Exposure to Engine Emissions

W. Addy Majewski

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Abstract: Several methods have been developed to estimate the exposure to engine exhaust emissions. Most methods are based on either ambient air quality surveys or emission modeling. One of the focus areas in exposure research has been diesel particulate emissions. Ambient exposures to diesel particulates are generally below 10 µg/m3, while occupational exposures can be in excess of 1000 µg/m3. Exposure to other components of engine exhaust emissions, such as polynuclear aromatic hydrocarbons, is also higher in occupational settings than it is in ambient environments.


Emission exposure information is of vital importance for epidemiological studies or for health risk analysis. However, since engine emissions constitute only a portion of the total air pollutant mix, accurate assessment of exposure to engine exhaust pollutants can be challenging. Combustion aerosols from diesel and spark-ignited engines are remarkably 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 and gasoline engine exhaust. There are also natural sources (combustion and non-combustion) of respirable particles and gaseous emissions that are found in ambient air. A further complication is presented by atmospheric transport and transformation processes that may change the original chemical composition and physical properties of emissions, creating new compounds of different toxic properties.

Historically, a number of studies investigated exposures to diesel exhaust with a focus on diesel particulate matter (DPM) emissions. This DPM focus was justified by several reasons. In environmental settings, old technology diesel engines were a major contributor to ambient levels of PM2.5—a pollutant of high public health concern. In occupational settings—especially where engines are used in confined spaces—the diesel engine is the predominant or the only type of engine due to its low CO emissions and safer handling of fuel compared to gasoline.

New, modern diesel engines not only produce less pollutants, but also produce pollutants of different chemical and/or physical properties, which has an effect on human exposure and the associated health risks. Diesel engines equipped with particulate filters produce less PM emissions than gasoline engines [5695] and the filtered particulates have a different composition and properties—for instance, they cause less oxidative stress suggesting a lower biological impact [5697]. It is important that models which estimate future exposure levels properly address the changing character of emissions from emission-controlled diesel engines.

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.

The following sections discuss ranges of exposure to diesel particulates and other diesel and non-diesel engine emissions typical for ambient (public health) and occupational environments. It should be noted 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 that puts exposures in perspective can be found in the literature [702].