About a third of U.S. residents receive tap water containing a previously unknown chemical byproduct, a new study finds. Some scientists are now concerned – and are actively investigating – whether that chemical could be toxic.

The newly identified substance, called ‘chlornitramide anion’, is produced when water is treated with chloramine, a chemical formed by mixing chlorine and ammonia. Chloramine is commonly used to kill viruses and bacteria in municipal water treatment systems.

Researchers said the existence of the byproduct was discovered about 40 years ago, but it was only now identified as analytical techniques have improved, eventually allowing scientists to determine the chemical’s structure.

It can take years to find out if chlornitramide anion is dangerous; it has never been investigated. The researchers reported their findings Thursday in the journal Science, in part to spur research to address safety concerns.

The scientists said they have no hard evidence to suggest the substance poses a hazard, but it does have similarities to other chemicals of concern. They think it deserves investigation because it has been discovered so widely.

“It has similarities to other toxic molecules,” said David Wahman, one of the study’s authors and an environmental research engineer at the Environmental Protection Agency. “We looked for it in 40 samples in 10 U.S. chlorinated drinking water systems in seven states. We found it in all samples.”

Chloronitramide anion is produced as chloramine decays over time. It is likely found in all drinking water treated by this method, he said.

The fact that a byproduct with unknown risks can be so ubiquitous and elude researchers for so long raises questions about the potential health effects of the chemicals used to treat tap water.

According to the study, approximately 113 million U.S. residents receive chloramine-treated water from their taps. The chemical has been used to disinfect water for about a century. Today it is often used to protect a system’s “residue”: the water that remains in the pipes for several days after leaving a water treatment plant.

Chloramine is increasingly preferred over chlorine because the latter also produces byproducts, some of which have been linked to bladder cancer and are regulated by the EPA.

David Reckhow, a research professor of civil and environmental engineering at the University of Massachusetts, Amherst, who was not involved in the study, said the finding was an important step. The ultimate goal, he said, is to understand whether the substance is a hazard; he agreed that it was probably poisonous.

“It is a fairly small molecule and can therefore probably enter biological systems and cells. And it is still a reactive molecule,” he said. “Those are the kinds of things you worry about.”

The authors of the new study arrived at their results after figuring out how to formulate high concentrations of the chemical for laboratory testing, said Julian Fairey, lead author and associate professor at the University of Arkansas.

“We don’t know the toxicity, but this study has allowed us to do that work now,” says Fairey, who researches drinking water byproducts. “Now we can do the hard work of finding out what its toxicological relevance is in our water systems.”

He added that some previous studies have suggested a link between drinking disinfected water and the increased risk of certain cancers.

“We don’t know what drives this. We have no idea if this substance is in any way related to these results,” Fairey said. “But we have unexplained incidents of certain types of cancer from treated drinking water.”

However, it will likely take years before any conclusions can be drawn about whether the newly identified substance is toxic. Potential regulation based on these final findings would take even longer.

“It’s a lot — probably 10 years of research once a funding source is found,” said Alan Roberson, executive director of the Association of State Drinking Water Administrators.

Reckhow said in the meantime, water utilities should pay close attention to ongoing research and try to reduce human exposure.

“You do what you can to keep it to a minimum,” he said. “You make the best judgment you can about the toxicity and you walk away with incomplete information. Unfortunately, that is the world we live in.”

The EPA regulates only a handful of disinfectant byproducts, including some related to the use of chlorine. Scientists say these regulations have prompted some water suppliers to increase the use of chloramine.

“This study raises the question of whether or not this disinfection process is safer from a health perspective,” said David Andrews, a senior scientist at the Environmental Working Group, an advocacy group pushing for more research on chemicals.

He added that hundreds of disinfection byproducts are found in water systems, but this one deserves further investigation.

“Many of these other pollutants are at lower concentrations or less common,” Andrews said.

Any treatment of drinking water carries some health risk, Roberson said. It’s a tradeoff: Disinfection processes have largely defeated waterborne diseases like cholera and typhoid, but research suggests some byproducts are linked to risks of cancer and miscarriage.

“The reason you add chloramine: you want to kill the bacteria and viruses, there is a real risk-risk trade-off,” he said.

Many American water companies announce on their websites whether they treat the water they supply with chlorine or chloramine. Wahman said research shows that activated carbon filters, such as those used in home water purifiers or refrigerator filters, can remove disinfectant byproducts, but more research is needed.