Measurement and Analysis
To identify and quantify chemical health hazards
Health effects can be acute or chronic, so there are different types of measurement to account for this: Long-term measurements to assess average
exposure over a given time period
Continuous measurements that can detect short-term acute exposure to high concentrations of contaminants
Spot readings to measure acute exposure if the exact point in time exposure is known
2 types of sampling:
Static sampling
Primary aim is to assess effectiveness of engineering controls or measure plant emissions into work environment
Levels should be set well below personal exposure limits
Personal sampling
Assess individual exposure
Factors influencing airborne concentrations:
No. of sources of contaminant
Rates of release from each source
Type and position of each source
Dispersion or mixing of contaminants
Ambient conditions e.g. wind speed, air temp.
Before devising sampling strategy, carry out initial appraisal. Info. required: Substances which occur in workplace
Airborne nature of substances
Hazardous properties of substances
Synergistic effects
Possible exposure by inhalation, ingestion, skin contact?
During which processes/tasks will exposure occur?
Groups/individuals at risk
Likely pattern and duration of exposure
Initial Appraisal:
Simple qualitative tests can be carried out:
Dust lamps
Smoke tubes
Stain tubes
Strategies:
First level for basic surveys
Second level for more detailed surveys
Third Level for surveys needing high degree of sophistication
First level strategies:
Used where crude quantitative information is required so that decisions can be taken as to whether problem actually exists, prior to conducting detailed survey
Divide population into groups in relation to work or degree of exposure
High risk groups can be studied in detail
Personal sampling, especially at peak periods
Second level strategies:
Appropriate for most detailed surveys and for routine monitoring
Emphasis on accurate measurement of average exposures and relating them to OEL’s
Entire period of exposure should be covered
Third level strategies:
Occasionally high degree of sophistication required
E.g. if all reasonably practicable steps have been taken and exposure is still close to OEL; or
Where OEL is very low so small changes in exposure are significant
Consideration of results should always lead to answers to 5 questions:
Is immediate action necessary to eliminate or reduce exposure?
Is immediate action necessary to re-establish adequate control?
Is a programme of planned improvements necessary?
Is a more detailed survey required?
Should routine monitoring be implemented or continued?
Series of detailed descriptions of analytical methods approved by HSE
Provide reliable and consistent methods
Covers sampling AND analysis
2 Fractions:
Inhalable fraction:
Total particles inhaled through nose and mouth
Respirable fraction
Less than 10 microns
Able to penetrate the respiratory system as far as the alveolar region
Fibrous particles:
Shape is as important as size
Long thin fibres more dangerous
Components: Sampling head
Size selection Cyclones to separate respirable fraction
Filters Used to collect sample before assessment
Wide range depending on contaminant
Pumps Must be able to be worn on body if personal sampling
required
Flow meter required
Static sampling:
Complete assembly includes filter, pump and flow meter
Left unattended in workplace
Use cyclones to remove larger particles
Personal sampling:
Preferred technique - more realistic measurement of exposure
Cyclones used to remove larger particles
Direct Reading Instruments:
Gives instantaneous result
Can be:
Light scattering
Beta particle attenuation
Oscillating micro-balance (quartz crystal oscillation changes with build up of dust)
Measured volume of air drawn through filter
Mass of dust collected is determined by weighing filter before and after sampling
Size selection devices can be used to measure respirable dust
Volume of air passing through filter is calculated by multiplying flow rate (cubic m per minute) by sampling time (minutes)
Weight gain of filter (mg), divided by the volume sampled, gives average dust concentration in mg per cubic meter of air (mg/m3)
Similar to dust, except: Open faced filter holder
Membrane filters which can be rendered transparent to allow fibre counting by phase contrast microscopy
Flow rate is specified and measurement must be over 4 hour period
Fibres are counted on membrane filter
Concentration is calculated by dividing total no. of fibres collected on filter by total volume of air to give fibres per millilitre (f/ml)
Grab Sampling: Sample taken in flask, bottle bag etc.
Useful for peak concentration or when concentrations are constant
Continuous or Long-Term Sampling: Sample removed from air over measured time
period and concentrated by passage through solid or liquid sorbent
Useful if: Concentration varies time with time
Concentration is low
TWA exposure is required
Evacuated flasks
Atmosphere drawn in
Gas/Liquid displacement container
Flask connected to a pump
Flexible plastic containers
Plastic bags
Hypodermic syringes
Liquid sorbents
Cold traps
Plastic sampling bags
Solid sorbents
Charcoal or Silica gel
Sampling equipment:
Pump
Adjustable flow rate
Able to be worn if personal sampling required
Flow measurement
Important to know flow rate to calculate exposure
Diffusive samplers
Badge or tube type
Factors affecting performance:
Temperature and pressure
Humidity
Concentration variations
Sorbent efficiency
Face velocity
Sample positioning:
General working atmosphere (grab sample)
Operator’s breathing zone (TWA sample)
Close to contaminant generation (continuous monitored sample)
Sampling frequency:
Statutory in some cases (asbestos every 4 hours)
Depends on level of risk
Most involved subjecting substance to burst of energy and examining way substance responds
Response is characteristic of substance and can be used as “fingerprint”
Magnitude of response can be used to estimate how much of agent is present
Techniques: Gas Chromatography
Atomic Absorption Spectroscopy
Infra Red Spectroscopy
X-Ray Diffraction
Optical Microscopy
Chemical is carried down an absorbent column by a carrier gas
The length of time the sample takes to travel down the column is unique to the substance
Size of spectrum peak indicates quantity of substance
Mixed substance can be separated
Used for metallic substances
If certain metals are heated to high temperatures in a flame, electronic changes in the metal atom cause a change in colour to the flame
Sample is injected into an air-acetylene flame and resultant spectrum is analysed by an atomic absorption spectrometer
Both identity and quantity of substance can be determined
Based on principle that chemical bonds that connect atoms into molecules are continuously vibrating and the energy of this vibration falls within the infra-red wavelength range
Infra-red radiation is passed through the sample and the absorption spectrum gives a characteristic fingerprint of the substance
Identifies and quantifies substance
Used for solid analysis
X-rays passed through a sample are diffracted in a characteristic fashion, which depends on the crystal structure and spacing between atoms
Gives characteristic fingerprint of substance
Mostly widely used for fibrous dust
Dust is collected on membrane filter, then counted under optical microscope
As sampling time and flow rate are know, fibres per unit volume can be calculated
Where it is necessary to determine type of asbestos, polarised light microscopy is used
Different types of fibre show different colours under polarised light
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