I favour strongly a hazard-based approach, as Europe has undertaken. Again, I'm going to draw on Joe Thornton to illustrate this problem.
One of the biggest barriers in toxics regulation is scientific uncertainty. Part of the problem is just identifying hazards. Is this a substance that causes cancer? The incredible work that's been done with Tox21 has made it easier to do that.
When they do risk assessment, they take that difficult project of figuring out whether the substance causes something, and then they add it to exposure estimates. Typically, what they try to do is estimate, say, if we're looking at cancer, how many excess cancers in a thousand will be caused by this substance. You compound the uncertainty. We've historically underestimated risk exposures.
To give you an example of how uncertain those can be, there's a famous risk assessment of trichloroethylene in drinking water from the U.S. EPA. They used four computer models using the risk, not the hazard, system, to estimate how many cancers it would cause. The results differed by orders of magnitude. In the preamble to the report, the EPA said that there was “an uncertainty equivalent of not knowing whether one has enough money to buy a cup of coffee or pay the national debt.” Hazard is a much more precautionary approach, in my opinion, than a risk-based approach.