Chlorination Byproducts in Tap Water and Fertility

Chlorination makes tap water safe to drink. Without it, waterborne diseases would remain a serious public health threat. The tradeoff is that when chlorine reacts with naturally occurring organic matter in water, it forms a class of chemicals called disinfection byproducts. The two main groups are trihalomethanes (THMs) and haloacetic acids (HAAs), and both have been studied for potential reproductive effects.

This is not a case for avoiding tap water or fearing your shower. It is a case for understanding the exposure and knowing which straightforward steps reduce it, especially during the preconception window when the reproductive system may be more sensitive to low-level endocrine-active compounds.

How Chlorination Byproducts Form

When chlorine or chloramine (a chlorine-ammonia compound used in some systems) is added to water at the treatment plant, it reacts with organic matter like decomposing plant material, algae, and humic acids. The reaction produces hundreds of chemical compounds, but THMs and HAAs make up the largest fraction and are the best studied.

Chloroform is the most common THM. Dichloroacetic acid and trichloroacetic acid are the dominant HAAs. Total THM levels in US tap water typically range from 20 to 80 parts per billion, with the EPA maximum contaminant level set at 80 ppb for total THMs and 60 ppb for total HAAs.

Byproduct levels are not fixed. They vary by season (higher in warm months when source water contains more organic matter), by how far you are from the treatment plant (byproducts continue forming as water travels through the distribution system), and by the disinfection method (chloramine systems produce fewer THMs but more of other byproduct classes).

What the Reproductive Research Shows

The evidence connecting disinfection byproducts to reproductive outcomes has been accumulating since the 1990s. The findings are consistent in direction but modest in magnitude, which means the risk for any individual is small but the exposure is near-universal.

Pregnancy outcomes

Multiple epidemiological studies have reported associations between higher THM exposure and increased risk of miscarriage, small-for-gestational-age birth, and some congenital anomalies. A 2019 meta-analysis of 16 studies found a statistically significant association between high total THM exposure and stillbirth risk. The effect sizes are small, and confounders are difficult to fully control in water-exposure studies, but the pattern is consistent across different populations and study designs.

Male fertility

A smaller number of studies have examined sperm parameters and found associations between higher THM exposure and reduced sperm motility and concentration, particularly in men with occupational or residential exposure at the upper end of the population distribution. The evidence here is less robust than for pregnancy outcomes but points in the same direction.

Female fertility

Time-to-pregnancy studies have shown mixed results, with some finding a modest association between higher THM exposure and longer time to pregnancy and others finding no significant effect. The inconsistency may reflect differences in how exposure was measured and the difficulty of capturing cumulative intake from drinking, bathing, and swimming.

The Three Exposure Routes

Most people think of drinking water as the only route, but THMs are volatile and also enter the body through inhalation and skin absorption. Understanding the three routes changes how you think about mitigation.

The shower contribution is consistently underestimated. Hot water releases dissolved THMs as gas, and the enclosed bathroom concentrates the vapor. A published study found that a 10-minute shower in water with typical US THM levels produced blood chloroform levels comparable to drinking two liters of the same water. The combination of warm skin (which absorbs more) and inhaled vapor makes the shower the single largest THM exposure event for most people.

Checking Your Own Water

Your water utility is required to publish an annual Consumer Confidence Report (CCR) that includes total THM and HAA levels. You can find your report by searching your utility's name and "water quality report" or through the EPA's CCR lookup tool. The numbers reported are running annual averages measured at distribution system monitoring points.

If your levels are consistently below 40 ppb total THMs (half the regulatory limit), your baseline exposure is relatively low. If levels approach or exceed 60 ppb, filtration becomes a higher-priority step. Levels above 80 ppb indicate a compliance issue with EPA standards and should be reported.

Private wells are not chlorinated and do not produce chlorination byproducts, but they are also not monitored or treated. Well users have different exposure concerns (nitrates, bacteria, naturally occurring metals) and should test independently.

Filtration: What Works and What Does Not

Carbon block and granular activated carbon

Activated carbon is highly effective at removing THMs. Carbon block filters (compressed carbon, under-sink installation, certified to NSF Standard 53) are the most reliable option for drinking water. Granular activated carbon pitchers (like Brita) offer some THM reduction but with shorter filter life and declining effectiveness as the carbon becomes saturated. Always replace pitcher filters on schedule.

Showerhead filters

Showerhead filters using KDF (kinetic degradation fluxion) media or granular activated carbon reduce THMs and free chlorine in shower water. Their effectiveness declines over the filter's lifespan, and they do not remove HAAs as effectively as THMs. For people in areas with higher THM levels, a showerhead filter combined with bathroom ventilation (running the exhaust fan during and after showering) is a reasonable and low-cost step.

Whole-house systems

A whole-house carbon filtration system treats all water entering the home, covering both drinking and bathing. This is the most comprehensive solution but also the most expensive, typically running several hundred to a few thousand dollars installed. For households on municipal water with THM levels consistently above 60 ppb, the investment may be warranted.

Reverse osmosis

RO removes THMs, HAAs, and nearly all other dissolved contaminants, but it is typically installed as a point-of-use system for drinking water only. RO is effective but may be overkill for byproduct reduction alone, since carbon block handles THMs efficiently at lower cost. RO also removes beneficial minerals and produces wastewater.

Practical Steps for the Preconception Period

• Read your annual water quality report. Know your baseline THM and HAA levels.
• Install an under-sink carbon block filter for drinking and cooking water. Look for NSF 53 certification for THM reduction specifically.
• Consider a showerhead filter, especially if your THM levels are above 40 ppb or you take long, hot showers.
• Run the bathroom exhaust fan during and after showering to ventilate volatile compounds.
• If you swim regularly in indoor chlorinated pools, choosing outdoor pools or well-ventilated facilities reduces inhaled byproduct exposure. The exercise benefits of swimming support fertility and should not be abandoned over this concern.
• Running cold tap water for 15 to 30 seconds before filling a glass allows some volatile THMs to dissipate and draws fresher water from the main, though this is a minor reduction compared to filtration.