Several advisory or regulatory authorities in North America and Europe, including IARC, NIOSH, MSHA, the Health Effects Institute, and the US Environmental Protection Agency (EPA), have concluded that sufficient evidence exists that exposure to DE causes an increased risk of cancer (Rogers, et al., 2005
). These evaluations were based on sufficient toxicological animal studies and limited evidence from almost 50 occupational epidemiologic studies. Among other limitations, the lack of quantitative exposure assessment has consistently been cited as a fundamental problem in determining causality from the existing epidemiological studies (Rogers, et al., 2005
). The purpose of this review was to provide a comprehensive overview of quantitative occupational exposure levels to DE that will allow for more accurate and consistent occupational exposure assessments in population-based epidemiologic studies.
For EC, the highest exposure levels were reported for underground mining (27–658 μg/m3
), tunnel construction (132–314 μg/m3
), and underground mine maintenance workers (53–144 μg/m3
). For maintenance workers of on-road and railroad equipment, distribution workers, fire fighters, and ship dock workers, exposure levels generally ranged from ND to 50 μg/m3
. Relatively low levels were reported for drivers of on-road vehicles, train crews, above ground mining, parking attendants, vehicle testers, utility service workers, above ground construction, and airline ground personnel (<25 μg/m3
). For airline personnel, jet exhaust may be another source of EC and more research is needed to investigate its contribution (Schauer, 2003
). EC is currently the preferred surrogate for DE in industries other than coal mines (Leeming, et al., 2004
), since it is relative simple to measure, has few chemical interferences and is the major component of diesel particulate matter (Groves, et al., 2000
; Schauer, 2003
There was little information available on PMS
to compare with the EC levels. Exposure levels of miners and underground construction workers were highest (154–1600 and 121 μg/m3
, respectively), followed by mechanics, above ground construction workers, and taxi drivers (10–35 μg/m3
has only a few interferences from non-diesel sources, i.e. oil mist and cigarette smoke (Hammond, et al., 1988
is a less suitable surrogate for DE since it is generated from more non-diesel sources, i.e. oil and grease mists, cigarette smoke, emissions from other combustion sources, and respirable inorganic matter such as mechanically aerosolized geological and fibrous materials (Hammond, et al., 1988
). These non-diesel sources are a likely explanation for the reported PMR
levels that were substantially higher than PMS
levels in all situations. Nonetheless, for PMR
, the highest levels also were reported for workers in underground mining and underground construction (710–3637 and 1160–1700, respectively).
For the gases, the highest mean levels generally were reported for workers in underground mining and underground construction. Similar to PM, the pattern of the gases among industries was generally consistent with the EC levels. However, relatively high mean concentrations for some of the gases also were reported in situations where reported EC levels were low, e.g. for DE exposed airline personnel, train crews, and utility service workers. These higher levels are likely the result of emissions from other combustion sources.
The results of this review suggest that enclosure of the work site and the type of diesel equipment used are the most important determinants affecting occupational DE exposure. Highest levels were found in underground mining, maintenance, and construction, where heavy equipment is used in enclosed underground work sites. Situations for which intermediate exposure levels were reported mostly involved smaller equipment, probably run intermittently, in above ground (semi-)enclosed areas that were more easily ventilated by natural or mechanical ventilation, i.e. mechanics in a shop, emergency workers in fire stations, distribution workers at a dock, and workers loading/unloading vehicles inside a ferry. Determinants that have been repeatedly reported for both above and underground (semi-)enclosed situations were ventilation (Cohen, et al., 2002
; Davis, et al., 2006
; Hammond, et al., 1988
; Madl, et al., 2002
; Sauvain, et al., 2003
; Zaebst, et al., 1992
; Zaebst, et al., 1991
) and the use of exhaust after treatment devices (Ambs, et al., ; Haney, et al., 1992
; Haney, et al., 1997
; Roegner, et al., 2002
; Zaebst, et al., 1992
). Lowest levels were found for workers in enclosed areas separated from the source or for workers who were outside. Airflow from outside the train or truck cab was reported to result in higher exposure levels for train crew and on-road drivers than exposure levels within a closed cab (Davis, et al., 2007
; Liukonen, et al., 2002
; Seshagiri, 2003
; Zaebst, et al., 1991
), suggesting that DE exposure in these situations occurs mostly via the outdoor air. The railroad studies indicated that the exposure is derived from preceding stacks of the same train (Liukonen, et al., 2002
; Seshagiri, 2003
). For drivers of on-road vehicles, higher levels were reported for inner city drivers than for drivers in rural or suburban areas, suggesting that emissions from other vehicles are probably responsible for most of the exposure (Garshick, et al., 2002
; Lewne, et al., 2006
Assessing occupational exposures in epidemiological studies in the general population is challenging. For chronic diseases such as cancer, the relevant exposure periods are usually decades ago, and exposure measurements for the relevant exposure period are often not available. In addition, exposures can vary widely depending on individual work environments. Thus, the availability of a comprehensive database of historical quantitative exposure levels including determinants of exposure is likely to result in a more accurate and consistent exposure assessment than when the assessment is based only on expert judgment.
However, there are some limitations to this approach. Because DE is a complex mixture of compounds, several agents were selected, complicating comparison across studies focusing on different agents. In addition, the composition of DE varies with engine technology, fuel type, operating conditions, and the presence of emission control systems, which have all changed over time (EPA, 2002
; Lloyd, et al., 2001
and the gases were selected to investigate time trends, since the more specific surrogates of DE, such as EC, were not developed until the 1990s. Recently, more advanced chemical techniques are being developed. However, these are not yet suitable for application in epidemiological and exposure studies because of the extensive number of samples and low air volume of the samples typical in these studies (Schauer, 2003
). Regulation of emissions has decreased emission levels (Bunn, et al., 2002
; Laden, et al., 2006
), yet the use of diesel engines has increased. However, not enough exposure data were available to assess the effect of these changes. Consequently, the incorporation of time trends in exposure assessment will be problematic. Another limitation of the complex composition of DE, is that the relevant toxic agent, which varies by health effect (Scheepers, et al., 1992
), may not be proportional to the chosen agent of study.
A further limitation of using published literature is the extraction and interpretation of exposure information from reports written by different authors for different purposes. The description of the measured jobs, the number of measurements, the duration of the measurements, and the exposure conditions was often unclear or absent. In addition, published reports may have been biased towards worst case scenarios and may not represent what is typical for the industry with regard to both the types of jobs reported and the concentrations measured. Finally, measurements on other industrial uses, such as farming and the military, have not been reported.
In spite of these limitations, contrast in exposure levels was found when comparing different jobs and industries, and several determinants of exposure have been identified. The data described in this study can be used to assess exposure levels based on job and industry title and certain exposure characteristics in population-based epidemiologic studies. Furthermore, these data can guide future exposure assessment efforts as well as the selection of study populations for future epidemiologic studies.