Health Assessment Document for Diesel Emissions - NSCEP | US ...

1 exposures. This is a complex question that is not rigorously addressed in this version of the
2 assessment. It is clear that newer technology engines will have somewhat different emission
3 composition (i.e., perhaps reduced NOx with increased fine particles), not to mention the
4 emission controls, which would reduce certain exhaust components, presumably larger particles.
5 Since particle mass is the surrogate dosimeter used to correlate toxicity with exposure and public
6 health impact, as the particle mass is changed by virtue of new technology or controls, so might
7 the applicability of the health assessment findings in this report. Further analysis of emission
8 changes may be a desirable research pursuit.
9
10 2.7. SUMMARY
11 Major research programs were carried out in the late 1970s and early 1980s to ascertain
12 the physical and chemical characteristics of emissions from diesel engines and the biological
13 effects of these emissions. New control technologies are being introduced into currently
14 manufactured diesel vehicles, and the effect of these changes on diesel emissions is likely to be
15 visible in the future. Diesel vehicles manufactured in the late 1970s and early 1980s are still on
16 the road and, in this sense, data collected from that period are still valid.
17 However, many ofthese data were collected using laboratory dynamometers with selected
18 new vehicles or vehicles well tuned to manufacturers' specifications. The well-controlled
19 conditions of the dynamometer tests have many benefits but do not necessarily represent vehicle
20 emissions under real on-road conditions, and the small number of vehicles tested in the
21 laboratory is not truly representative of the distribution within the on-road vehicle fleet.
22 Although several roadway and tunnel emission measurements were performed in the past, the
23 database on mobile-source emission rates necessary to assess the role of vehicle emissions in air
24 · pollution problems is still not sufficient. More measurements carried out under realistic on-road
25 .conditions are necessary, in particular for gaseous and particulate-phase organic compounds
26 present in vehicle emissions.
27 Once released into the atmosphere, diesel emissions are subject to dispersion and
28 transport and, at the same time, to chemical and physical transformation into secondary
29 pollutants, which may be more harmful than their precursors. Thus, a knowledge of diesel
30 . emissions at or near their sources is no longer sufficient to assess fully the impact of these
31 emissions on human health and welfare. The understanding of physical and chemical changes
32 that primary diesel emissions undergo during their transport through the atmosphere is equally
33 important. As a result of the past two decades of laboratory_ and ambient experiments and
34 computer modeling, a comprehensive set of data now exists concerning the atmospheric loss
35 processes and transformation of automotive emissions, but knowledge concerning the products of
36 ·these chemical transformations is still limited. Study .is required to determine the products from
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