Chlorine Byproducts in Your Tap Water: What Rochester Residents Need to Know About THMs and HAAs

What Are Disinfection Byproducts?

Disinfection byproducts form when chlorine (or other chemical disinfectants) reacts with naturally occurring organic matter in the source water. This organic matter -- dissolved plant material, algae, humic and fulvic acids -- is present in virtually all surface water, including Lake Ontario and the Hemlock-Canadice reservoir system that supplies Rochester.

When chlorine encounters these organic compounds, it doesn't just neutralize them. It transforms them into hundreds of new chemical compounds. The two most studied and regulated groups are:

Trihalomethanes (THMs): A group of four compounds -- chloroform, bromodichloromethane, dibromochloromethane, and bromoform. Chloroform is typically the most prevalent in Rochester's water. THMs are volatile, meaning they evaporate readily. You inhale them in the shower, in steam from cooking, and in the warm air above a filled bathtub.

Haloacetic acids (HAAs): A group of five regulated compounds (HAA5) including monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. HAAs are less volatile than THMs but are ingested directly through drinking water.

Beyond THMs and HAAs, chlorination produces dozens of other byproducts that are less studied and currently unregulated, including haloacetonitriles, haloketones, and chloral hydrate. Research suggests some of these unregulated DBPs may be more toxic than the ones currently monitored.

Rochester's DBP Levels: The Numbers

Rochester Water Authority and the Monroe County Water Authority regularly test for THMs and HAAs as required by the EPA. Here's what the data shows:

Trihalomethanes (THMs):

• Rochester's reported range: 12-81 ppb (parts per billion)
• EPA Maximum Contaminant Level (legal limit): 80 ppb
• EWG health guideline: 0.7 ppb
• Rochester's highest readings approach the EPA legal limit and exceed the EWG guideline by more than 115 times

Haloacetic acids (HAAs):

• Rochester's reported range: 20-60 ppb
• EPA Maximum Contaminant Level: 60 ppb
• EWG health guideline: 0.3 ppb
• Rochester's upper readings approach the EPA limit and exceed the EWG guideline by up to 200 times

These numbers require context. The EPA's legal limits were set in the early 2000s as a balance between maintaining effective disinfection and limiting DBP exposure. They represent what the EPA considers feasible for water treatment plants to achieve, not necessarily the level at which no health effects occur. The EWG guidelines are based on more recent cancer risk assessments and represent the level at which one additional case of cancer per one million people would be expected over a lifetime of exposure.

The Seasonal Factor: Why Summer Water Is Different

If you've noticed that your Rochester tap water tastes and smells more strongly of chlorine in summer, you're not imagining it. DBP levels in Rochester follow a predictable seasonal pattern:

Summer peaks (June-September): Lake Ontario and the reservoir system warm up, increasing biological activity and organic matter in the source water. More organic matter means more raw material for DBP formation. Simultaneously, the water authority increases chlorine dosing to maintain disinfection in warmer water (where bacteria multiply faster). The combination of more organic matter and more chlorine produces significantly higher DBP levels. Rochester's highest THM readings (60-81 ppb) typically occur in late July through September.

Winter lows (December-March): Cold water suppresses biological activity, reducing organic matter. Lower water temperatures also slow the chemical reactions that form DBPs. Winter THM levels in Rochester often drop to 12-25 ppb -- still above health guidelines but substantially lower than summer peaks.

Spring and fall transitions: Shoulder seasons show moderate DBP levels, typically 30-50 ppb for THMs. Spring runoff can temporarily increase organic matter, causing brief spikes.

This seasonal variation means your actual DBP exposure depends partly on when you consume the most unfiltered tap water. Ironically, summer -- when people drink the most water -- is when DBP levels are highest.

Distribution System Effects

DBPs don't stop forming at the treatment plant. Chlorine continues reacting with organic matter as water travels through Rochester's distribution network. Homes at the end of long distribution lines -- particularly in outer suburban areas served by the MCWA -- often have higher DBP levels than homes near the treatment facility. This is because the water has more time in the pipe system for reactions to continue.

If your home is in a more distant suburb like Hilton, Brockport, or the eastern edge of Wayne County, your DBP levels may be higher than what's reported at the treatment plant. A home water test is the most accurate way to know your actual exposure.

Health Research: What the Science Shows

The health effects of long-term DBP exposure have been studied extensively over the past 30 years. Here's a summary of the major findings:

Cancer risk. The strongest evidence links long-term DBP exposure to bladder cancer. A 2004 meta-analysis in the American Journal of Epidemiology found that people with the highest THM exposure had a 21% increased risk of bladder cancer. Subsequent studies have strengthened this association. The International Agency for Research on Cancer (IARC) classifies chloroform (the primary THM) as "possibly carcinogenic to humans." More recent research has also identified associations with colorectal cancer and kidney cancer, though the evidence is less definitive.

