![]() ![]() 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. 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. 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. A key assumption here is that the contaminant is thoroughly mixed throughout each compartment. ![]() ![]() 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. 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. Lange poses the question “are personal and static samples related?”. 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. The median ratio between personal and static concentrations was 1.5, although the individual data points ranged from 0.4 to 10. In this analysis more than 80% of the personal measurements exceeded the corresponding static sample concentration. This showed, as Lange asserts in his letter to this journal, 3 that personal samples are “generally higher in concentration than static samples”. 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 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. 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. In 1957 the personal sampling pump had just been invented by Jerry Sherwood and Don Greenhalgh from the UK Atomic Energy Authority. The paper from Harrison and his co-workers 1 and the subsequent correspondence 2, 3 has reignited a debate about the relation between personal and static sample measurements that started more than 40 years ago. ![]()
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