaust ventilation system which can be

classified according to the type of hood used to receive the contaminant:

Receptor Hoods

These are usually large structures and depend mainly upon the contaminant being

directed naturally into the hood by thermal draughts, directional movement (for

solids), or by local generation. The natural movement is aided by exhaust draughts

produced by fans.

Except in spray booths where almost total enclosure is used, the design of smaller

receptor hoods should be such that the point of contaminant generation is well inside

the hood. Where thermal draughts are involved, rectangular cone hoods with ample

side cover of the contaminant and supported vertically over the hazard area are

satisfactory. This type is illustrated in Figure 6.8.


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Receptor hoods are placed over the work like a canopy (see Figure 6.8) and the

airborne contaminant enters the system without inducement (i.e. via thermal currents

or by low density contaminants naturally rising).

Hoods should not be placed over the top of the work if operators have to lean over into

the path of the air as it rises, and the general design principle should be that of

preventing operators placing themselves between the hood and the contaminant

source.

In order to avoid uneven air velocity distributions across large hoods, the face of the

hood can be divided up into a series of slots to ensure even distribution. Also, the air

volume rate can be minimised by placing the hood as close as possible to the point of

release of the contaminant. It is important that the air flow rate should be able to

overcome extraneous air currents such as side draughts or those caused by the

movements of the operator.

Captor Hoods

Captor hoods are designed so that the air stream captures the contaminant outside the

hood and induces a flow into the hood and ductwork. Consequently, the air flow rate

must be capable of capturing the contaminant at the furthermost point of release and


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the important design parameter is the capture velocity required for the particular

contaminant in question.

For example, an air flow rate of 0.5 m/s would be adequate to capture vapour

evaporating from a tank, whereas a much higher capture velocity of 10 m/s would be

required for particles released during grinding.

These hoods are placed as near as possible to the hazard generation area.

Contaminants are captured by air flow into the hood, which can be achieved by two

methods: firstly, by using shaped hoods in conjunction with a large air volume flow at

a specific velocity, operating at low pressure; or secondly by using a low volume/high

velocity (LVHV) air flow system operating at high pressure (pressure being given in

negative units).

The shaped hoods have two forms, either a round bell or round flanged cone as shown

in Figure 6.9 or a slit opening as shown in Figure 6.10, where L , the aspect ratio, is

less than 0.2.

Figure 6.9

Based on comparative structures, slit hoods have a more effective capture capacity

than round hoods.

Slots

Hazardous substances -pre

36 B4 Hazardous Substances -Preventive and Protective Measures

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