International Journal of lndustrial Ergonomics, 10 (1992) 331-340 331 Elsevier

S h o r t c o m m u n i c a t i o n

Health effects of vibration in agriculture - Pilot for an epidemiological study

Nige l R o b e r t s o n a n d L o r r a i n e G a r d n e r

Research Institute for Consumer Ergonomics, Swingbridge Road, Loughborough, Leicestershire, LEl l OJB. UK

(Received January 17, 1992; accepted in revised form August 1992)

A b s t r a c t

A pilot study has been conducted to prepare for an epidemiological study into the effects of vibration on tractor drivers. The reason for the study is that, despite a large volume of existing data on health and vibration (over 43,000 exposed workers having been studied) there is still no good basis for estimating dose-response relationships between vibration and health. There have been three main difficulties: - Many studies have not measured vibration, or have not measured it thoroughly enough. - Confounding variables such as posture have not been entirely controlled for, because well-matched control groups do not exist. - Health data have tended to be binary incidence data rather than continuous scale data, reducing the sensitivity of the studies. The pilot study has produced procedures to overcome these difficulties.

R e l e v a n c e t o i n d u s t r y

The methodology should enable the subsequent epidemiological study to identify dose-response relationships between tractor driving and vibration damage to health. This would enable informed policy-making for vibration standards.

K e y w o r d s

Vibration; agriculture; tractor; driving; epidemiology; methodology; health; spine.

I n t r o d u c t i o n

The work repor ted here is the Pilot study for a major epidemiological invest igat ion on the heal th effects of v ibrat ion in agriculture. The complex s t ructure of the study is a result of the many sources of heal th damage in agriculture. The pos- sible sites of whole-body-vibrat ion damage in- clude not only the lower back, where the evi- dence of ill effect is quite strong, but also the

Correspondence to: Nigel Robertson, Research Institute for Consumer Ergonomics, Swingbridge Road, Loughborough, Leicestershire, LEll 0JB, UK.

digestive system, the geni to-ur inary system, the cardiovascular system, and musculo-skeletal dis- orders in the neck and shoulders (Bongers and Bushuizen, 1990). All these body parts are ex- posed to risks as a result of the agricultural env i ronmen t and the tasks that have to be per- formed. The lower back and the whole musculo- skeletal system are at risk both from s lumped and twisted sitting postures, and from mechanica l loads imposed by lifting and general heavy me- chanical work. Other body parts are exposed to risks from chemicals, stress and the poor diet which tends to arise from constraints on eat ing dur ing the 10- to 16-hour days often worked in the industry.

0169-1936/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

332 N. Robertson, L. Gardner / Health effects of cibration

The purpose of the study is to gather data which will enable the vibration-induced damage to be identified from the rest. The role of the Pilot study has been to develop a practical methodology which is capable of achieving that goal.

Basic des ign of the study

The results of previous studies indicate some key points about the design of epidemiological studies of lower-back trouble and vibration. These are: (1) Vibration exposure must be measured or esti-

mated, together with exposure duration, to calculate the vibration dose. It is clear that prolonged exposure to 'severe ' levels of vibra- tion increases the incidence of lower-back trouble. What are needed are dose- response data.

(2) Confounding factors must be dealt with at the design stage. For tractor drivers the most difficult of these is posture: so far no-one has identified a control group of workers who sit for 8 to 12 hours while repeatedly twisting to look behind, but who are n o t exposed to vibration. All the resulting symptoms have a number of possible causes, such as stress and diet for cardiovascular symptoms, chemical exposures for digestive disorders, and heavy lifting for lower-back trouble.

(3) There is a wide range of data from which damage to the low back has been inferred. They include sick-leave, disability pension data, the number of days reported for back pain, and incidence of back pain over 7-day and 12-month periods. These data have the disadvantage that although studies have gen- erally collected a number of different types of data, the individual data types cannot be ag- gregated. This produces results which com- prise several separate coarse measures, such as incidence of lower-back trouble within the last 7 days, and incidence of visits to a health professional within the last year (Nordic Questionnaire, Jonsson, 1986; Kuorinka et al., 1987). Data like these need large sample sizes to give reliable results, like those obtained by

analysis of health records, or by an extensive postal survey; however this study is to use a relatively small sample with interview, in or- der to estimate the vibration dose accurately.

(4) Radiographic diagnosis is a rather coarse tool (Nachemson, 1976; Wikstrom, 1978), and is now considered unethical for non-clinical use. However, the use of questionnaires is rela- tively simple yet effective - self-reported pain is a valid measure (Williams, 1988) and physi- cal examination adds little to the symptoms identified by interview and questionnaire (Hildebrandt, 1985).

