Disinfection Byproducts (THMs and HAAs): What Are They?

The Tradeoff of Water Disinfection

Chlorine disinfection is one of the greatest public health achievements of the 20th century. It eliminated waterborne diseases like cholera, typhoid, and dysentery from US drinking water. But chlorine has a side effect: when it reacts with natural organic matter in water (decomposed leaves, soil particles, algae), it creates disinfection byproducts (DBPs).

The two regulated categories are trihalomethanes (TTHMs) and haloacetic acids (HAA5). Both are associated with increased cancer risk at high, long-term exposure levels. The EPA regulates these because eliminating them entirely would mean eliminating disinfection, which would cause far more harm.

EPA Limits and What They Mean

Contaminant	EPA MCL	What It Measures
Total Trihalomethanes (TTHMs)	80 ppb	Annual average of four THM compounds: chloroform, bromodichloromethane, dibromochloromethane, bromoform
Haloacetic Acids (HAA5)	60 ppb	Annual average of five HAA compounds: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, dibromoacetic acid

These limits are based on annual running averages, not single measurements. A city might have seasonal spikes above these levels (especially in summer) while still being in compliance on an annual basis. Check your city\'s water to see reported DBP levels.

How DBPs Form

The formation of disinfection byproducts depends on three factors:

• Amount of natural organic matter (NOM): More organic material in the source water means more raw material for DBP formation. Surface water (rivers, lakes, reservoirs) typically has more NOM than groundwater
• Chlorine dose: Higher chlorine levels create more byproducts. Utilities must balance enough chlorine to kill pathogens with limiting DBP formation
• Temperature and contact time: Warm water accelerates the chemical reactions that form DBPs. DBP levels are typically highest in summer and at the far ends of distribution systems where water has traveled the longest

Chloramine vs. chlorine

Some utilities have switched from free chlorine to chloramines (chlorine combined with ammonia) as a secondary disinfectant. Chloramines produce fewer TTHMs and HAAs, which is why they are used. However, chloramines create their own category of byproducts, including N-nitrosodimethylamine (NDMA), which is also a suspected carcinogen. The switch is not a complete solution; it shifts the problem.

Health Risks of DBP Exposure

The health risks from disinfection byproducts are based on long-term chronic exposure, not occasional consumption. The EPA\'s MCLs are set to limit lifetime cancer risk to approximately 1 in 10,000.

Cancer risk

Epidemiological studies have found associations between long-term DBP exposure and bladder cancer. The evidence is strongest for bladder cancer, with some studies estimating a 15-30% increased risk for people exposed to high DBP levels over decades. Associations with colorectal cancer and kidney cancer have also been reported but are less consistent.

Reproductive and developmental effects

Some studies have linked high DBP exposure to adverse pregnancy outcomes including low birth weight, preterm delivery, and certain birth defects. The evidence is not conclusive, but the EPA notes these as areas of concern warranting further research.

Exposure routes

DBP exposure is not limited to drinking water. Inhalation and skin absorption during hot showers and baths are significant exposure pathways. THMs are volatile, meaning they evaporate from hot water and are inhaled. Some researchers estimate that showering accounts for as much or more DBP exposure as drinking water.

Which Cities Have the Highest DBP Levels

Several factors make certain cities more prone to elevated DBPs:

• Surface water sources with high organic content: Cities drawing from rivers or lakes surrounded by vegetation have more raw material for DBP formation
• Warm climates: Southern and southeastern cities with warm source water see higher DBP levels, particularly in summer
• Long distribution systems: Sprawling systems where water travels far from the treatment plant accumulate more DBPs as chlorine continues reacting with organic matter
• Older treatment plants: Plants without advanced organic removal (like ozonation or GAC treatment) before chlorination produce higher DBP levels

How to Reduce Your Exposure

Activated carbon filtration

Carbon filtration is the most effective and practical method for removing DBPs at home. Carbon adsorbs THMs and HAAs as water passes through. Options include:

System Type	DBP Removal	Cost	Coverage
Whole-house carbon filter	90-99%	$500-1,500	All taps, showers, and baths
Under-sink carbon block	95-99%	$100-250	Kitchen drinking/cooking water only
Countertop carbon filter	90-95%	$50-150	Drinking water only
Carbon pitcher filter	70-90%	$20-40	Drinking water only
Shower carbon filter	50-80%	$25-50	Shower water only

Because showering is a significant exposure route, a whole-house carbon filter provides the most complete protection. It removes chlorine and DBPs from all water entering your home. If a whole-house system is not feasible, combining an under-sink filter for drinking water with a shower filter covers the two main exposure routes.

Ventilation during showers

Running the bathroom exhaust fan during and after hot showers reduces airborne THM concentration. Opening a window also helps. This does not eliminate exposure but reduces the inhalation dose.

Shorter, cooler showers

Hot water releases more volatile THMs into the air. Reducing shower temperature and duration lowers both dermal absorption and inhalation exposure. This is a free, immediate step.

Letting water sit before drinking

THMs are volatile and will partially evaporate from an open container. Filling a pitcher and letting it sit uncovered for a few hours reduces THM levels. However, this does not reduce HAAs, which are not volatile.

What About Boiling

Boiling has a complicated relationship with DBPs. Boiling for 1-3 minutes can volatilize and remove some THMs. However, it concentrates non-volatile contaminants like HAAs. It also does not address the fundamental issue if you are using the water for showering, washing, and other purposes. Filtration is a far more practical approach.

DBPs vs. Chlorine: Different Problems

Chlorine and disinfection byproducts are related but distinct issues. Chlorine causes taste and odor problems; DBPs are the health concern. A carbon filter addresses both, but they are worth understanding separately. Removing chlorine at the point of entry (whole-house) also prevents additional DBP formation in your hot water heater, where warm water and residual chlorine continue to react. For more on chlorine, see our chlorine guide.

The Bottom Line

Disinfection byproducts are an unavoidable consequence of making water safe to drink. The risk from DBPs is real but significantly lower than the risk from untreated water. The practical response is not to avoid chlorinated water but to filter it at home. A whole-house carbon filter or an under-sink carbon block removes the vast majority of DBPs while keeping the benefits of disinfected water. Check your city\'s water to see the DBP levels in your supply and determine which level of filtration makes sense.