Disinfection Byproducts and Health Risks

How Disinfection Byproducts Form

When chlorine is added to water, it reacts with naturally occurring organic compounds — decomposing plant matter, algae metabolites, humic acids — to form chlorinated compounds. The main groups are:

• Trihalomethanes (THMs): Chloroform, bromodichloromethane, dibromochloromethane, and bromoform. These are volatile — they evaporate readily and can be inhaled during showering or absorbed through the skin in addition to being consumed in drinking water.
• Haloacetic acids (HAAs): Less volatile but also regulated under the U.S. Safe Drinking Water Act. More water-soluble and primarily ingested.
• Chlorate and chlorite: Formed when chlorine dioxide is used as a disinfectant (a practice in some European systems).

THM concentrations in tap water depend on several factors: the amount of organic matter in the source water, the season (warmer temperatures accelerate byproduct formation), water pH, and disinfectant dose. Levels vary widely — some systems routinely stay well below regulatory limits while others approach or exceed them.

The Bladder Cancer Connection

Of all the health effects studied in relation to THMs, bladder cancer has the most robust epidemiological support. Bladder cancer is the sixth most common cancer in the United States, and the bladder's role in concentrating urine means prolonged contact time with any water-derived contaminant.

A 2003 meta-analysis synthesizing the best available case-control studies found that long-term consumption of chlorinated drinking water was associated with bladder cancer in men, with consistent findings across multiple study populations [1]. A landmark 2020 study estimated that THMs in drinking water are attributable to approximately 4.9% of bladder cancer cases across the European Union — roughly 6,500 additional cases per year [2]. In countries with the highest THM levels, the attributable fraction exceeded 17%.

The relationship appears stronger for men than women, possibly due to differences in fluid intake, bladder anatomy, or metabolic activation of THM compounds. Risk is primarily associated with long-term, high-level exposure — occasional consumption of tap water is very different from drinking it exclusively over decades.

Reproductive and Developmental Concerns

A second area of active research involves pregnancy outcomes. Several studies have observed associations between elevated THM exposure during pregnancy and increased rates of miscarriage, low birth weight, and certain congenital anomalies. A population-based Italian case-control study found that chlorite and chlorate exposure via drinking water was associated with neural tube defects, cardiac anomalies, and urinary tract malformations, though THM levels in that study were low [4]. Systematic reviews of the broader literature have found consistent, if modest, associations between THM exposure and small-for-gestational-age births and preterm delivery, particularly at THM concentrations exceeding 50 µg/L.

For pregnant women, the precautionary principle argues for using filtered water during the first trimester, when organogenesis is most sensitive to chemical exposures.

Showering and Skin Exposure

Drinking is not the only route of exposure. THMs are volatile — they evaporate rapidly from warm water, making a hot shower an inhalation event as well as a dermal exposure event. Some researchers have argued that showering in chlorinated water may contribute more to blood THM levels than drinking it, because the lungs absorb volatile compounds efficiently and gut metabolism partially degrades THMs before they reach systemic circulation.

Chlorine also directly affects skin health. Research has shown that even low concentrations of residual chlorine in bathing water reduce the water-holding capacity of the stratum corneum — the skin's outer barrier layer — particularly in people with atopic dermatitis [5]. Chlorine strips natural oils and disrupts the skin's acid mantle, which can exacerbate eczema, increase transepidermal water loss, and alter the skin microbiome by reducing beneficial bacterial populations.

What You Can Do

Reducing exposure to chlorine and its byproducts is straightforward with the right filtration. Key approaches:

• Activated carbon filters (pitcher, under-sink, or whole-house) effectively remove THMs from drinking water. Look for NSF/ANSI Standard 53 certification.
• Shower filters with activated carbon or KDF (kinetic degradation fluxion) media reduce chlorine at the point of contact. Less effective than whole-house carbon filters but meaningful for skin and inhalation exposure.
• Whole-house carbon filtration addresses both drinking and bathing water in one system — the most comprehensive approach.
• Letting water sit uncovered or running cold water briefly before using it allows some volatile THMs to off-gas, though this is an imperfect solution.

The goal is not to avoid all chlorinated water — the microbiological safety it provides is genuinely important. The goal is to reduce long-term exposure to byproducts, particularly for high-risk groups: pregnant women, people who drink large volumes of tap water, and those with skin conditions.

See our Water Filtration page for detailed filter comparisons, and our Lead in Water page for guidance on broader tap water safety.