A major Swedish study from Karolinska Institutet shows a clear link between chlorination byproducts in drinking water and an increased risk of colorectal cancer — especially in men exposed to high levels over a long period.
A 22-year follow-up of 60,000 Swedish men shows that those exposed to high levels of trihalomethanes (THM) in drinking water had a 20 percent higher risk of developing colorectal cancer. The study was published by Karolinska Institutet in March 2026.
What does the study from Karolinska Institutet show?
In March 2026, Karolinska Institutet published a study that followed over 60,000 Swedish men for 22 years. Researchers measured the levels of trihalomethanes (THM) in the drinking water available to participants and then compared this data against cancer registries.
The result: Men who were exposed to the highest levels of THM in their drinking water had a 20 percent higher risk of developing colorectal cancer compared to men with the lowest levels. The risk was dose-dependent — the higher the exposure, the higher the risk.
The study is published in Karolinska Institutet's news portal and has received attention in medical press, including Läkartidningen, which has covered the link between chlorine in drinking water and cancer.
Read Karolinska Institutet's news about the study
What are trihalomethanes and how are we exposed?
Trihalomethanes (THM) are formed when chlorine reacts with organic matter in water. The most common THMs are chloroform, bromodichloromethane, and dibromochloromethane.
An important exposure that is often underestimated is showering. When hot water (35–45°C) passes through chlorinated water, a fine aerosol is formed, which you inhale. Research published in Environmental Science & Technology has shown that a 10-minute shower can result in greater chlorine exposure via inhalation than drinking eight glasses of chlorinated water.
THMs are fat-soluble and partially accumulate in the body. With regular exposure over years and decades, the accumulated effects can become significant.
What about Swedish drinking water?
The Swedish National Food Agency sets limit values for chlorine and THM in Swedish drinking water. The limit for free chlorine is 0.5 mg/L, and the guideline value for THM is 0.03 mg/L — among the strictest in Europe.
Several Swedish municipalities use chloramine to reduce the formation of THM. Stockholm, Gothenburg, and Malmö all use chloramine, but chloramine can also form THM at high temperatures.
Swedish Water and Waste Management (SVAVA) publishes regional water reports showing water quality per municipality.
Swedish National Food Agency's limit values for drinking water
What can you do?
1. Install a shower filter with calcium sulfite
A shower filter with calcium sulfite chemically neutralizes chlorine and chloramine on contact, before the water reaches your skin and before THM can form and aerosolize in the shower.
2. Shorten showering time
Shorter showers dramatically reduce overall THM exposure via inhalation.
3. Ventilate the bathroom
Having ventilation on during and after showering dilutes the THM concentration in the air.
4. Keep an eye on your municipality's water report
Your municipality is obligated to publish its water quality report annually. Check the levels of chlorine and THM measured in your area.
Frequently Asked Questions
What are trihalomethanes (THM)?
Trihalomethanes are a group of chemical compounds formed when chlorine reacts with organic matter in water. Common THMs are chloroform, bromodichloromethane, and dibromochloromethane. When showering in hot water, THMs are aerosolized, and you inhale them — providing additional exposure beyond what you drink.
Is Swedish drinking water dangerous to drink?
No. The Swedish National Food Agency sets limit values for chlorine and THM in drinking water (free chlorine ≤0.5 mg/L, THM ≤0.03 mg/L). The Swedish values are among the strictest in Europe. However, the study shows that long-term exposure can have health consequences. A shower filter that reduces THM can minimize exposure.
Does the study also apply to women?
The study focused on men and showed a statistically significant correlation in men. The authors note that this may be due to differences in exposure (men shower longer on average), metabolism, or hormonal factors. More research is needed.
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