Pediatric Environmental Health Center | Total Trihalomethanes (TTHMs)

Total trihalomethanes (TTHM) are a group of disinfection byproducts. They form when chemicals and organic material in the water react with disinfectants. Common disinfectants include chlorine and ozone. When TTHMs form, they can evaporate and enter the air.

Seasonal changes can impact the amount of TTHM in water. TTHM levels may increase when water systems are dealing with repairs or contamination.

You can be exposed to TTHM through:

• Drinking water that contains high levels of TTHM
• Eating food prepared with TTHM-contaminated water
• Breathing in air that TTHM has evaporated into
• Bathing or showering in water than contains TTHM

The length of time and amount of TTHM exposure influences the severity of health effects.

Scientists are still studying the health effects of TTHM. Some animal studies have shown links between exposure to very high levels of TTHM and health impacts including:

• Kidney and liver damage
• Neurological and reproductive issues
• Some evidence of cancer

Children are likely at a greater risk for health impacts from TTHM. They consume more water per bodyweight than adults. Infants can also be exposed to TTHM through breastmilk.

It is necessary to disinfect drinking water. Exposure to contaminated drinking water is more dangerous than exposure to disinfection byproducts.

There are many ways to reduce exposure if you are told there is TTHM in your water:

• Using carbon filters that are certified to remove TTHMs
• