In the US, municipal drinking water is protected by the Safe Drinking Water Act, which compels utilities to monitor things like microorganisms and the disinfectants used to subdue them. In 1998 the EPA tightened its standards on disinfectants, many of which can have their own toxic byproducts. One of the worst offenders is a classic: chlorine. Its main replacement, a chemical called chloramine (really just a mix of chlorine and ammonia), has lower levels of carcinogenic breakdown products, but it also makes the water corrosive—enough to eat through metal. ... Lead is insidiously useful. It’s hard but malleable, is relatively common, melts at a low enough temperature to be workable, and doesn’t rust. The Romans used it for plumbing—in fact, that word derives from the Latin word for lead, plumbum. Even the Romans noticed, as early as 312 BC, that lead exposure seemed to cause strange behaviors in people. But as Werner Troesken, an economist at the University of Pittsburgh, explains in his book The Great Lead Water Pipe Disaster, lead pipes solved a lot more problems than they caused. The hydrologists of the 19th century knew that lakes and wells could harbor cholera; they needed large, clean bodies of water that they could pump into the city. Lead made those pipes possible. ... in 1991 the EPA instituted the Lead and Copper Rule, requiring utilities to check water regularly. The critical level has changed over the years as new science has come to light, but today officials are required to take action if lead exceeds 15 ppb in more than 10 percent of residents’ taps. The metric is utilitarian, scaled to spot trouble just before it turns into disaster. It’s a good rule, as long as utilities follow it.