Americans have relied on tap water for over a century, but new research suggests it may pose hidden health risks. A Swedish study found that chemicals present in public drinking water systems across the U.S. could significantly increase the risk of cancer.
The research, published in Environmental Health Perspectives, highlights a concerning link between trihalomethanes (THMs)—a group of chemical byproducts formed when chlorine disinfects water—and a heightened risk of colorectal and bladder cancer. According to the study, individuals exposed to THMs in drinking water had a 15% higher risk of developing colorectal cancer and a 33% higher risk of bladder cancer compared to those with lower exposure levels.
The study, conducted by researchers at the Karolinska Institute in Sweden, reviewed data from 30 scientific studies involving over 90,000 participants. It found that even relatively low THM levels—starting at just 40 parts per billion (ppb)—were associated with increased cancer risks.
For reference, THM levels in New York City’s water supply were measured at 38.1 ppb between 2013 and 2019, according to the Environmental Working Group (EWG). The U.S. legal limit for THMs in drinking water is 80 ppb, but the EWG strongly recommends that levels should not exceed 0.15 ppb—a threshold that American water systems exceed many times over.
The use of chlorine to disinfect drinking water in the U.S. dates back to 1908 in Jersey City, New Jersey. The practice was widely adopted after proving effective in eliminating deadly waterborne diseases such as cholera and typhoid fever. However, by the 1970s, scientists discovered that chlorine treatment also produces harmful byproducts, including THMs and haloacetic acids (HAAs).
While regulatory agencies like the Environmental Protection Agency (EPA) have set legal limits on these chemicals, critics argue that the standards are outdated and fail to reflect modern scientific understanding of long-term health risks. The EWG has warned for years that exposure to THMs and HAAs may lead to an increased risk of cancer and reproductive issues.
Men appear to be at higher risk than women, according to the Swedish study, though further research is needed to understand the full extent of the health consequences.
The study’s lead author, Emilie Helte, stated that more high-quality studies are needed to confirm these findings but acknowledged that the issue presents a difficult challenge. While reducing chlorine use in water treatment may seem like a solution, it could lead to dangerous outbreaks of waterborne diseases. At the same time, upgrading water filtration systems to remove THMs and other harmful byproducts would come at a significant cost.
For now, Helte advises Americans to continue drinking tap water but recommends using granular activated carbon filters to help reduce exposure to THMs and other contaminants.
This research comes amid growing concerns over the safety of the U.S. water supply. Just last year, studies revealed that “forever chemicals”—man-made compounds linked to various health problems—are widespread in American drinking water. A U.S. government report also found that high fluoride levels in drinking water may lower IQ in children, adding to concerns about chemical exposure through the public water system.
As evidence continues to mount, questions remain about whether federal and state agencies are doing enough to protect public health. While disinfection is necessary to ensure safe drinking water, the long-term effects of chemical byproducts like THMs are becoming harder to ignore.