Researchers have identified a by-product of a chemical used to disinfect the tap water of millions of Americans. But the discovery, reported Thursday in the journal Science, cannot yet say if the chemical —  chloronitramide anion — is toxic to our health.  Separately, Robert F. Kennedy Jr. has advocated the removal of fluoride from drinking water. President-elect Donald Trump’s pick to lead the Department of Health and Human Services has claimed that fluoride consumption is associated with medical conditions such as bone fractures and neurodevelopmental disorders. The presence of these chemicals — chloronitramide anion and fluoride — has led to some concerns about the safety of tap water in the United States. But researchers, including those who discovered the anion, said that these concerns, while understandable, are unnecessary. Tap water generally is safe to drink, they said.