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The paper from Harrison and his co-workers1 and the subsequent correspondence2,3 has reignited a debate about the relation between personal and static sample measurements that started more than 40 years ago. In 1957 the personal sampling pump had just been invented by Jerry Sherwood and Don Greenhalgh from the UK Atomic Energy Authority.4 They compared their new personal sampler with the conventional static sampler and showed that personal exposures were generally higher than those made at a fixed location. This classic paper has recently been reproduced in the electronic edition of the Annals of Occupational Hygiene together with a commentary on its significance to the science of human exposure assessment.5 In this commentary, information concerning personal and static measurement results from papers published in that journal over the past 10 years was reviewed. This showed, as Lange asserts in his letter to this journal,3 that personal samples are “generally higher in concentration than static samples”. In this analysis more than 80% of the personal measurements exceeded the corresponding static sample concentration. The median ratio between personal and static concentrations was 1.5, although the individual data points ranged from 0.4 to 10. It is reasonable to expect that in general, personal exposure would be greater than static samples if on average workers spend a proportion of their time close to sources of the hazardous substance.
Lange poses the question “are personal and static samples related?”.3 The answer must be “yes”, but perhaps a more pertinent question is: how can the relation between personal and static samples be useful in epidemiological studies or risk evaluations? A simple conceptual model, shown in fig 1, is sufficient to convince us that there must be a relation between personal and static monitoring data.
Here the air compartments represent the local external environment, room air (where static samples are obtained), breathing zone air (where personal air samples are collected), and the inhalation of contaminants into the nose or mouth. A key assumption here is that the contaminant is thoroughly mixed throughout each compartment. In addition, in the model we have the potential for airborne contaminants to adsorb or sediment onto room surfaces and for this contamination to become resuspended in the air. In general there is the potential for contaminants to be exchanged to and from each compartment; for example, as air flows from the breathing zone to the body of the room, it is replaced by air from the room. The purpose of showing this model is to illustrate the complexity of the processes relating room and breathing zone air concentrations; this is almost certainly the main reason why there is such a wide range in the ratio of personal and static air concentrations. Key factors in determining the relation between personal and static measurements in any situation will include the volume of the room, the quantity of general ventilation, the time the person spends in the proximity of sources of hazardous substances (that is, with a source in their breathing zone), the presence of other internal or environmental sources of the contaminant, and others. In most circumstances, without knowing something about each of these factors it is impossible to predict what the relation between personal and static concentrations might be.
There is one class of situations where room samples and personal samples are likely to be very similar. Using a simple mathematical model, Cherrie6 showed that in small poorly ventilated rooms it makes little difference whether the concentration is measured in the breathing zone compartment or in the room compartment because the contaminant quickly mixes throughout both spaces. In most domestic situations it is likely that this is the case since the rooms are generally small and the ventilation rate is likely to be low, probably less than one air-change per hour. Therefore, I think almost uniquely, in epidemiological studies where we want to assess the exposure of people in houses it probably does not make a lot of difference whether we use samplers located in the room or samplers located in the person’s breathing zone. This will mostly not be the case in occupational epidemiological studies, where spaces are typically larger and ventilation rates greater.