In early January, the Elk River near Charleston, W.V., began to smell of licorice. The source of the strange odor was a steel tank with a small hole that leaked thousands of gallons of crude 4-methylcyclohexane methanol, a chemical used in coal processing, into the river, just upstream of an intake for Charleston's municipal water system.

State officials told residents in nine counties not to drink, cook with or bathe in tap water. They lifted the ban after four days, but later advised pregnant women to stick with bottled water. Doctors cautioned parents regarding young children, too. A month later, NPR's Melissa Block asked West Virginia Sen. Jay Rockefeller, "Is the water safe to drink?" "I wouldn't drink the water if you paid me," he answered bluntly.

The truth was, nobody knew if the water was safe, because nobody knew much about MCHM's toxicity. When the 1976 Toxic Substances Control Act passed, MCHM was among 62,000 chemicals that had been used for years. Because they weren't known to make people sick, they were presumed safe and left unregulated. The situation isn't much different for more than 20,000 chemicals put into use since.

"Chemicals in our society today are innocent until they're proven to harm us," Charleston Gazette reporter Ken Ward Jr. said recently. The Environmental Protection Agency can only regulate chemicals if there's evidence of danger, and not many have been tested for safety. That helps explain why, as Ward reported, the Centers for Disease Control determined the safe level of MCHM in drinking water "on the fly" after the spill, basing its assessment on one late '90s study done by the chemical's manufacturer. West Virginians, journalist Deborah Blum wrote, became "this month's guinea pigs in our national toxicity experiments."

The residents of Fallon, Nev., can probably relate. Just over a decade ago, when the most significant cluster of childhood leukemia on national record appeared there, they also began to wonder about their water. There was reason to suspect that something in the environment was to blame: Fallon's water had high levels of naturally occurring arsenic. An aging jet fuel pipeline ran through town and probably leaked. Jet fuel was dumped on Fallon's outskirts and could have blown in with the wind. A mill that processed tungsten, a metal whose health effects are unknown, operated beside an elementary school.

Health officials launched a major investigation into the cancer cluster's cause. But as contributing editor Sierra Crane-Murdoch reports, they came up short, in part because scientists didn't know enough about the health effects of jet fuel or tungsten. Plus, it is extremely difficult to trace disease directly to exposure to pollutants – especially a disease like leukemia, whose causes are still poorly understood.

More than answers, the investigation produced additional questions – including one being asked in West Virginia today: Our nation puts the responsibility on the public to prove that chemicals are harmful, which is hard to do before damage is done. Uncertainty justifies lax regulation. Should it?