Pregnancy and reproductive outcomes. Multiple studies have found associations between elevated DBP exposure and adverse pregnancy outcomes. Research published in Environmental Health Perspectives and Epidemiology has linked higher THM exposure to small-for-gestational-age babies, low birth weight, and increased risk of miscarriage. A 2022 meta-analysis found consistent associations between DBP exposure and neural tube defects. These findings are particularly relevant for pregnant women in Rochester, especially during summer months when DBP levels peak.

Respiratory effects. Because THMs are volatile and are released into air during showering and bathing, inhalation exposure is significant. Studies of competitive swimmers show elevated risks of asthma and respiratory irritation. While residential exposure levels are lower, the cumulative effect of daily showers, baths, and indoor water use contributes to total DBP intake.

Routes of Exposure: It's Not Just Drinking Water

An important and often overlooked fact: drinking water accounts for only about one-third of total DBP exposure. The other two-thirds come from inhalation and skin absorption:

Showering and bathing. A 10-minute hot shower exposes you to significant THM levels through both inhalation (THMs evaporate into the shower steam) and skin absorption. Studies have shown that blood THM levels increase measurably after showering, sometimes more than after drinking several glasses of tap water. Enclosed shower stalls concentrate airborne THMs.

Cooking. Boiling water for pasta, rice, or hot beverages releases THMs into kitchen air. Using tap water in cooking also means ingesting DBPs with food.

Dishwashing and cleaning. Hot water use throughout the home releases volatilized THMs into indoor air.

This multi-pathway exposure is why a whole-house filtration approach -- not just a drinking water filter -- provides the most comprehensive DBP reduction.

Filtration Solutions: What Actually Works

The good news is that DBPs are among the easiest contaminants to remove with appropriate filtration. Here's what works:

Activated Carbon Filtration: The Most Effective Option. Activated carbon (also called activated charcoal) is remarkably effective at adsorbing THMs, HAAs, and chlorine from water. This is the go-to technology for DBP removal:

• Whole-house carbon filters ($800-1,500 installed) treat all water entering the home, reducing DBPs in drinking water, shower water, and cooking water. This addresses all three exposure pathways -- ingestion, inhalation, and skin absorption. For comprehensive DBP protection, this is the best single investment.
• Under-sink carbon block filters ($100-400 installed) treat drinking and cooking water at the kitchen tap. Effective for ingestion but doesn't address shower and bathing exposure.
• Pitcher filters and faucet-mounted filters ($20-100) provide moderate DBP reduction for drinking water. Less consistent than installed systems but better than no filtration.

Quality carbon filters remove 95-99% of THMs and 90-95% of HAAs. The key is ensuring adequate carbon contact time -- water needs to spend enough time in contact with the carbon media for adsorption to occur.

Reverse Osmosis: Also Effective. RO systems remove THMs and HAAs along with dozens of other contaminants. If you're installing an RO system for lead or fluoride removal, it will handle DBPs as well. However, RO is more expensive and produces wastewater, so if DBPs are your primary concern, activated carbon alone is usually sufficient and more cost-effective.

What Doesn't Work for DBPs:

• Boiling -- actually increases THM concentration by evaporating water but not all dissolved THMs
• UV treatment -- designed for microorganisms, not chemical compounds
• Water softeners -- designed for mineral removal, not chemical filtration
• Standard sediment filters -- designed for particles, not dissolved chemicals

A Practical Approach for Rochester Homeowners

Given Rochester's DBP levels and the multi-pathway nature of exposure, here's what we recommend:

Best protection: A whole-house activated carbon filter treats all water in the home, addressing ingestion, inhalation, and skin absorption simultaneously. This is the most effective single solution for DBP reduction. Cost: $800-1,500 installed, $75-150 per year for cartridge replacement.

Good protection on a budget: An under-sink carbon block filter at the kitchen tap addresses the drinking and cooking water pathway. Add an inexpensive carbon shower filter ($20-40) to reduce THM inhalation during showers. Combined cost: $150-450 installed.

Comprehensive protection: Whole-house carbon filter plus an under-sink RO system. The carbon filter handles chlorine and DBPs throughout the home, while the RO provides the highest level of contaminant removal for drinking water (including lead, fluoride, and microplastics in addition to DBPs). This is the setup we recommend for families with infants, pregnant women, or anyone who wants maximum protection.

Schedule a free water test with our team. We'll measure THMs, chlorine residual, and other parameters at your tap, show you how your levels compare to health guidelines, and recommend a targeted solution that fits your budget. Reducing DBP exposure is straightforward with the right equipment -- and given the health research, it's an investment worth making.