Ann. occup. Hyg., Vol. 47, No. 3, pp. 179-185, 2003
© 2003 British Occupational Hygiene Society
Published by Oxford University Press
Commentary
The Beginning of the Science Underpinning Occupational Hygiene
University of Aberdeen and the Institute of Occupational Medicine, Edinburgh EH8 9SU, UK
Received 8 January 2003; in final form 20 January 2003
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ABSTRACT |
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 TOP ABSTRACT THE INVENTION OF A...PERSONAL AND AREA SAMPLINGPERSONAL SAMPLING PUMPS AND...THE LEGACYREFERENCES |
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Sherwood and Greenhalgh’s 1960 paper is a seminal one for the development of the science of human exposure. There are three key elements in the paper that deserve to be highlighted: the development of the first personal sampling pump and sampling head; the first comparison between personal sampling and static sampling; the first observation of the possible effect of personal sampling on the individual being sampled.
Keywords: human exposure; personal sampling pump; personal sampling; static sampling
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THE INVENTION OF A PERSONAL SAMPLING PUMP |
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TOPABSTRACTTHE INVENTION OF A...PERSONAL AND AREA SAMPLINGPERSONAL SAMPLING PUMPS AND...THE LEGACYREFERENCES |
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Around 4 p.m. on a Friday afternoon during 1957, Jerry Sherwood was quietly reading the journal Wireless World in the library of the UK Atomic Energy Research Establishment (AERE) in Harwell. Browsing through the small advertisements he came across one company offering miniature DC motors for sale. This rather unremarkable circumstance was the genesis of personal sampling for hazardous substances.
In the late 1950s there were a few large organizations committed to monitoring dust and vapours in workplaces. Jerry Sherwood and his colleagues were concerned to monitor radioactive dust in the emerging UK nuclear power industry. Others were involved in monitoring dust in coal mines or gases and vapours in the oil industry. The instruments that were used were portable, but not sufficiently reliable or lightweight to allow for personal sampling. For aerosols the instruments being used included the thermal precipitator (developed 1936–37), the Pneumoconiosis Research Unit hand pump (1948), konimeter (1927), the Owens jet sampler (1923) and others.
The thermal precipitator was typical of the type of instrument available to occupational hygienist in the 1950s (Fig. 1). The complete unit, including lead acid battery, was carried in a metal box that was 38 cm tall with a 19 cm square base. The whole thing weighed 7.25 kg. The operating principle of the thermal precipitator is ingenious; air is drawn through a narrow chamber where a heated wire is sandwiched between two glass microscope coverslips. The particles in the air stream are deflected towards the glass surfaces because of the temperature gradient induced by the wire (a process known as thermophoresis). This type of sampler has high collection efficiency for particles from 
5 µm down to 0.01 µm, perhaps even smaller. It also has a low resistance to airflow and this enabled a simple water displacement system to be used to draw air through the sampling head. The aerosol concentration was obtained by subsequently counting the number of particles on the coverslip using a microscope.
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Some occupational hygienists had tried to collect personal samples using the thermal precipitator in a backpack, but the weight made it uncomfortable and the water used to aspirate the sample often leaked and trickled down the worker’s back. The normal use was for the hygienist to shadow the worker and try to collect a sample from the general vicinity of the breathing zone. All of the effort of the hygienist was needed to collect a single sample using this instrument, making it a costly and inefficient procedure.
A good example of the effort put into environmental sampling comes from a research study that was carried out in 25 British collieries (Fay and Rae, 1959). This was the Pneumoconiosis Field Research (PFR) study involving chest radiographs of up to 35 000 miners, about 5% of the total UK mining population at that time. In parallel, airborne respirable dust measurements were being made from samples collected by thermal precipitator and then analysed by microscopic counting of particles <5 µm in diameter. During each sampling shift an investigator would shadow a miner collecting a sequence of samples from their breathing zone. In the four surveys of the study population that were completed by 1959 the researchers had collected about 60 000 samples from about 14 000 work shifts.
