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

1 impacts. Use of these measures requires caution and recognition of the biological and risk
2 assessment uncertainties that are present.
3 For the acute effects of DE exposure, there is no specific recommendation from this
4 assessment for a concentration not to be exceeded in order to avoid acute effects, because of an
5 absence of sufficient data. Clearly, if acute symptoms are noted one would want to remove
6 oneself from the locale as soon as practicable, if for no other reason than personal comfort. As the
7 level of exposure increases, the acute symptoms can become more annoying and be indicative of
8 temporary impairment. With the inherent variability of susceptibility an? sensitivity in the hum.an
9 population, it is not possible to judge the outcome of a specific exposure incident. The same
1 0 statements could generally be made about exposure to gasoline exhaust and many other agents as
11 well.
12 For the chronic effect hazards, EPA believes that for many people, keeping long-term
13 exposures at or below 5 Jlg/m 3 of diesel particulate matter provides an adequate margin of safety
14 for noncancer respiratory hazards. This level also includes a 10-fold margin to account for
15 variability in the human population .. This is not an absolute demarcation of acceptable versus
16 unacceptable exposure, since an order-of-magnitude range of uncertainty is thought appropriate
17 for this recommendation. For practical purposes, the belief is that as the long-term average
18 exposure concentration exceeds this value, the likelihood of respiratory distress increases. TJle 5
19 Jlg/m 3 value comes from test animals who experienced respiratory distress at higher experimental
20 exposures to which a margin of safety (e.g., uncertainty factor) and animal-to-human equivalence
21 factors have been used to derive the 5 J.!glm 3 level for humans. It was necessary to depend on
22 animal studies because the human database was not robust enough. While children should not, a
23 priori, be assumed to be protected by adult recommendations, at this time there is no separate
24 recommendation for children, except for the general wisdom to minimize exposure as much as
25 possible.
26 For carcinogenic hazard and risk of cancer over a lifetime, EPA is recommending that
27 exposure be viewed as likely to pose a risk at low levels, as well as high levels, and is offering a
28 crude range ·of cancer risks per unit of lifetime exposure in order to gauge the public health
29 acceptability of exposures. The risk values provide an upper bound to the possible human risk,
30 rather than a true estimate; the true estimate is undefinable and could be much lower. A range of
31 risk estimators was provided because the available cancer data had too many uncertainties to
32 justify the selection of one scientifically best estimate. The risk range is thought to bracket the
33 upper limits of possible risk, and these values would not likely underestimate the true risk.
34 Assuming that D.E is a cancer hazard for humans, EPA believes that the cancer risks for DE
35 are not likely higher than 1 X 1 o- 5 to 200 X 1 o- 5 per Jlg/m 3 of diesel particulate. These values
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