TheGuardian.com – Tom Perkins,
Bladder cancer risk increased 33% and colorectal cancer by 15% in using chlorine to disinfect water
Chlorinating drinking water at levels common in the United States and European Union probably increases the risk of several cancers, a new analysis of recent research from across the globe finds.
The process of disinfecting water with chlorine creates trihalomethane (THM), a byproduct found in virtually all public drinking water systems across the US and EU – nearly 300 million people in the US have concerning levels in their water, by one estimate.
While the chlorination process is a “cheap, effective, and readily available” method for killing organisms and infectious disease, it comes with trade-offs, the study’s authors wrote, including a 33% increased risk of bladder cancer and 15% increased risk of colorectal cancer.
“What we see is alarming and we need some more high quality studies,” Emilie Helte, a lead author with Karolinska Institute in Sweden, said.
The process of disinfecting water is an essential public health measure that dramatically increased life expectancy when the US began chlorinating drinking water in the early 1900s because it significantly reduced microbial infections and waterborne illnesses, like cholera and typhoid fever.
It wasn’t until the 1970s that researchers discovered the process came with consequences. When chlorine is added to water, it reacts with organic compounds, like decaying plant material, to create any number of hundreds of potentially toxic byproducts.
Some of the most common – chloroform, bromoform, bromodichloromethane, and chlorodibromomethane – are known to be genotoxic and carcinogenic to rats.
The US and EU set limits on byproducts at 80 parts per billion (ppb) and 100ppb, respectively, but the new research points to increased cancer risks at levels as low as 40ppb, which is around what they have been found at in New York City. The EPA reports levels are typically in the 40 to 60ppb range and the public health advocacy non-profit Environmental Working Group estimates the safe level at 0.15ppb.
The new meta study is among the most emphatic evidence because it looked at data from about 30 studies and 90,000 participants, and found men were more at risk than women. The authors only looked at bladder and colorectal impacts because there is a dearth of research on other cancers. Researchers are not sure why the chemicals seem to most frequently target the large intestine and bladder, Helte said.
The problem creates a difficult tension for regulators. Surface water typically has higher THM levels than groundwater because it has more organisms and organic matter for the disinfectants to react with. Water utilities could clean some of the organic matter out of the water before disinfecting, and it is also potentially possible to lower the amount of chlorine added, but “it’s really important not to use too little disinfectant”, Helte said.
Alternatives such as treating the water with ultraviolet light or installing new filtration systems are also possible, but are expensive, Helte said.
She stressed that people should continue to drink municipal water. Granulated activated carbon is among the best filtration systems that can be used at home to remove the contaminants.
Source: https://www.theguardian.com/us-news/2025/feb/17/water-chlorination-cancer-risk-us-eu
