What Are THMs?
Trihalomethanes (THMs) are a group of chemical compounds that form as a byproduct when chlorine, a common water disinfectant, reacts with organic matter in water. The four primary THMs found in drinking water are chloroform, bromodichloromethane, dibromochloromethane, and bromoform. These chemicals have been detected in nearly all public water systems in the United States and the European Union.
Why Are THMs in Our Drinking Water?
For over a century, chlorine has been the standard method for disinfecting public water supplies. The practice began in 1908 in Jersey City, New Jersey, and quickly spread across the country due to its effectiveness in eliminating deadly waterborne diseases like cholera and typhoid fever. However, by the 1970s, researchers discovered that chlorination also produces harmful byproducts, including THMs and haloacetic acids (HAAs), which have been linked to various health risks.
How Might THMs Cause Cancer?
THMs can enter the body through ingestion, inhalation, or skin absorption during activities such as drinking, showering, or swimming. Once inside the body, these chemicals may cause damage at the cellular level. Studies suggest that THMs can lead to DNA mutations, which increase the risk of cancerous growths. Animal studies have shown that prolonged exposure to high levels of THMs leads to the development of tumors, particularly in the bladder and colon.
Recent research from the Karolinska Institute in Sweden has linked THM exposure to a 33% increased risk of bladder cancer and a 15% increased risk of colorectal cancer. The mechanism behind this effect remains unclear, but scientists believe that THMs may disrupt cell replication and repair processes in the digestive tract, increasing the likelihood of malignant tumors.
The Statistics: How Common Are THMs in Drinking Water?
A review of 30 scientific studies involving over 90,000 participants found that cancer risks were significantly elevated when THM levels in drinking water exceeded 40 parts per billion (ppb). Despite this, the U.S. Environmental Protection Agency (EPA) allows up to 80 ppb of THMs in public water supplies.
In major U.S. cities, THM levels often approach or exceed the 40 ppb threshold associated with increased cancer risk:
- New York City: 38.1 ppb (2013-2019 data)
- Los Angeles: 28.3 ppb
- Dallas: 8.85 ppb
- Chicago: 29.8 ppb
- Boston: 24.2 ppb
By contrast, the Environmental Working Group (EWG) recommends a far lower limit—just 0.15 ppb—to minimize health risks.
What Can We Do About It?
Reducing exposure to THMs in drinking water is challenging because chlorination remains a crucial public health measure to prevent bacterial infections. However, individuals can take steps to lower their personal risk:
- Use Home Water Filters – Granular activated carbon (GAC) and reverse osmosis filtration systems can significantly reduce THM levels in tap water.
- Limit Shower Time and Use Ventilation – Since THMs evaporate easily, minimizing exposure to steam and ensuring proper air circulation can reduce inhalation risks.
- Avoid Bottled Water as a Primary Alternative – Studies indicate that bottled water does not necessarily offer a safer alternative, as it may contain other harmful contaminants.
- Advocate for Better Regulations – Public pressure on policymakers to lower THM limits in drinking water could lead to improved safety standards.
- Consider Alternative Disinfection Methods – While expensive, technologies like ultraviolet (UV) purification and advanced filtration at municipal plants could help reduce THM formation.
While chlorine disinfection has played a crucial role in preventing deadly waterborne diseases, the byproducts it creates, including THMs, pose a potential long-term health risk. With colorectal cancer rates rising, especially among younger populations, addressing this hidden hazard in our drinking water should be a public health priority. By taking individual precautions and advocating for regulatory changes, we can work toward safer drinking water for all.
HNZ Editor: Chlorination has made drinking water safer for a very long time. This risks of drinking water without chlorination are vastly higher than this added risk of colon cancer, so in the absence of a reasonable substitute, I’m thinking we are way better off for now.
