Water Chlorination in the US Likely Increases Cancer Risk, New Study Suggests

Nearly all public drinking water in the United States and Europe is treated with chlorine. However, a new study suggests that the current level of chlorination may raise the risk of multiple cancers.

In late January, researchers from the Karolinska Institutet in Sweden published their analysis in the Environ Health Perspect, showing that “chlorination is a widespread method for drinking water disinfection that has the drawback of introducing potentially carcinogenic chemical by-products to drinking water.”

Here's what you need to know.

It's not the chlorine itself — it's the byproducts

The team explained that chlorination is a “cheap, effective, and readily available method for preventing waterborne infectious disease” and is widely adopted worldwide.

As Safe Water explained, it's a chemical disinfection process that uses “various types of chlorine or chlorine-containing substances” to disinfect a potable water source. The Centers for Disease Control and Prevention added that some public utilities switch between chlorine and chloramine at different times of the year or for other “operational reasons,” while some that use underground water do not use any at all, as testing shows it has lower levels of germs and contamination.

Chlorine can be added at various steps. Pre-chlorination is when chlorine is added to raw water immediately after it enters a treatment facility, or more commonly, as the final step in the water treatment process. This is done to “disinfect the water and maintain chlorine residuals that will remain in the water as it travels through the distribution system,” Safe Water explained. The amount added varies depending “on the impurities in the water that need to be treated.”

However, the process can produce by-products when “chlorine or other chemical disinfectants react with natural organic matter in the raw water.” The most pressing of which, the authors state, is trihalomethanes (THMs), which “may have carcinogenic properties.” 

THMs are regulated, but may still be harmful 

It's critical to note that the level of THMs in drinking water is currently regulated in the U.S. and the EU. Stateside, the limit on byproducts is 80 parts per billion (ppb), while the limit in the EU is 100 ppb. However, according to this research, the increase in cancer risks comes in as low as 40 ppb. 

Who's at risk?

To determine who is most at risk and for what, the team conducted a meta-analysis of more than 2,000 records, narrowing it down to 29 studies assessing the risk of 14 different cancers. It summarized the relative risks for bladder cancer and colorectal cancer based on “5,860 and 9,262 cases and 84,371 and 90,272 participants, respectively.” After looking at that truly extensive amount of data, the team concluded that men are more at risk than women for bladder and colorectal cancers. It also showed a “statistically significant 33% and 15% increased risk of bladder cancer and colorectal cancer, respectively, when comparing the highest with the lowest category of THM exposure.” The researchers added in their conclusion that the current legal limits “may fail to protect against cancer in the general population.” 

Why water should still be disinfected 

Emilie Helte, a lead author on the study, shared with The Guardian, “What we see is alarming, and we need some more high-quality studies.” However, Helte also noted that people should still continue drinking municipal water. That's because disinfecting water remains essential for the greater public good. According to the Population Reference Bureau, “The introduction of water filtration and chlorination in major U.S. cities between 1900 and 1940 accounted for about one-half of the 30% decline in urban death rates during those years.” The bureau noted that's because it significantly cut waterborne illnesses, including typhoid fever.

How you can protect yourself at home 

There are ways to improve your home water quality. As Helte told The Guardian, granulated activated carbon, which is used in several home filtration systems, works wonders to remove contaminants.

Dr. Laura Purdy, a board-certified family medicine physician, also noted that this doesn't mean it's time to turn to bottled water. “I would absolutely recommend avoiding water bottled in plastic; this only adds more cancer-contributing PFAS to your toxic load,” Purdy shared. “I would look into a water filtration system and reverse osmosis for your whole house if possible and definitely the sink or refrigerator where your drinking water comes from.” 

Why you shouldn't panic 

As the authors note in this study, the information they gathered provided “limited-suggestive evidence,” a research term that means the data suggests an effect, it doesn't determine it, or as  Claudia Cometa, a clinical pharmacist and author, pointed out, it “means there is a suggested direction of effect, but there are enough limitations present that a causality can not be reached.” However, Cometa noted that this isn't the first study to evaluate the carcinogenic potential of chlorination in drinking water, so it is critical to keep an eye on future research. 

“While Americans should be aware of these potential risks, it is important to avoid fixation on any one cause of cancer,” Cometa added. “Environmental toxins are a growing concern, including our water supply, food supply, and even air quality. Attention should be given to limiting toxin exposure when possible.”