The federal Environmental Protection Agency sets limits for drinking water on more than 90 contaminants. Each state is responsible for making sure that public water systems meet these standards and may add additional criteria.

But there are tens of thousands of industrial chemicals that have never been tested for safety. And new ones are constantly being introduced.

The regulatory process is long. Periodically, the Environmental Protection Agency adds chemicals to a watchlist of unregulated contaminants that are potentially harmful. Researchers work their way down the list to determine whether the chemicals pose a risk and at what levels, and the feasibility of regulating them. Often, data on occurrences and health effects just isn’t available.