The study was designed with these factors in mind. The problem that no control group is avail- able is addressed by concentrating on agricultural contractors, and obtaining an estimate of the vibration dose. If there is sufficient variation among the contractors' exposures, as a function of doing different driving tasks, then a dose-re- sponse relationship may be observed, with mini- mum interference from non-driving tasks. Both dose and response data will be continuous nu- merical data (as opposed to binary or categorical data). If this approach is unsuccessful, an at tempt at establishing a control group can be made; however, many researchers have failed to find a dose- response relationship by these means. An interview was considered essential to obtain the level of detail required about the tasks which make up the tractor-drivers' jobs. The question- naire design was made variable-based rather than question-based, to ensure that useful and analysable data are collected. A set of variables was established, with reference to the literature, visits to farms, and expert advice.

This set of variables was chosen to meet sev- eral criteria: - It had to include all the causative components

of vibration, as far as was practicable. - The set had to enable subsequent insertion of

measured vibration values or factors. - It has to include some means of accounting for

the many confounding factors in lower-back pain causation, which are mainly in heavy man- ual work and posture, but also include acci- dents and diseases.

- It needed to be collectable by interview, in a reasonably short period of time - ideally half- an-hour.

N. Robertson, L. Gardner / Health effects of vibration 333

- It needed to be suitable for computerised data handling and analysis. This meant binary, cate- gorical or numerical forms for the data.

The questionnaire and interview documentation were then designed, to collect only the specified data.

The interview documentation has three sec- tions: (1) The Job Profile enables detailed estimation

of the vibration exposure in the current job. (2) The 12-Month Back History records the

severity of lower-back pain (if any) experi- enced on each day of the preceding year. It uses a pain-scale from 'no pain' to 'could not be worse'. The 7-point severity scale can be treated statistically as a ratio scale, and the area under the curve (e.g. severity x days) can be used as a single aggregated numerical indi- cator of back pain over the year.

(3) The Questionnaire addresses confounding variables such as impact damage, previous jobs, and hobbies. It also records the amount of time in the current job, at the current vibration exposures, uses questions from the Nordic Questionnaire for comparison with other studies, and enables consistency checks to test the 12-month Back History which fol- lows.

older workers, other factors being the same. In addition, workers who leave their jobs because of back pain are a serious source of difficulty or error in a study such as this; this behaviour will be less frequent among younger workers.

Use full-time agricultural contractors rather than farm workers generally. The non-driving tasks done in agriculture are often heavy manual tasks, such as lifting, moving equipment and so on. Because these are risk factors for lower-back trouble in their own right, the hypothetical vibra- tion effects may be less damaging than the non- driving alternatives. Bongers and Boshuizen (1990) describe vibration exposure effects in terms of an equivalence with heavy manual jobs. The object in using agricultural contractors is to have subjects who spend a minimum of time doing non-driving tasks, but whose vibration dose is variable.

The study was also influenced in this decision by early information that 'most contractors are under 30'; this has since proved to be incorrect. The subject age limit, initially set at 30, has therefore had to be raised to 40 years. However the principle of young subjects will be used as far as is practicable, and the use of agricultural con- tractors remains desirable.

Subjec t s e l ec t ion

A decision was taken to place certain limita- tions on the scope of the study, to help control of some of the confounding variables. These were:

Use young subjects, up to and including 40 years old. The ageing process, and the accumulating exposure to risk factors for lower-back trouble, raises the incidence of lower-back trouble with age (Gruber and Zipperman, 1974; Jonsson, 1986). A consequence of this is that the incidence of lower-back trouble is so high among older farm workers (Sjoflot, 1980) that the influence of vibra- tion would be difficult to isolate - ' they all have bad backs'. Another factor is that the amount of heavy manual work has tended to decrease in agriculture in the last few decades. The risk expo- sure for load-related lower-back trouble has probably been lower for younger workers than for

It has not been possible to control for bias in response rates, such as a predisposition to volun- teer among back-pain sufferers. However, be- cause the study is looking for dose-response rela- tionships, rather than incidence data, this is not thought to be an insurmountable problem. The main criterion will be to include a range of back- pain severities and durations, with which to corre- late vibration doses.

T a s k a n a l y s i s

The task analysis had two objectives: (1) To identify confounding sources of lower-back

trouble or other health problems. (2) To enable estimation of vibration dose. The pilot study addressed these aims mainly by designing into the interview a section gathering detailed data about tasks. Initial background in- formation about tasks was obtained by making

334 N. Robertson, L. Gardner /Health effects of t'ibration

visits to farms, and to the Nottingham School of Agriculture.

