Many public water systems in the United States use inorganic chloramines to disinfect drinking water, but their decomposition products have long been a mystery. In a new study, researchers report the discovery of chloronitramide anion – a compound whose existence, though not identity, has been known for 30 years. Detected in the tap water of millions of Americans, this compound’s toxicity remains untested, prompting calls for immediate toxicological evaluation and raising questions about the safety of chloramine in public water supplies. For over a century, chemical disinfection of public water supplies has effectively reduced waterborne disease by killing pathogens in drinking water. Inorganic chloramines, like monochloramine (NH₂2Cl) and dichloramine (NHCl₂), have become widely used in the U.S. for this purpose and are used to treat the tap water of nearly one-third Americans. However, for decades, chloramine decomposition has been suspected of producing elusive chemical by-products, including potential nitrogen-containing compounds with unknown toxicity. One such disinfection by-product, referred to simply as an “unidentified product”, remains uncharacterized despite being first identified more than 40 years ago. By combining classic synthesis methods with advanced analytical techniques like high-resolution mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy, Julian Fairey and colleagues isolated and identified chloronitramide anion (Cl–N–NO₂^-) as a previously unidentified product of inorganic chloramine decomposition. Fairey et al. measured chloronitramide anion content in a range of chloraminated water systems in the U.S., detecting levels as high as ~100 micrograms per liter (μg/l), which surpasses the typical regulatory limits for many disinfection by-products (60–80 μg/liter). Notably, this compound was absent in water systems that used alternative disinfectants. Although direct toxicological studies have not yet been conducted, the authors caution that computational analyses suggest that chloronitramide anion may not be benign, emphasizing the need for an immediate toxicological assessment and quantification in source waters, finished drinking waters, and wastewater effluents. “The findings of Fairey et al. may trigger a reevaluation of the wisdom of chloraminating public water supplies,” writes Daniel McCurry in a related Perspective. “Regardless of whether chloronitramide anion is found to be toxic or not, its discovery warrants a moment of reflection for water researchers and engineers.”

***A related embargoed news briefing was held on Tuesday, 19 November, as a Zoom Webinar. Recordings can be found at the following links:

Video: https://aaas.zoom.us/rec/share/9vgXlijMh1GT2RaVczoOs8HNjHQ1KXnZkpXCeBt29jVfTtS0pbSGcYUW3hCUd_Ym._bps7Fb6Rg80Jzff

Audio: https://aaas.zoom.us/rec/share/z30gghwhhhBZNqf3Syz1M19V9v40_lu7U8lD__tenAtWx8fo5ZBDOK4AjfEYsSMN.HpRiuXBzkb0zx0v9

The passcode for both is mD?+Fm9k