Water Chlorination: Is Your Tap Water a Cancer Cocktail?

Most Americans take water chlorination for granted. This is how the majority of municipalities ensure that their tap water won’t make people sick. After all, chlorine is very effective at killing microbes that cause waterborne diseases. That is why public health officials consider water chlorination one of the greatest public health advances of all time. But a Swedish study published in Environmental Health Perspectives (January 21, 2025) suggests that there may be unexpected downsides to water chlorination.

The CDC Says Water Chlorination is Safe:

The Centers for Disease Control and Prevention (CDC) is considered the premier public health organization in the world. It states:

“Chlorine or chloramine [‘compounds that contain chlorine and ammonia’] levels up to 4 milligrams per liter (mg/L), or 4 parts per million (ppm), are considered safe in drinking water. These low levels kill harmful germs and are unlikely to make people sick.”

What About Water Chlorination and Cancer-Causing By-Products?

The Swedish scientists introduce their research this way (Environmental Health Perspectives, Jan. 21, 2025):

“Chlorination is a cheap, effective, and readily available method for preventing waterborne infectious disease and is widely adopted across the globe. Nevertheless, chemical drinking water disinfection may also give rise to disinfection by-products that are formed when chlorine or other chemical disinfectants react with natural organic matter in the raw water, and the most prevalent class of by-products in chlorinated drinking water is the trihalomethanes (THMs). There are concerns that THMs may have carcinogenic properties.”

In their comprehensive systematic review and meta-analysis, the researchers found that exposure to THMs was associated with 14 different cancers. In particular, people with the highest exposure were 33% more likely to develop bladder cancer and 15% more likely to receive a diagnosis of colorectal cancer. Not surprisingly, the authors call for more research to establish the risk of disinfection by-products.

The authors sum up their study results this way:

“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 summary RR [relative risk] estimate for bladder cancer in men and women was statistically significant at THM concentrations >41μ⁢g⁡/L. THMs are currently regulated not to exceed 80μ⁢g⁡/L in the US and 100μ⁢g⁡/L in the EU, and our findings suggest that these limits may fail to protect against cancer in the general population. For other cancer outcomes evaluated, there were insufficient data available to draw any firm conclusions. Our review highlights the need for further research on disinfection by-products and cancer, particularly some cancers for which the few studies available indicated associations with increased risk, such as female hormone-related cancers and malignant melanoma.”

Water Chlorination Has Been Controversial For Years:

Most people take tap water for granted. They assume that public health authorities would never put anything in the water that might cause harm. I recently wrote about the fluoride controversy at this link.

“Does Fluoride in the Water Impact Children’s IQ?”

We just hosted a 2-part series on our nationally syndicated public radio show. It provided a balanced perspective on fluoride with two experts who had differing perspectives on the safety of fluoride.

Show 1417: Examining the Pros and Cons of Water Fluoridation (Part 1)

Show 1418: More About the Pros and Cons of Water Fluoridation (Part 2)

The Downsides of Water Chlorination:

There have been signals that chlorination by-products might pose risks dating back decades. Animal studies from the 1980s suggested that trihalomethanes (THMs) might be carcinogenic (National Toxicology Program technical report series, Oct., 1987).

A study published in the International Journal of Epidemiology (June, 2010) concluded:

“The study findings provide limited evidence of a positive association between colorectal cancer and exposure to DBPs [disinfection by-products] in drinking water. The small number of studies and limitations in study quality prevent causal inference.”

Don’t you find it a bit disturbing that there were so few studies on the safety of chlorination byproducts in 2010? Chlorine has been added to municipal water sources in the US for more than a century.

Another Problem with Water Chlorination: Phenols

Another study suggests that chlorine forms toxic chemicals in the presence of phenols (Environmental Science and Technology, Jan. 6, 2020). Such phenols are common in water supplies. The source may be personal care products or pharmaceuticals that end up in waste water and may be incompletely removed during purification. Plastic water pipes may also shed phenols into drinking water.

The authors point out that

“chlorine reacts with organic and inorganic water constituents to produce a variety of disinfection by-products (DBPs) that pose potential health risks.”

They note that over 700 DBPs have been identified, including some known carcinogens. The researchers found that phenols from consumer products interact with chlorine to cause some of these potentially toxic chemicals.

How Important Is Chlorine?

Do we really need water chlorination? Although it is a good way to guard against water-borne infections, chlorination is not the only technique that is effective. European public health officials often use other measures including filtration, reverse osmosis and microfiltration, ozone or ultraviolet treatment to purify water. There is no indication that people drinking such water are more likely to suffer illness as a consequence.

What Can You Do About Water Chlorination and Dangerous Disinfection By-Products?

If you live in a city or municipality that provides water to your home or apartment, you are almost assuredly getting some disinfection by-products (DBPs) in your tap water. We generally do not like to recommend bottled water. What’s left?

Water filtration can be an effective strategy for removing DBPs from tab water. There are lots of ways to do this. There are lots of products on the market. Some use reverse osmosis or granular activated carbon.

The Environmental Working Group (EWG) offers this recommendation:

“What can I do to remove disinfection byproducts from my tap water?

“EWG recommends using a home filtration system to treat disinfection byproducts in your tap water. Simple filtration methods, such as countertop carbon filters, can decrease the levels of common disinfection byproducts in water, such as trihalomethanes and haloacetic acids.”

EWG makes some recommendations about countertop water filters at this link.

Consumer Reports also rates water filters. CR offers a variety of suggestions from water filter pitchers and under-sink water filters to reverse osmosis systems and countertop water filters at this link.

We have been very happy with our AquaTru reverse osmosis water purifier. These folks sponsored our podcast for a few weeks and sent us this device so we could try it our for ourselves. It’s not cheap, but we really like it for convenience and purification. I have always preferred glass carafes to plastic and Aquatru has a very nice glass pitcher. It uses a pre/carbon filter, an ultra-fine reverse osmosis filter and an activated coconut block carbon filter to reduce volatile organic compounds. Because we are on a well system, we don’t worry about chlorine very much, but we suspect there could be other nasty chemicals in the aquifer that provides us tap water.

Final Words:

Chlorine has been used for more than 100 years. It definitely kills nasty pathogens that would otherwise cause serious illness. But we find it shocking that there have been surprisingly few well-conducted research studies to test the safety of disinfection by-products (DBPs) such as trihalomethanes (THMs).

Such research takes a long time because exposure to DBPs can be for a lifetime. If someone develops bladder or colorectal cancer, most physicians would not know that tap water might have contributed to the malignancy.

We would like to know how you deal with the issues we have raised around water chlorination or fluoridation. Please share your thoughts in the comment section below. If you have friends or family that might find this article of interest, please send it along via email or social media.

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