Forms and questionnaire design

The interview documents are shown in figures 1, 2, and 3. A set of desired variables was drawn up, taking account of all the factors described. Then the interview documentation was prepared from the list of variables, and was designed specifically for use by a skilled interviewer famil- iar with the study. Each of the three sections (Job

Profile, 12-Month Back History and Question- naire) has its own design. The Job Profile (figure 1) is a matrix of tasks by other variables, matching the layout of the spreadsheet into which the data are entered. The heading of each column speci- fies the valid data options, which the interviewer reads out, if necessary, to the subject. Capital letters are valid abbreviations for the words con- taining them. The Questionnaire (figure 3) is conventional, and the 12-Month Back History (figure 2) is a matrix of reported back pain sever- ity by time.

S.bj=,.~ C"

Task

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I.D~I L L I M 6-

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Tractor 4wd / make/ritual 2wd or

11011-

llractor

text 2/4/0

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. I 4

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Job Proff ie . Weights Tractor on front age

~111 years

P;A l

Y y 3

~vl~ 3 Y 3 Y q y 3

Road Reed Task Tyres Tool time speed speed Viba per day

h o f s m J mph psi h/n~i

6 o go l/C, t_

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o,2s" ,~ ~ 6 L

o.5- /'/ g 6 M o . f t~.( f ~ f /4 #,o ,, ~ g, ¢" 2 ~ /'1

Fig. 1. Job profile.

N. Robertson, L. Gardner / Health effects of vibration 335

Pilot results

Subject recruitment

Subjects were recruited by phone, using the listings in the industrial and commercial tele- phone directory for 'Agricultural Contractors'. A script was prepared, for use as a guide during phone calls, setting out who the caller represents, the purpose of the study, and the request for an

interview with full-time agricultural contractors up to 30 (later 40) years old. All the contractors listed in the 3 counties of Leicestershire, Notting- hamshire and Derbyshire in England were called. In all about 200 phone calls were made, from which seven interviews were arranged. The small number of acceptances was mainly the result of very high seasonal workloads in farming during July and August. The main study will need a much higher acceptance rate, as well as a wider

Time Swivel Seat looking scat

back No Fixed

Used Aircush % Disused ~ S T g

2 0 5 Io to Zo ~ s

~ o 5

Seat setting:

Ba~k As found

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5

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Shovel, Horns Conscc Days hours per -mive pet per stint clays year stint

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hough Medium Sraomh

S S B

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Um' m mPmUlm

Total number of days driving per year_.~. 0 ~ Fig. 1. (continued).

336 N. Robertson, L. Gardner / Health effects of cibration

sea rch area . T h e f a rming work load falls o f f a f te r S e p t e m b e r , and is very qu ie t in N o v e m b e r and D e c e m b e r : it is h o p e d tha t the a c c e p t a n c e ra te will t h en rise sufficiently.

Data capture and handling

T h e da ta were e n t e r e d into a s p r e a d s h e e t pro- g ram on a P C for stat ist ical analysis. T h e da ta

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C o u l d n o t b e w o r s e

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O a l e ~ a r a o o u m t e ? ~ . ~ / ? (circle). Date: . . . . . . . . . . . . . . . . . . . . . . . . .

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Fig . 2. 1 2 - M o n t h b a c k h i s t o r y .

N. Robertson, L. Gardner / Health effects of t~ibration 337

from the 12-Month Back History were entered as a string of numbers with a minimum value of 1 (no pain) and a maximum of 7 (could not be

worse). 1 was then subtracted from all the scores, leaving a range of 0 to 6. This procedure allows the interview form to present a full range 7-point

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Calendar accurate? Y/N/? (circle). Date: . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 2. (continued).

338 N. Robertson, L. Gardner / Health e/fects of cibration

scale to the subject, with a visible zero (which is already filled in). The pain-scale values were then summed, and also squared then summed; they

can easily be raised to any power. The pain duration is simply the count of entries over zero, i.e. the number of days on which pain is reported.

How long have you been doing this job (or a near-identical one)?

How old are you?

Weight

Height

Did you have another job before this one? If so 1 how many? I~sc~:~ion of the p~vious jobs, when. for how Ionlk ¢hiving %

Low back risk estimate

Have you done any sport or hobbies outside work? Description of sport/hobby, and frequency & intensity, dates eg motor sport, rugby, fishing

Low bac~ risk estimate

Have you ever had low back trouble? (demonswate low back area)

How long ago was the last time?

Talk about it, fill in where possible: How long ago was your first spell of low back Irouble?

Was it caused by an accident? (collision, fall)

How long did it last? (or has it lasted)

tO

zs"

s'l o

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¥ q-

Stme

Ptl h

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Y/N

¥/N

, m a m t m

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daye

Have you been seen by a doctor or osteopath or anyone like that. because of low 7__

back trouble? If so, how long ago7

Have you ever been hospitalised because of low back trouble? I~J

(If either) What did they say was the problem? (ie diagnosis) [ ~ L L ~

Her di /non-m hani I Fig. 3. Questionnaire.

