But is all the drinking water in the United States causing cancer? Of course it's not that simple. In the US, the federal Environmental Protection Agency (EPA) sets maximum contaminant levels (MCLs) as part of the Safe Drinking Water Act. Because it found these lacking, the EWG based its analysis partially on its own standards.
"When official guidelines are not available or are insufficient to protect public health, we developed our own health benchmarks using publicly available scientific research," reads EWG's data sources and methodology page.
Similarly, on its its "EWG Standards" page, the organization notes the standards were devised using "the best and latest scientific evidence," but does not link to or mention any specific scientific studies it used. On the methodology page, EWG also notes that it relied on EPA standards, California public health goals, and an assessment from the Minnesota Department of Health.
A spokesperson from the EWG said in an email that it got the figure from a 2010 draft California Office of Environmental Health Hazard assessment, which indicated 1 ppb of chloroform is low enough for a "one-in-a-million" lifetime risk of cancer. In other words, there is some basis for choosing a number so low, but EWG cherry-picked the lowest health guideline it could find even though the document was never finalized.
"Some people look at this and say you're causing alarm or scaring people," Bill Walker, the EWG's vice president and managing editor told me over the phone. "Well we think people are smart enough to take in information and act on it intelligently."