A Swedish study from Karolinska Institutet with over 60,000 participants and 22 years of follow-up found a link between chlorination byproducts in drinking water and cancer risk.
The Karolinska Institutet study (2024) found that men with the highest exposure to THM (trihalomethanes) in drinking water had a 20% increased risk of colorectal cancer. THM form when chlorine reacts with organic matter in water. Showering creates a unique exposure route since THM are volatile and inhaled — levels in shower air can exceed those in the water itself.
What the Karolinska Institutet Study Found
A comprehensive study published by Karolinska Institutet (2024) followed over 60,000 Swedes for 22 years. Researchers measured trihalomethane (THM) levels in participants' drinking water and compared them with cancer incidence.
The result: men with the highest THM exposure had a 20% increased risk of developing colorectal cancer compared to men with the lowest exposure. A smaller but still present risk difference was observed for women.
The study controlled for other risk factors including smoking, alcohol, diet, and socioeconomic status — and the association remained significant.
Karolinska Institutet — Read the original study
What Are Trihalomethanes and How Are We Exposed?
Trihalomethanes (THM) are byproducts formed when chlorine reacts with organic matter (humus, algae residue) in water. The most common THM species are chloroform, bromodichloromethane, dibromochloromethane, and bromoform.
There are three exposure routes: drinking water consumption, skin absorption during showering, and inhalation of THM vapors. Multiple studies have shown that inhalation and skin absorption during showering can be a significant exposure pathway.
THM are volatile organic compounds that evaporate when water is heated. In a typical shower, airborne THM levels can be several times higher than in the water itself. This means you can inhale more THM during a 10-minute shower than you'd ingest from drinking water all day.
THE SCIENCETHM are volatile and evaporate when water is heated. In a typical shower, airborne levels can be several times higher than in the water. You inhale more THM during a 10-minute shower than you'd get from drinking water all day.
Swedish Regulations for THM
The Swedish Food Agency sets the guideline value for THM in drinking water at 0.03 mg/L (30 µg/L), which is lower than the WHO guideline of 0.1 mg/L.
Swedish water utilities generally maintain these levels, but THM concentrations can vary seasonally. Highest levels are typically measured during autumn when organic matter in the water is at its peak.
While Swedish drinking water maintains high quality standards, showering creates a different exposure pathway than drinking water consumption. THM levels in shower air can be significantly higher than in the water.
REGULATIONSWhat You Can Do
While the research may sound concerning, there are simple steps you can take to reduce your THM exposure during showering.
Nordisk Shower Filter — triple media with GAC for THM reduction
Install a shower filter with GAC
- Granular Activated Carbon (GAC) reduces THM effectively through adsorption
- Choose a filter combining GAC with calcium sulfite and KDF-55D for full protection
Shorter showers, lower temperature
- THM evaporation increases with temperature and time
- Shorter showers at lower temperature reduce exposure
Ventilate your bathroom
- Use an exhaust fan or open a window during and after showering
- Reduces the concentration of volatile compounds in the air
Being aware of THM is no reason for alarm — Swedish drinking water meets high quality standards. But for those who want to minimize exposure, a shower filter with GAC and good bathroom ventilation are simple, effective measures.
Vanliga Fragor
Is Swedish drinking water dangerous?
No — Swedish drinking water is among the world's most strictly regulated, with THM levels well below the Swedish Food Agency's guidelines. However, showering creates exposure via inhalation and skin absorption, which can result in higher total exposure than drinking.
What are THM in water?
Trihalomethanes (THM) are byproducts formed when chlorine reacts with organic matter in water. The most common types are chloroform, bromodichloromethane, and dibromochloromethane. THM are volatile and can be inhaled during showering.
Do shower filters remove THM?
Yes — Granular Activated Carbon (GAC) effectively reduces THM through adsorption. A triple-media filter with GAC alongside calcium sulfite and KDF-55D provides the broadest protection.
Should I be worried about the Karolinska study?
The study is important research that highlights the need for awareness, not alarm. Swedish water quality is excellent. Simple measures like a GAC shower filter and bathroom ventilation can significantly reduce your exposure for those concerned.
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