Y/N

~ N A

N. Robertson, L. Gardner / Health effects of cibration 339

Have you ever had to change jobs or duties because of low back trouble?

Do you avoid some tasks because of low back trouble?

Have you ever had leg pain associated with your back pain?

Or any other sensations in your legs, like numbness or tingling?

How far down your leg did/does the trouble extend?

J~ Y/N

Y/N

~de/NA

How many times a year did/do you get back Irouble?

Did/does it come on at w o ~ or because of work? (ever)

Have you had to have any time off work because of your back? If so, how long

ago?

For how many days in the last 12 months

Y/N~/NA i

0=no

In the last 12 months, has your back pain affected any of the following activities?

Standing still for 15 minutes

Getting up from a low chair

Putting on socks Other Health Ouestions

Do you get headaches?

Do you get problems with your digestion, or constipation7

Do you get piles?

Do you have any heart trouble?

Do you have trouble with your neck?

Do you have trouble with your shoulders or upper back? i

Do have any other health problems? [

Y/Nn

¢

Y/Nft

N Y/Sft

t m ~

Do you know anyone who left the job or this farm because of trouble with their back? [

Get name, address, age etc i f poss.

Fig. 3. (continued).

N

340 N. Robertson, L. Gardner / Health effects of cibration

This var iable can also be ra ised to any power, enabl ing the re la t ive weight ing of pain dura t ion and severity to be changed at will for analysis.

r e la t ionsh ip with v ibra t ion exposure and dose than has been poss ible with b inary inc idence data .

Conclusions

No da ta a re p r e s e n t e d as resul ts of the pi lot study, because the object ive was to deve lop the p r ocedu re s for use in the main study, r a the r than to es tabl ish any initial statist ics. The ou tcome of the pi lot s tudy has been: - The use of interviews and of a very busy popu-

lat ion has caused a major p rob lem of subject r ec ru i tment , but there a re g rounds for expect - ing that p rob l em to d i s a p p e a r if the da ta col- lect ion is unde r t aken outs ide the main fa rming months .

- The study d e p e n d s on f inding a var ia t ion in the v ibra t ion dose expe r i enced by agr icul tura l cont rac tors , o the r major factors be ing con- stant. If a sufficient range is found, then vibra- t ion dose and exposure var iables can be corre- la ted with hea l th var iables . If not, then a con- trol g roup will have to be used. This would be a funct ion of the way the world is r a the r than fault in the study.

- A range of control and convent iona l symptom var iables will enab le t ight cont ro l of the o the r var iables , and compar i son with the f indings of o the r s tudies. If the 12-month back history expe r imen t fails, this will p rovide a sound fall- back posi t ion.

- The 12-month back his tory may be a useful s tep in this a rea of research. It d e p e n d s on the memory of subjects over the previous year. The ques t ionna i re da ta will al low some val ida t ion of the results. If it is successful in y ie ld ing a real numer ica l indica tor of lower -back t rouble , it will enab le more sensit ive analysis of the

Acknowledgements

The Hea l th and Safety Execut ive of the Uni ted Kingdom are gra teful ly acknowledged for spon- sor ing this research.

W e also thank Dr Jack Sandover of Loughbor - ough Universi ty, and Dr Paul ien Bongers of T N O , Hol land , for the i r extensive help and suppor t .

References

Bongers, P.M. and Bushuizen, H.C., 1990. Back disorders and whole-body vibration at work. Unpublished Ph.D. thesis, University of Amsterdam.

Gruber, G.J. and Ziperman, H.H., 1974. Relationship be- tween whole-body vibration and morbidity patterns among motor coach operators. US DHEW (NIOSH) Contract no. HSM-00-72-047.

Hildebrandt, V.H., 1985. Gezondheidsonderzoek stukadoors. Leiden: NIPG-TNO.

Jonsson, B., 1986. Standardised questionnaires as tools in the study of work-related musculo-skeletal complaints. Swedish National Board of Occupational Safety and Health.

Kuorinka, I., Jonsson, B., Kilbom, A., Vinterberg, H., Biering-Sorensen, F., Andersson, G. and Jorgensen, K., 1987. Standardised Nordic questionnaires for the analysis of musculoskeletal symptoms. Applied Ergonomics, 18(3): 233-237.

Nachemson, A., 1976. The lumbar spine: An orthopaedic challenge. Spine, 1(1): 59-71.

Sjoflot, L., 1980. The tractor as a workplace. Proceedings of the Ergonomics Society conference: Ergonomics in Agri- culture, 1980, pub 1981.

Wickstrom, G., 1978. Effect of work on degenerative back disease: A review. Scandinavian Journal of Work Environ- ment and Health, 4 (suppl 1): 1-12.

Williams, R.C., 1988. Toward a set of reliable and valid measures for chronic pain assessment and outcome re- search. Pain, 35: 239-251.