Public Health Assessment Public Comment Release
human differences, and/or the use of the lowest adverse effect level. For noncancer health
effects, the following health guideline values are used.
Minimal Risk Level (MRLs) - Developed by ATSDR
An MRL is an estimate of daily human exposure by a specified route and length of time to a
dose of chemical that is likely to be without a measurable risk of adverse, noncancerous effects.
An MRL should not be used as a predictor of adverse health effects. A list of MRLs can be
found at http://www.atsdr.cdc.gov/mrls.html.
Reference Dose (RfD) - Developed by EPA
An RfD is an estimate, with safety factors built in, of the daily, life-time exposure of human
populations to a possible hazard that is not likely to cause noncancerous health effects. RfDs can
be found at http://www.epa.gov/iris.
If the estimated exposure dose for a chemical is less than the health guideline value, then the
exposure is unlikely to cause a noncarcinogenic health effect in that specific situation. If the
exposure dose for a chemical is greater than the health guideline, then the exposure dose is
compared to known toxicologic values for that chemical and is discussed in more detail. These
toxicologic values are doses derived from human and animal studies that are summarized in the
ATSDR Toxicological Profiles. A direct comparison of site-specific exposure and doses to
study-derived exposures and doses that cause adverse health effects is the basis for deciding
whether health effects are likely or not.
Calculation of Risk of Carcinogenic Effects
The theoretical risk
of developing cancer resulting from exposure
to the contaminants was
calculated by multiplying the site-specific adult exposure dose by EPA=s corresponding oral
slope factor (found at http://www.epa.gov/iris ). The results estimate the maximum increase in
risk of developing cancer after 70 years of exposure to the contaminant. The risk was then
multiplied by the fraction (25/70) because the exposure was assumed to last 25 years.
The actual risk of cancer is probably lower than the calculated number, which gives a worst-case
excess cancer risk. The method used to calculate EPA=s oral slope factor assumes that high-dose
animal data can be used to estimate the risk for low dose exposures in humans. The method also
assumes that no safe level exists for exposure. Little experimental evidence exists to confirm or
refute those two assumptions. Lastly, the method computes the 95% upper bound for the risk,
rather than the average risk, suggesting that the cancer risk is actually lower, perhaps by several
U.S. Environmental Protection Agency (EPA), Office of Emergency and Remedial Response. Risk assessment
guidance for Superfund, volume 1, human health evaluation manual. Washington (DC): U.S. Environmental
Protection Agency; 1989.
Agency for Toxic Substances and Disease Registry (ATSDR). Cancer policy framework. Atlanta (GA): U.S.
Department of Health and Human Services; 1993.