National Research Council Rejects EPA Risk Assessment Methodology
July 20, 2006
Trevor Butterworth
Risks from dioxin overstated: EPA not justified in relying solely on linear risk assessment for suspected carcinogens. Decision undercuts many environmental health scares

In a devastating blow to many advocacy groups, the National Research Council has called into question the Environmental Protection Agency’s (EPA) method of assessing cancer risks from chemicals. The criticism, which was unanimous, came in the NRC’s review of the EPA’s latest risk assessment of dioxin. As Michael Fumento reports on TCS Daily, the EPA assumed that because extremely high doses of dioxin in animals caused cancer, there was a linear risk for humans: very low exposure levels would still present a risk. The NRC concluded that the “EPA’s decision to rely solely on a default linear model lacked adequate scientific support.”

As STATS guide to evaluating health risks explains:

“[I]n order for a chemical to produce an effect, toxic or otherwise, it needs to interact with receptors or enzymes in a cell molecule. If the amount of the chemical is insufficient to bind itself to most or all of the receptors in a cell there is no effect. As a consequence, most regulatory bodies that assess the risk from chemicals work on the principle that there is a threshold below which there will be no effect. This is known as the “no observed adverse effect level” (NOAEL). Regulatory bodies then create exposure guidelines that add a margin of safety 100 to 1000 times lower than the NOAEL.

The upshot is that it is simply wrong to assume that the risk from a “toxic” chemical is always linear — meaning that it decreases as exposure to the chemical decreases, but never disappears.

There is an important caveat to note here, namely that in the case of chemicals considered to be carcinogenic, some scientists believe there is no threshold below which they are safe: the process of molecular interaction is genotoxic, meaning that the chemical damages the DNA of the cell no matter how low the exposure. The risk, in other words, is linear. The Environmental Protection A gency issues exposure guidelines based on this theory.

This is a hyper-cautious approach to risk and it is controversial for the following reasons: not all carcinogens are genotoxic; there is experimental data which suggests that there are thresholds for genotoxic carcinogens; and more generally, there are barriers that reduce absorption into the body and mechanisms which repair cellular damage.” (read more.)

The NRC concluded that new data on dioxin showed that it was not genotoxic, therefore the EPA needed to conduct a non-linear risk assessment.

“So where does this leave us regarding both dioxin and other potentially carcinogenic substances?” asks Fumento. “Given the poor evidence for low-dose carcinogenesis of dioxin in humans and that according to the EPA dioxin emissions in the environment have been reduced by 92% since 1987 it would seem time to call off the dioxin dogs. That includes both new government regulations and the environmental groups demanding them.”

What this means for the media
Given that many environmental health scares (such as PCBs in salmon) are driven by linear risk assessments, it is important for reporters to determine whether the bulk of the evidence points to the chemical being genotoxic (thus warranting the linear risk assessment) or non-genotoxic.

Because its default position has been that all carcinogenic risks are linear, the EPA is often at odds with other international regulatory bodies (and indeed, the Food and Drug Administration here in the U.S.) in its risk assessments. The media should be wary of falling for advocacy campaigns that make the same assumptions.

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