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

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from stationary dioxin sources, e.g., incinerators, have been noninhalation exposures associated
with ingestion of certain foods, e.g., beef and vegetables, contaminated by the deposition of stackemitted
dioxin. There is a potential for deposition of stack emissions from trucks to soil and
- water adjacent to some highways resulting in similar impacts. We recommend that appropriate
data be collected.
The changing composition of DE (i.e., older engines vs. newer technology ones, heavyduty
vs. light-duty, and engines run under varied operating conditions) gives rise to questions
about how the health data and the risk a§sessment findings in this report, which are based on pre-
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1998 engines, can be applied to present-day engine exhaust emissions and resulting ambient
exposures. This is a complex question that is not rigorously addressed in this assessment. It is
clear that newer technology engines will have somewhat different emission composition (i.e.,
perhaps reduced NOx with increased fine particles), not to mention emission controls, which
would reduce certain exhaust components, presumably larger particles. Since particle mass is the
·surrogate dosimeter used to correlate -toxicity with exposure and public health impact, the
implication is. that we have a basis for scaling to account for exhaust changes. This relationship
may be too simplistic, and thus further investigations of current-day emissions, and how they
average out across the fleet or stationary engine use, may be warranted.
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19 12.5. DIESEL EXHAUST HEALTH RISKS-A PLAIN-LANGUAGE OVERVIEW OF
20 KEY INFORMATION
21 This section reviews key information about diesel emission hazards and risks by posing and
22 answering simple questions. It is mostly duplicative of information found earlier in this chapter,
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though some added explanation has been added for background purposes.
25 What is diesel exhaust (DE)? What happens when it enters the environment?
26 Diesel engines are very durable and have performance characteristics that make them a
27 desirable alternative in certain uses. The diesel engine, whether it be in an automobile, truck, off-
28 road equipment, locomotive, or ship, produces exhaust from the combustion of diesel fuel. The
29 exhaust is a mixture of organics and inorganic constituents (i.e.,. products of incomplete
30 combustion). The exhaust can be invisible or be seen as a gray or black smoke. When visible,
31 what is seen is the particle fraction of the exhaust mixture, usually from anengine that is not
32 required to control its emission or one that is not well maintained. In the simplest terms, DE is a
33 mixture of carbon core particles that have a coating of various inorganic/organic compounds, as
34 well as gases and semivolatiles. The identifiable organic and inorganic compounds number in the
35 hundreds. The particles have a spongelike structure and a very large surface area per gram, which
2/1/98 12-31 DRAFT --DO NOT CITE OR QUOTE