Reconstructing the past : estimating exposure to hazardous substances in occupational epidemiology
The aim of the work described in this thesis was to develop a reliable method for retrospective assessment of occupational exposure, for individual workers, to substances hazardous to health. A review of existing methods for reconstructing exposure has shown a diverse range of approaches, partly dictated by the availability of measurement records and other documentation about work activities. These methods show little evidence of a coherent theoretical basis for exposure assessment. Progress has been made in defining a theory of exposure to hazardous substances and in elaborating this for substances where the main route of exposure is by inhalation. A new metric for exposure assessment, i.e. uptake, is proposed. For inhalation this corresponds to the cumulative amount of material inhaled during the exposure period, e.g. for a dust this would correspond to the time integral of the product of exposure level (i.e. concentration) and breathing rate. Analogous definitions are provided for dermal and ingestion uptake, and total uptake as the sum of these individual measures. It is argued that uptake should provide the best metric for reconstructing past exposure. For inhalation exposure several stages have been identified linking emission of hazardous substances from sources to inhalation of the substance by an individual worker. Emission from sources into the work environment has been divided into three parts (i.e. intrinsic emission, plus the effects of handling and local controls), which are assumed to be independent of each other. It is further assumed that the total emission from a source is determined by the product of these terms.