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Diesel exposure information is of vital importance for diesel epidemiological studies or for diesel risk analysis. However, since diesel emissions constitute only a small portion of the total air pollutant mix, accurate assessment of exposure to diesel exhaust or its components is very difficult. Combustion aerosols from diesel and spark-ignited engines are remarkable similar in many ways; once they are mixed together, it is very difficult to determine the source of the exposure. Combustion of other materials, such as liquid and gas fuels, coal, or tobacco, as well as industrial emissions, produce many of the same chemical components that are present in diesel exhaust. In addition, there are several natural, both combustion and non-combustion, sources of respirable particles that are found in the ambient air. Further complications are caused by atmospheric transport and transformation processes that may change the original chemical composition and physical properties of diesel emissions, creating new compounds of different toxic properties.
People spend most of their time indoors. Therefore, accurate estimates of average exposures require a consideration of the amount of time spent in different environments and the respective concentrations of pollutants. Since exposure levels may change over time, they are often averaged and presented as “time-weighted exposures”. Time-weighted averaging provides satisfactory representation for human exposure, however, it may not account for short duration exposure spikes or “burst modes” of exposure. The burst mode of exposure, which may happen, for example, in the case of failure of a ventilation hood, spill container, enclosure, etc., is very important as the concentrations of pollutants can increase by orders of magnitude.
From the historical perspective, changes in the diesel engine and fuel technologies also influence human exposures and toxic properties of diesel emissions. New engines not only produce less pollutants, but also may produce pollutants of different chemical and/or physical properties. For example, the particulate emissions from different engines may have different size distribution or different proportion of metallic ash from lube oil additives. In engines equipped with particulate filters, PM emissions are almost exclusively composed of volatile material, mostly SOF. If oxidation catalysts are used, the NOx emissions may have increased proportion of nitrogen dioxide. It is important that models which estimate future exposure levels properly address the changing character of emissions from new diesel engines.
This paper discusses ranges of exposure to diesel emissions typical for both ambient (environmental) and occupational settings. It should be emphasized that human health response is not necessarily directly related to exposures. Rather, it is dependent on the dose to a particular target organ, e.g., to the lungs. Discussion of exposure and dose issues which puts exposures in perspective can be found in the literature .