Dr B.M. Wright, from the Pneumoconiosis Research Unit of the UK Medical Research Council, speaking at the Second British Occupational Hygiene Society Conference, summed up the key problem with dust sampling as follows (Wright, 1954):
Dust concentration and composition vary widely from place to place and from time to time, but the development of pneumoconiosis results from prolonged exposure in such varying environments. A satisfactory estimate of exposure therefore requires the integration of samples taken over a long period of time. Instruments in present use are hand-operated and take relatively short-term samples. Such samples are therefore very expensive to obtain and evaluate and each one gives relatively little information.
An instrument is required which is robust, portable, self-contained and automatic, so that it can be carried with the worker wherever he goes, and which gives a single sample over a period of at least a week, representing his average exposure. The sample should give a measure of respirable dust and some estimate of its composition. The instrument should be relatively cheap and easy to produce, so that it can be used on a wide scale as a routine sampler.
Until such an instrument has been produced and used over a period of many years, safe limits of dust exposure cannot be established with any precision.
Jerry Sherwood and his colleague Don Greenhalgh were the first to build a practical personal sampling pump. Their paper ‘A personal air sampler’ was published in 1960 in the second volume of the Annals of Occupational Hygiene (Sherwood and Greenhalgh, 1960). The concept was simple: a miniature DC motor was used to drive a PTFE diaphragm pump, with the motive power coming from a mercury cell. The prototype was completed in September 1957 and it was able to run successfully for 8 h at 0.5 l/min. This was followed by the Mk1 pump, shown at the top right of Fig. 2, which weighed 0.45 kg and was able to run continuously for about 30 h. The authors suggested it ‘may be worn in the pocket of a laboratory coat or hip pocket of trousers’. In true ‘Heath Robinson’ fashion, the pump was fitted into a bicycle lamp case and the original sampling head was made from an outlet fitting for an electrical cable assembly. The Mk2 pump, also shown in Fig. 2, incorporated a higher capacity battery (up to 100 h operation) and a timer. The pump was commercialized by C.F. Cassella in the UK, who introduced rechargeable nickel-cadmium batteries and other technical innovations into the sampler.
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Part of the genius in this design was the separation of the pump from the sampling head. This enabled the subsequent development of new sampling heads to proceed fairly independently of the pump capabilities. Jerry Sherwood was himself involved in the development of a personal sampler for sulphur dioxide and in pioneering personal sampling for benzene (Sherwood, 1969; Sherwood and Carter, 1970). In addition, the sampling of aerosols was given a considerable boost by others at AERE who developed high efficiency glassfibre filters for use in the air samplers (Stevens and Hounam, 1961). The research team at AERE were clearly the pioneers of personal sampling at this time.
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PERSONAL AND AREA SAMPLING |
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TOPABSTRACTTHE INVENTION OF A...PERSONAL AND AREA SAMPLINGPERSONAL SAMPLING PUMPS AND...THE LEGACYREFERENCES |
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The invention of the sampler would on its own have been a landmark in occupational hygiene, but Sherwood and Greenhalgh immediately began to compare the results from the new samplers with those from conventional static installed instruments. Some of the early data are presented in their 1960 paper. There were data for 39 week-long samples from a worker and corresponding data from the ‘installed samplers’. On 16 occasions the measured concentrations were low on both samplers. However, on the remaining samples the measured concentrations were in general higher on the personal samplers (65% of occasions) and on average the personal sampler concentrations were five times that of the fixed location samplers.
On the face of it there is no reason why personal measurements should be greater than those made at a fixed location; if the static samplers were positioned closer to a source than the person worked then they might consistently produce measured concentrations greater than the personal sampler. However, this observation of Sherwood and Greenhalgh has been reproduced in many diverse environments with all types of hazardous substance. Cherrie (1999) summarized data from eight studies, including the Sherwood and Greenhalgh paper, which showed that the ratio of the geometric mean personal exposure level to the corresponding data from static samples ranged from 1.2 to 8.5. The largest ratios were seen in large workrooms, whereas the lowest ratios were from small poorly ventilated rooms.
