Summary of Health Effects Evaluation

Now, let’s summarize the toxicological evaluation results for our case exercise on carbon tetrachloride. We might have something like this:

Noncancer Health Effects

The oral sub-chronic 12-week study used for the basis of the EPA’s RfD reported a BMDL of 3.9 mg/kg/day. This means that the highest site-specific exposure dose from our case example (0.037 mg/kg/day) is 105 times lower than the BMDL, the lowest level estimated to show effects in animals exposed to carbon tetrachloride.
We are confident in using EPA’s RfD to assess noncancer effects for our site. EPA performs many scientifically defensible steps to derive its values, and as such, ATSDR approves of their use in the public health assessment process.
We must, however, consider there is some uncertainty with comparing a chronic (one year or longer) exposure dose to one based on a study conducted over a shorter, sub-chronic duration. Also, EPA acknowledges other limitations of the study, such as relatively small group size and evaluation of only two target organs (kidney and liver).
After a consideration of the uncertainty and a review of all the scientific data available, ATSDR is confident in comparing the estimated dose to this BMDL.
Based on this dose comparison and evaluating the evidence, ATSDR believes that the detected levels of carbon tetrachloride are not likely to harm PW-3 residents under the site-specific conditions evaluated.

Cancer Health Effects

EPA’s cancer assessment of carbon tetrachloride includes some uncertainties, such as modeling being used to estimate the oral slope factor. Even so, EPA indicates that available evidence supports the conclusion that experimental findings of liver cancer seen in animals are relevant to humans.
It is important to provide additional perspective on ATSDR’s estimated carbon tetrachloride increased cancer risk of two to three extra cancer cases per 10,000 people. For instance, the American Cancer Society (2018) reports that one in three (or 3,333 in 10,000) persons will develop cancer at some point in their lifetime.
ATSDR’s evaluation suggests that drinking the carbon tetrachloride contaminated water from PW-3 for many years results in an increased lifetime risk for cancer.
The estimated cancer risk represents a theoretical increase in cancer cases should 10,000 persons drink contaminated water with 262 ppb carbon tetrachloride for 33 years. It is not the actual number of cancers in the community from exposure to carbon tetrachloride. This numeric estimate is used as a guide for ATSDR recommending public health actions.
ATSDR suggests that homeowners lower their exposure to carbon tetrachloride in their drinking water by using a different source of drinking water (such as bottled water) or by installing a treatment system that will reduce the amount of carbon tetrachloride in their water.