In 1908, Jersey City, New Jersey became the first city in the United States to continuously chlorinate its municipal water supply. Within a decade, typhoid fever rates in American cities had dropped by more than 50%. Chlorination is, without exaggeration, one of the most consequential public health interventions in human history.
It is also producing a set of chemical byproducts that have been associated, in epidemiological studies, with bladder cancer and adverse pregnancy outcomes.
This is not a reason to stop chlorinating water. The alternative — waterborne typhoid, cholera, and dysentery — is far worse. But it is a reason to understand what's in your municipal water and what you can do about it.
How Disinfection Byproducts Form
When chlorine is added to water, it doesn't just kill bacteria. It also reacts with naturally occurring organic matter in the water — decomposed plant material, algae, sediment — to form a family of compounds called disinfection byproducts (DBPs).
The two main categories are trihalomethanes (THMs) — chloroform is the most common — and haloacetic acids (HAAs). Both are regulated by the EPA under the National Primary Drinking Water Regulations. The maximum contaminant level for total THMs is 80 parts per billion; for HAAs, it's 60 ppb.
The formation of DBPs depends on two factors: the amount of chlorine added, and the amount of organic matter in the source water. Surface water — rivers and reservoirs — contains more organic matter than groundwater. Connecticut's major municipal water systems draw primarily from surface water sources: the MDC system draws from the Barkhamsted and Nepaug reservoirs; the South Central Connecticut Regional Water Authority draws from Lake Whitney and Lake Gaillard; Aquarion draws from a mix of surface and groundwater sources.
How Dangerous Are Disinfection Byproducts?
The epidemiological evidence on DBPs and health effects has been accumulating for decades. A 2010 meta-analysis in Epidemiology found that long-term consumption of chlorinated drinking water was associated with a 35% increased risk of bladder cancer. A 2004 study in the American Journal of Epidemiology found associations between THM exposure and adverse pregnancy outcomes, including low birth weight and preterm birth.
These are associations, not proven causation. But the consistency of the findings across multiple studies and populations has led the EPA to set and enforce DBP limits, and has led the International Agency for Research on Cancer to classify chloroform (a THM) as a possible human carcinogen. The pregnancy data is particularly concerning — if you or someone in your household is pregnant, our guide on water quality risks during pregnancy in Connecticut covers what to watch for.
The EPA's limits — 80 ppb for THMs, 60 ppb for HAAs — are set to reduce risk to an acceptable level while maintaining effective disinfection. They are not zero-risk thresholds. The EWG's Tap Water Database sets much stricter health guidelines for THMs and HAAs, based on more recent cancer research.
What Can Connecticut Residents Do to Reduce DBP Exposure?
The most effective way to reduce DBP exposure from municipal water is activated carbon filtration. Granular activated carbon (GAC) and solid block carbon filters both effectively reduce THMs and HAAs. A standard under-sink carbon filter or a pitcher filter with a carbon block will significantly reduce DBP levels in your drinking water.
For DBPs, look for filters certified to NSF/ANSI Standard 53 for VOC reduction — most quality carbon filters meet this standard.
The irony is that the same filter that reduces DBPs from municipal water also reduces chlorine taste and odor — the most common complaint about municipal water. The health benefit and the quality-of-life benefit are the same filter. If you're weighing your options, our guide to what reverse osmosis removes explains how RO compares to carbon filtration for different contaminants.
What's in the water you're drinking every day? Enter your ZIP code at CheckYourTap.com to see what's in your tap water — free, in 30 seconds.
Frequently Asked Questions
Are chlorine byproducts in Connecticut tap water dangerous?
A 2010 meta-analysis in Epidemiology found long-term consumption of chlorinated water was associated with a 35% increased risk of bladder cancer. The EPA regulates THMs at 80 ppb and HAAs at 60 ppb, but EWG's health guidelines are significantly stricter. Surface-water systems like those serving most Connecticut cities tend to produce higher DBP levels.
Does boiling water remove chlorine byproducts?
THMs (like chloroform) are volatile and partially evaporate during boiling, but HAAs do not evaporate and may actually concentrate as water boils away. The most effective removal method is an activated carbon filter — either a faucet-mount, under-sink, or pitcher filter certified to NSF/ANSI Standard 53 for VOC reduction.
Which Connecticut water systems have the highest disinfection byproducts?
Systems drawing from surface water — rivers and reservoirs — produce more DBPs because surface water contains more organic matter that reacts with chlorine. The MDC system (Barkhamsted/Nepaug reservoirs), South Central CT Regional Water Authority (Lake Whitney/Gaillard), and Aquarion's surface water sources tend to have higher DBP levels than groundwater-fed systems.
Keep Reading
- You're Pregnant in Connecticut. Here's What to Know About Your Water Before Your Next Glass.
- Reverse Osmosis Removes Almost Everything. Here's What Connecticut Homeowners Need to Know Before Buying One.
- The Connection Between Your Water, Your Thyroid, and Your Hair That Nobody Is Making
Sources: Epidemiology, DBP and bladder cancer meta-analysis, 2010; American Journal of Epidemiology, THM and pregnancy outcomes, 2004; EPA Disinfection Byproducts Rule; IARC Monographs on Chloroform; CT DPH Water Quality Reports.