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Fact
Sheets: Diesel Exhaust
What is diesel exhaust
(DE)?
The diesel
engine, whether it be in an automobile, bus, truck,
off-road equipment, locomotive, or ship, produces
exhaust from the combustion of diesel fuel. The exhaust
is a mixture of organic and inorganic constituents
(i.e., products of incomplete combustion). The exhaust
can be invisible or be seen as a gray or black smoke.
When visible, what is seen is the particle fraction of
the exhaust mixture, usually from an engine that is not
required to control its emission or one that is not well
maintained. In the simplest terms, DE is a mixture of
carbon core particles that have a coating of various
inorganic/organic compounds, as well as semi-volatile
gases. The identifiable organic and inorganic compounds
number in the hundreds.
How are individuals exposed to DE? How does it enter
and leave the human body? Is there a test to determine
whether exposure has occurred?
Individuals may be exposed to DE when they are in an
area where diesel engines are in use and the exhaust
mixture is breathable. Some occupational settings may be
prone to more frequent and higher exposures, such as in
engine maintenance shops, heavy equipment operations
tunnels, or bus terminal operations, to suggest a few. A
non-occupational setting that may have a higher than
average ambient exposure could be, for example, among
those who spend a notable part of their day in the
vicinity of diesel roadway traffic, such as in or around
highways or urban street. The odor threshold, at least
according to one study, is about 200 ug/m3 of
particulate, or greater.
DE exhaust most easily enters the body by breathing,
though in some occupations portions of the exhaust may
cling to skin or hair and there after possibly be
ingested as a consequence of hand-to-mouth activity. By
far, the major exposure pathway is from breathing. Once
inhaled some DE mixture components could be deposited or
absorbed into the nasal and respiratory tract, but most
of the mixture travels into the lungs where the
particles and gases are deposited on lung tissue.
Eventually absorbed into the lung tissue, then the
bloodstream, and therafter begin a process of normal
detoxification by various body organs followed by
removal from the body via urine/feces.
There is no single medical test to determine if a DE
exposure has occurred. Many symptoms of episodic DE
exposure are similar to symptoms caused by other agents
or, in some cases, the onset of a common cold. Invasive
sampling of particle deposits in the upper respiratory
tract or lung could be done, yet such particles may not
be readily distinguishable from particulate matter from
other sources.
How does exposure affect human health and how certain
are we about these effects?
One way to consider the possible harmful effects of
DE is to consider acute exposure (i.e.,
episodic/infrequent contact) versus chronic exposure
(i.e., fairly continuous over long periods of time, such
as years). As the exposure frequency and/or duration of
the contact increase, acute exposure and its effects
give way to chronic exposure and its consequences. Most
health studies are designed to evaluate either acute or
chronic effects.
The pure carbon core DE particle, the organic coating
the particle, and the gas/vapor phase components of the
mixture all have health study evidence that shows
toxicity, and thus potential to be hazardous under some
regime of exposure. Taken individually, both the
particles and some of the chemical compounds can be
irritants and cause inflammation in the respiratory
system, or in larger amounts cause more permanent
harmful effects. For example, among the many
hydrocarbons found in DE, 19 of them are believed or
know to pose a human carcinogenicity hazard, with the
magnitude of the risk thought to be proportional to
total exposure over a lifetime.
What recommendations exist to protect human health?
For the acute effects of DE exposure, there is no
specific recommendation because of an absence of
sufficient data. Clearly, if acute symptoms are noted
one would want to remove oneself from the locale as soon
as practicable, if for no other reason than personal
comfort.
For the chronic effect hazards, EPA believes that for
many people, keeping long-term exposures at or below 5
ug/m3 of diesel particulate matter provides an adequate
margin of safety for non-cancer respiratory hazards.
This level also includes a 10-fold margin to account for
variability in the human population. For practical
purposes, the belief is that as the long-term average
exposure concentration exceeds this value, the
likelihood of respiratory distress increases.
For carcinogenic hazard and risk of cancer over a
lifetime, EPA is recommending that exposure be viewed as
likely to pose a risk at low levels, as well as high
levels, and is offering a crude range of cancer risks
per unit of lifetime exposure in order to gauge the
public health acceptability of exposures. The risk
values provide an upper bound to the possible human
risk, rather than a true estimate; the true estimate is
undefinable and could be lower.
The National Institute for Occupational Safety and
Health (NIOSH) recommends that whole diesel exhaust be
regarded as "a potential occupational carcinogen."
Work Practices:
-
When diesel equipment
is not in use, the engines should not be allowed to
idle. Equipment should be turned off and restarted
when needed again. Enforce bus/truck idling
restrictions.
-
Any diesel equipment
that is producing visibly smoky exhaust should be
immediately removed from service until the condition
has been corrected.
-
When feasible,
alternative sources of power or fuel with a lower
potential health risk should be substituted for
diesel.
-
Check all depot and
garage ventilation systems to ensure proper
functioning.
-
Enforce the use of bus
exhaust collection systems (tailpipe hoses).
-
At key problem areas
(e.g., fueling areas, bus/truck pull in/out lanes)
ensure that all possible natural ventilation is
used.
-
Continue routine
maintenance of engines.
-
Have routine air
monitoring conducted. Post this information in all
locations where diesel engines operate.
-
On your yearly physical
tell your doctor you work in an area with diesel.
Note: Exposure to diesel exhaust and smoking may
significantly increase your risk.
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