Water Chlorination Might Be Raising Our Risk of Certain Cancers

An important tool for keeping our drinking water clean may be riskier than we thought. New research finds link between water chlorination and an increased risk of certain cancers.

Scientists at the Karolinska Institute in Sweden led the research, which is a review of past studies looking at chlorination and cancer. They found evidence that people exposed to the highest levels of chlorine byproducts were significantly more likely to develop bladder and colorectal cancer than people exposed to the lowest levels. This associated risk was seen starting at levels below the safety thresholds established in the U.S. and Europe, suggesting that current guidelines aren’t enough to protect the public, the researchers say.

Chlorine has been routinely used to disinfect drinking and recreational water since the early 20th century. It’s helped eradicate or reduce the spread of dangerous diseases like typhoid fever and cholera. But chlorine and other disinfectants are known to have their drawbacks. One major downside is the formation of disinfectant byproducts, created by these chemicals mixing with organic compounds in raw water, and the most prominent byproducts from chlorine are called trihalomethanes (THMs). Past research has shown that THMs can be cancer-causing, at least in rodents, but studies examining whether THMs in chlorinated water are tied to cancer in humans have been more mixed.

Earlier reviews of the data generally found limited evidence of a link between THMs and bladder and colorectal cancers. But those reviews are now over a decade old, and there have been newer, potentially more informative studies on the topic published since then. So the Karolinska scientists sought to conduct their own updated meta-analysis of the research.

They ultimately analyzed data from 29 papers, the latest published just last year. Though most of the studies looked at bladder and colorectal cancer, 14 cancers in total were evaluated. The researchers failed to find a significant link between THM exposure and any other cancers besides the two. But they found that the highest THM levels (relative to the lowest) were associated with a 33% higher risk of bladder cancer, and a 15% higher risk of colorectal cancer. Importantly, this added risk appeared at THM levels starting at 41 parts per billion (ppb)—below the 80 ppb regulatory limit in the U.S. and the 100 ppb limit in the EU.

“In conclusion, in this systematic review and dose–response meta-analysis, we found limited-suggestive evidence that exposure to THMs in drinking water increases the risk of bladder cancer and colorectal cancer,” the researchers wrote in their paper, published this January in Environmental Health Perspectives.

There are other technologies that can disinfect drinking water nowadays, such as ultraviolet light treatment. And practices such as removing organic matter from water before it’s treated with chlorine could potentially lower THM levels. But the researchers fully admit that the data collected so far isn’t enough to prove a cause-and-effect link between chlorination and cancer. They also aren’t telling the public to go cold turkey on drinking tap water based on their findings. At the same time, they are urgently calling for more well-conducted research to look into and confirm this possible risk.

“What we see is alarming and we need some more high quality studies,” lead researcher Emilie Helte told the Guardian.