An examination of papers published in the Annals of Occupational Hygiene over the last 10 years indicates a further 12 papers where there was data from personal and static measurements. These papers are listed in Table 1, with a brief description of the data. Figure 3 shows a log-probability plot of the ratio of the average personal exposure level to the corresponding static data for 40 different work situations described in these papers. Data where it was stated that the authors deliberately placed the sampler at the source were excluded. The median ratio was 1.5 and 80% of the data were 
1.
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It is rare in occupational hygiene that observations can be widely generalized, but it seems that the early data of Sherwood and Greenhalgh identified something important. Cherrie (1999) argued that if the source was close to the worker, i.e. in their near-field, and the static sample was collected away from the worker, i.e. in their far-field, far from other sources, then the personal exposure level would always be greater than or equal to the static concentration. The magnitude of the ratio between the two measurements was shown to be mainly dependent on the room size and the quantity of room general ventilation. Data on personal and static samples from Purdham et al. (1993) support this analysis. They summarized their data in terms of workplaces with good, fair and poor general ventilation (Table 2). The ratio of average personal to static concentration ranged from 3.2 to 1.5, with the lowest values for the poor ventilation conditions.
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In any case, Sherwood and Greenhalgh concluded that exposure could not be reliably assessed from static samples, which is confirmed by the later experimental results and subsequent theoretical analyses (Cherrie, 1999; Esman and Hall, 2000). It has become an accepted part of occupational hygiene practice that only personal samples are representative of human exposure and risk. In the UK and most other countries, measurements made for comparison with occupational exposure limits are almost always to be made using personal sampling techniques.
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PERSONAL SAMPLING PUMPS AND WORKER BEHAVIOUR |
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At the end of the results section of their paper, Sherwood and Greenhalgh made a passing remark about the psychological impact of the new personal sampling on those being sampled. They had attempted to correlate the personal exposure measurements of inhaled uranium of two men with the amount of uranium excreted in their urine. They noticed that ‘On the introduction of personal samplers the urinary levels showed an immediate reduction by a factor of five to statistically insignificant amounts’. And they suggested that ‘It is possible that this was due in part to the extra care induced by wearing the samplers’.
Despite this early observation and subsequent anecdotal evidence there has been very little systematic investigation of the effect that taking personal samples may have on the behaviour of those being sampled. One study has compared exposure of five groups of workers on days when they wore personal sampling pumps with days when they did not. In both cases personal diffusive samplers were used to ensure that there were no differences due to the different sampling methods (Cherrie et al., 1994). At one site the average exposures were 50% higher when pumps were worn, while at the remaining sites there were no clear differences between the different sampling regimes. At the time we hypothesized that rather than the personal sampling pumps inducing workers to behave more carefully, they prompted workers who spent some of their time seated in quiet (clean) environments to spend a greater proportion of their work shift in areas where they were more highly exposed. However, no further work to test this hypothesis has been carried out.
Are the observations of Sherwood and Greenhalgh of more careful working when being sampled inconsistent with the higher exposures from personal samples in the latter studies? Social psychologists have shown that the presence of others may either improve or impair the performance in a given task depending on the ease with which the behaviour may be accomplished. This is known as ‘social facilitation’ (Smith and Mackie, 2000). So it may be argued that the presence of a hygienist conducting a monitoring exercise could cause workers to become apprehensive because they believe they are being evaluated in some way or the hygienist could distract the workers from their tasks. In either case the hypothesized effect is a heightened arousal in the worker, perhaps in extreme cases a stressful response. If the work behaviours that result in lower exposures are well learned and commonly used (the psychologists would term this type of behaviour ‘accessible’) then the worker’s performance should improve in the presence of the hygienist, just as a concert pianist performs better in front of an audience. However, if the personal behaviour that reduces exposure is not commonly practiced by the workers then the arousal induced by the presence of the hygienist will produce other more ‘accessible’ behaviours and that may cause exposures to increase. The corresponding musical analogy is the novice pianist who may make more errors than normal at their first public recital.
