THE CALTOX MODEL
CalTOX utilizes data on a pollutant's physical-chemical properties and the landscape characteristics of the environment receiving a release to model how that chemical will be distributed among seven connected environmental compartments (e.g., soil, water, air). CalTOX predicts the chemical concentrations in these compartments which will result from a continuous release of a pollutant, taking into account transport and transformation processes that affect the pollutant. CalTOX applies a multi-pathway exposure assessment model to these ambient concentrations to estimate the total chemical dose people may receive from a release. CalTOX produces an estimate of the average daily dose that is associated with a unit release of a chemical to air or water in a model environment, expressed in milligrams of a chemical per kilogram of body weight.
CalTOX was then used to combine estimated doses with Scorecard's risk assessment values to conduct screening-level risk assessments of releases of one pound of a chemical to air or water. In many cases, risk assessment values are available for only one of the three exposure routes CalTOX considers (ingestion, inhalation, and dermal contact). As a general rule, any available risk assessment value is applied to other routes of exposure lacking toxicity data , unless there is a clear toxicological rationale against making this assumption. This assumption of cross-route applicability is made to avoid treating chemicals as if they pose no health risk at all if exposures occur via a route that lacks data.
CalTOX characterizes potential cancer and noncancer risks using standard regulatory and scientific assumptions. For carcinogens, the increased risk of cancer associated with exposure is:
Added Lifetime Cancer Risk = Average Daily Dose from Exposure x Cancer Potency Value
For non-carcinogens, risks are characterized using a hazard index:
Hazard Index = Average Daily Dose from Exposure/Reference Dose
For chemicals that possess the physical-chemical and toxicity data required to run CalTOX, this method yields a set of up to four health risk estimates associated with a unit release of a chemical. This set can include cancer and/or noncancer risks posed by a unit release to air, and cancer and/or noncancer risks posed by a unit release to water.
CALCULATING TEPS USING THE RISK ESTIMATES FROM THE CALTOX MODEL
Once CalTOX has produced estimates of the health risks posed by a unit release of a chemical to air or water, these can be used to calculate TEPs for a chemical. TEPs are simply the ratio of the risk posed by a one pound release of chemical X to the risk posed by a one pound release of a reference compound. Separate TEPs are calculated for chemical releases to air and water.
For carcinogens, TEPs are expressed as pounds of benzene-equivalents:
TEP = [Added Cancer Risk/Unit Release of Chemical X]/[Added Cancer Risk/Unit Release of Benzene]
For noncarcinogens, TEPs are expressed as pounds of toluene-equivalents:
TEP = [Hazard Index/Unit Release of Chemical X]/[Hazard Index/Unit Release of Toluene]
MORE
On the assumptions made to deal with data gaps and modeling issues.
Technical details of how media and health endpoint-specific TEPs are calculated:
Cancer Risk Score
- Air Releases
Cancer Risk Score
- Water Releases
Noncancer Risk
Score - Air Releases
Noncancer Risk
Score - Water Releases
REFERENCES
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Hertwich, E.G. and T.E. McKone. The spatial scale of pollutants in multimedia models and its implications for the potential dose. Environmental Science & Technology 35(1):142-148. 2001.
Hertwich, E.G., T.E. McKone, and W.S. Pease. A systematic uncertainty analysis of an evaluative fate and exposure model, Risk Analysis 20(4):437-452. 2000.
Hertwich, E.G. Toxic Equivalency: Addressing Human Health Effects in Life Cycle Impact Assessment. A dissertation submitted for the degree of Doctor of Philosophy in Energy and Resources in the Graduate Division of the University of California, Berkeley. 1999. http://www.is4ie.org/dynamic/listing.php?id=21
http://www.tev.ntnu.no/edgar.hertwich/CV.html
Hertwich, E.G., T.E. McKone, and W.S Pease. Parameter uncertainty and variability in evaluative fate and exposure models. Risk Analysis 19(6):1193-1204. 1999.
Hertwich, E.G., W.S. Pease, and T.E. McKone. Evaluating Toxic Impact Assessment Methods: What Works Best? Environmental Science & Technology 32(5): 138A -145A. 1998.
Science Advisory Board, U.S. Environmental Protection Agency. Human Exposure Assessment: A Guide to Risk Ranking, Risk Reduction and Research Planning. EPA/SAB/IAQC-95-005. Government Printing Office, Washington, DC. 1997.
McKone, T.E. CalTOX, A Multimedia Total Exposure Model for Hazardous-Waste Sites UCRL-CR-111456PtI-IV, U.S. Department of Energy, Lawrence Livermore National Laboratory, Government Printing Office, Washington, DC. 1993.