By Chris Adams

The U.S. Environmental Protection Agency calls the chemical tetrachloroethylene “likely to be carcinogenic to humans.” The National Toxicology Program says it is “reasonably anticipated” to be one.

And the American Conference of Governmental Industrial Hygienists says “available evidence does not suggest the agent is likely to cause cancer” – except under some extreme circumstances.


If regulators are, imagine what the public is thinking.

That gets to the basic core of risk analysis: determining whether something is risky and what to do about it.

George Gray, a professor in the Department of Environmental and Occupational Health at George Washington University, led National Press Foundation fellows through the field of risk analysis, which is often fraught with uncertainty.

“Science is messy and takes time,” Gray said. “And scientists are people – they get invested in their own science.”

Gray previously served as an EPA science advisor and assistant administrator for the agency’s Office of Research and Development. He explained the different standards used by the EPA and other governmental bodies in the U.S. and other nations. He described one study that he conducted to compare what was considered as acceptable by those different bodies – and found that what the different agencies considered as acceptable often widely divergent.

“How do you make a decision within that range?” he asked. “There is a lot of disagreement on that.”

He gave guidance to journalists on how to digest and interpret conflicting scientific studies – including the meanings of words such as “possible,” “likely” and “probable” to describe whether something is a carcinogen or not.

He also cautioned that “risk” means many things: something causing harm, the likelihood that it will cause harm, and the true consequences of that harm.