The effect of the hygienist making measurements of personal exposure is therefore likely to be unpredictable, perhaps increasing, decreasing or having no effect on the worker’s exposure. Any effect may also differ from one worker to another depending on the accessibility of the safety behaviours that modulate exposure. Also, the effect may change over time as the hygienist gains the trust of the workforce and ceases to be viewed as an ‘outside observer’. From the data of Sherwood and Greenhalgh and those of Cherrie et al. (1994) we might tentatively conclude that the average bias in measured exposure induced by the presence of an occupational hygienist would generally be small but could be as great as a 5-fold difference, either higher or lower.
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THE LEGACY |
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TOPABSTRACTTHE INVENTION OF A...PERSONAL AND AREA SAMPLINGPERSONAL SAMPLING PUMPS AND...THE LEGACYREFERENCES |
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Sadly, the heyday of measuring personal exposure to hazardous substances may have passed. In the UK the HSE National Exposure Database (NEDB) contains more than 80 000 measurements for about 400 different substances. However, the majority of these data were collected between about 1985 and 1990, when between 5000 and 10 000 measurements were added to the database each year. Since then there has been a fairly steady decline, with currently only a few hundred personal exposure measurements recorded by HSE each year (Philips, personal communication). There are also very few companies measuring exposure to chemicals. In a recent study designed to retrospectively collect exposure data for a small number of commonly used hazardous substances from companies identified as likely users of chemicals, we found that only 3.6% reported having relevant monitoring data (Cherrie et al., 2001). Most surprisingly, in this survey only 41% of professional occupational hygiene consultants had any relevant measurement data.
Early occupational hygiene pioneers such as Sherwood and Greenhalgh recognized the importance of exposure measurement in coming to a robust conclusion about the likely risks from chemical exposure. As with many of their observations, they were right. Experienced occupational hygienists may be able to make a valuable contribution to controlling exposure without collecting any personal samples and this is perhaps one reason why the number of personal exposure measurements being made has decreased. However, their expertise is based on previous experience of measuring exposures in analogous situations. Without a reliable knowledge base it will become increasingly difficult for them to make reliable judgements.
Over the last decade there have been considerable developments in the science of human exposure assessment and in particular in the development of theoretical models to describe exposure to hazardous substances, for example Cherrie (1999) and Schneider et al. (2000). Some of these models have been implemented as computer-based predictive tools to help estimate exposure in the absence of measurements, such as the EASE software described by Friar (1996). However, in science it is impossible to divorce theory and measurement, both are necessary. Models will always need to be verified by exposure measurements and in turn measurements of personal exposure can be made more efficiently with the benefit of exposure predictions.
Some have argued that the development of predictive models has caused or at least contributed to the decline in the use of personal sampling techniques (Kromhout, 2002). However, as we have seen from the exposure data collected by HSE the downward trend in exposure monitoring started more than 10 yr ago. It is still unclear why there has been this change and whether it is confined to the UK or is a reflection of a wider change. Certainly the peak period of monitoring by HSE (between 1985 and 1990) corresponded to a time when the control of substances hazardous to health had high legislative priority in the UK and since then it has become less important.
The development of the personal sampling pump by Sherwood and Greenhalgh heralded the beginning of modern occupational hygiene and provided the foundation for a proper scientific underpinning of professional practice. It led to a period of enthusiastic monitoring of personal exposure, which not only helped control exposures on a case-by-case basis but provided the knowledge base for subsequent developments. It is therefore paradoxical that as personal exposure measurement has become easier, more reliable and cheaper, it has also become less common. If we are to continue to efficiently manage the risks from chemicals then we must all make more use of the personal sampling techniques pioneered in the 1960s.
Acknowledgements—I am grateful to Sean Semple and Rob Aitken for their helpful comments on the draft commentary.
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FOOTNOTES |
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To celebrate the BOHS 50th anniversary this year, we are reproducing in our on-line edition ‘classic papers’ from past issues of the Annals, with accompanying commentaries in the print and on-line edition. For this issue, the classic paper we reproduce is Sherwood RJ, Greenhalgh DMS. (1960) A personal air sampler. Ann Occup Hyg; 2: 127–32. The Beginning of the Science Underpinning Occupational Hygiene
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TOPABSTRACTTHE INVENTION OF A...PERSONAL AND AREA SAMPLINGPERSONAL SAMPLING PUMPS AND...THE LEGACYREFERENCES |
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