5.6 Toxicological profiles

The Agency for Toxic Substances and Disease Registry (ATSDR) data and IRIS infor­mation are used to prepare toxicological profiles for COPCs. The toxicological profiles will include specific toxicological information (e.g., toxicological effects, target organs, critical effect).

5.7 Uncertainties related to toxicity information

The RfDs established for COPCs are a major source of uncertainty in a BLRA. The RfD is the estimate of daily exposure likely to be without an appreciable risk of deleterious effects during a lifetime. The RfD is derived by the application of uncertainty factors to selected exposure levels identified in animal or human studies. Identified exposure levels are divided by these uncertainty factors to assure that the RfD will not be overestimated.

For example, an uncertainty factor of 10 is used to account for variations in human sensi­tivity when using data from valid human studies involving long-term exposure of average, healthy subjects. Additional uncertainty factors of 10 are applied to account for uncertain­ties in extrapolating from observation of toxicity in animals to predicted toxicity in humans, to account for uncertainties in identifying threshold dose from experimental data, and to account for uncertainties in extrapolating from subchronic to chronic studies. Any additional uncertainty factor or modifying factor ranging from >0 to £10 may be applied to reflect professional assessment of other uncertainties that may exist in the toxicity data­base for a specific compound.

Considerable uncertainties are involved in identifying whether or not a compound is a likely potential human carcinogen and at what level of exposure an increased risk of can­cer may exist.

Uncertainties in quantifying the exposure level that may result in elevated carcino­genic risk for specific compounds are compensated for by using the 95% UCL of the esti­mated slope. This slope refers to the line that relates chemical exposure to the probability of developing cancer—thus the term slope factor for carcinogens. Using the 95% UCL is a statistical path to assure that the actual SF is highly unlikely to be greater than the SF listed for that chemical. These dose-response assumptions provide an upper, but plausible, esti­mate of the limit of risk when the SF is used to estimate risk associated with an estimated level of exposure.

5.8 Potentially exposed populations

For the future land-use scenario the assumption is that the site property will remain a site. The population to be considered includes on-site workers, site visitors, and possible future construction workers. The population may be exposed to surface soil, subsurface soil, and groundwater through inhalation, ingestion, and dermal contact.

The monitoring results of the well survey efforts are used to determine completed exposure pathways. The site does not have on-site residential or adjacent residential prop­erty; therefore, the soil exposure pathway is not applicable (i.e., complete) for on-site or adjacent sites.

Recreational receptors to be evaluated will include site trespassers and users of the site.