For Diesel Particulate Matter (diesel PM), the NJDEP Risk
Assessment uses the Reference Concentration (RfC) divided
by 10 for the Health Benchmark. Normally for a pollutant
like diesel PM which has been declared a carcinogen, NJDEP
would use a slope factor (or unit
risk factor URF) to develop a Health Benchmark. However,
the studies that were used to determine carcinogenicity
were not able to quantify the amount of diesel PM that led
to the cancers, so USEPA has not set a unit risk factor
for this pollutant yet.
In the NJDEP, as well as other regulatory agencies, there
is a standard policy that recommends dividing the RfC
by 10 to set a benchmark if the pollutant is a Class C
carcinogen (i.e., a probable human carcinogen based on
animal studies). In the case of diesel PM, the evidence
for carcinogenicity is even stronger, with both human
and animal evidence, thus warranting a B1 (possible human
carcinogen) classification. Therefore, it seems reasonable
to use at least the RfC divided by 10 for the health benchmark.
Some might argue that an even higher adjustment factor
(e.g. RfC/50 or RfC/100) might be warranted for use in
this evaluation. However, there is no precedent for developing
a health benchmark from an RfC for a B1 carcinogen since
it rarely happens that a B1 carcinogen does not have a
URF.
The California Air Resources Board (CARB) has done some
work to develop a URF for diesel PM. They identified a
range of URFs and have recommended a mid-range URF of
3 x 10-4 (ug/m3)-1 for use in their state. However, the
studies upon which those URFs were based had difficulty
in quantifying the exposure that the workers received,
leading to a great deal of uncertainty in these values.
Therefore, NJDEP has chosen not to use any of these URFs
at this time.
It should be noted that in the draft Health Assessment
Document for Diesel Exhaust (USEPA 2000) it is concluded
that "The magnitude of the estimated lifetime cancer
risk (between 10-5 and 10-4), derived from using a high-end
occupational to environmental exposure difference, establishes
a reasonable basis for concern that the general population
faces possible risks higher than 10-6." This is consistent
with the conclusion drawn by the NJDEP risk assessment
that diesel PM should be on the list of Air Toxics of
Greatest Concern. In spite of the problems with quantifying
the risk posed by exposure to diesel PM in New Jersey,
it is clear that current exposures are too high and that
readily available methods for reducing exposure should
be explored.
REFERENCES
US Environmental Protection Agency (USEPA), 2000. Health
Assessment Document for Diesel Exhaust: SAB Review Draft.
EPA/600/8-90/057E (p. 8-13).
California Environmental Protection Agency, Office of
Environmental Health and Hazard Assessment (CalEPA/OEHHA),
May 1998. Proposed Identification of Diesel Exhaust as
a Toxic Air Contaminant, Part B: Health Risk Assessment
for Diesel Exhaust.
Cal EPA/OEHHA, April 1999. Air Toxics Hot Spots Program
Risk Assessment Guidelines, Part II: Technical Support
Document for Describing Available Cancer Potency Factors
(table of Hot Spots Unit Risk and Cancer Potency Values,
page 4).
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