But the discovery of a new and previously unknown chemical, called chloronitramide anion, could have implications for municipal water systems that use a class of chlorine-based disinfectants called chloramines. For decades, these disinfectants, derived from the mixture of chlorine and ammonia, have been added to many municipal water supplies to kill bacteria and prevent waterborne illnesses.

"We need to investigate it. We don't know the toxicity," said environmental engineer Julian Fairey, an associate professor at the University of Arkansas and lead author of the paper. "This work was 40 years in the making in terms of trying to identify the compound, and now that we have identified it, we can delve into how toxic is this thing."

Initial computational modeling of the compound shows similarities to other chemicals with demonstrated toxicity, which "suggests that it is probably not harmless," Fairey said.

Water-quality experts not involved in the new study said such modeling is an initial method of trying to understand a chemical's toxic potential, and urged people not to be alarmed.

"The water is still safe to drink. Tap water is more regulated, with more people working on it, than bottled water," said Lisa Ragain, a principal water resources planner at the Metropolitan Washington Council of Governments.

Scientists and policy makers have long known that disinfecting water requires a balance of risks and benefits. Clean drinking water is a public health triumph, making a major contribution to increasing life expectancy over the past century across much of the planet. Before the implementation of filtration and chlorination of water in US cities in the early 1900s, waterborne illnesses such as typhoid fever and cholera were common.

But those benefits have to be weighed against the risk of chemical by-products created when a disinfectant reacts with organics in the water or decomposes.

Scientists realized in the 1970s that chlorine added to the water supply not only killed germs but reacted with organic compounds, such as those produced by decaying plant material, to create "disinfection by-products." Some of those contaminants have been linked to cancers and miscarriages, and are now regulated by the Environmental Protection Agency, which limits their concentration in the water supply.

Chloramination provided an alternative way to disinfect water. But it created by-products of its own, and one compound has for decades remained mysterious, its formula and structure eluding characterization. Researchers had no name for it, and just called it "unidentified product."

That product turns out to be chloronitramide anion, a compound of chlorine, nitrogen, and oxygen atoms.

This is a novel chemical. It doesn't appear in the Chemical Abstracts Service, a registry of 219 million substances. "It's like the number of stars we have in the sky for chemistry," said Beate Escher, a toxicologist at the Helmholtz Center for Environmental Research in Leipzig, Germany, who was not involved in the study.

"Do not panic," Escher said. "I wouldn't be worried too much. It would be nice to test it toxicologically."

The EPA issued a statement responding to the new report.