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Water: Water Quality Standards Academy

Basic Course: Supplemental Topics - Health (s)

Toxicological Parameter for Cancer Effects (Nonlinear): POD/UF

Human Health Ambient Water Quality Criteria

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The following equation is used for deriving nonlinear cancer human health AWQC (mg/L):


The nonlinear approach is only an option if the mode-of-action information supports a zero slope at a dose of zero. In other words, there are some doses where there is no risk.

In such an instance, a Point of Departure or an estimate of a dose near the bottom of the dose-response curve for tumors or a precursor effect is selected and divided by an Uncertainty Factor to arrive at the value that will be used for the calculation. The POD/UF term in the equation for a carcinogen with a nonlinear response to dose is analogous to the RfD in the noncancer human health AWQC equation except that it applies to a tumor precursor event or tumor incidence as the POD. Cytotoxicity and regenerative hyperplasia are examples of precursor events that can serve as a POD for a carcinogen with a nonlinear response to dose.

key point
Key Point. A mode of action is an experimentally documented sequence of key events starting with the interaction of an agent with a cell, proceeding through functional and anatomical changes, and resulting in cancer formation. In other words, there is no cancer risk at doses below those that cause the precursor cellular events.

For informational purposes only–Not official statements of EPA policy.

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