hexachlorobutadiene - Davidborowski.com

HEXACHLOROBUTADIENE 10
2. HEALTH EFFECTS
attempt at distinguishing between “less serious” and “serious ” effects. The distinction between “less
serious” effects and “serious” effects is considered to be important because it helps the users of the
profiles to identify levels of exposure at which major health effects start to appear. LOAELs or
NOAELs should also help in determining whether or not the effects vary with dose and/or duration,
and place into perspective the possible significance of these effects to human health.
The significance of the exposure levels shown in the Levels of Significant Exposure (LSE) tables and
figures may differ depending on the user’s perspective. Public health officials and others concerned
with appropriate actions to take at hazardous waste sites may want information on levels of exposure
associated with more subtle effects in humans or animals (LOAEL) or exposure levels below which
no adverse effects (NOAELs) have been observed. Estimates of levels posing minimal risk to humans
(Minimal Risk Levels or MRLs) may be of interest to health professionals and citizens alike.
Levels of exposure associated with carcinogenic effects (Cancer Effect Levels, CELs) of
hexachlorobutadiene are indicated in Table 2-2. Because cancer effects could occur at lower exposure
levels, Figures 2-l and 2-2 also show ranges for the upper bound of estimated excess risks, ranging
from a risk of 1 in 10,000 to 1 in 10,000,000 (10 -4 to 10 -7 ), as developed by EPA.
Estimates of exposure levels posing minimal risk to humans (Minimal Risk Levels or MRLs) have
been made for hexachlorobutadiene. An MRL is defined as an estimate of daily human exposure to a
substance that is likely to be without an appreciable risk of adverse effects (noncarcinogenic) over a
specified duration of exposure. MRLs are derived when reliable and sufficient data exist to identify
the target organ(s) of effect or the most sensitive health effect(s) for a specific duration within a given
route of exposure. MRLs are based on noncancerous health effects only and do not consider
carcinogenic effects. MRLs can be derived for acute, intermediate, and chronic duration exposures
for inhalation and oral routes. Appropriate methodology does not exist to develop MRLs for dermal
exposure.
Although methods have been established to derive these levels (Barnes and Dourson 1988; EPA
1990h), uncertainties are associated with these techniques. Furthermore, ATSDR acknowledges
additional uncertainties inherent in the application of the procedures to derive less than lifetime
MRLs. As an example, acute inhalation MRLs may not be protective for health effects that are
delayed in development or are acquired following repeated acute insults, such as hypersensitivity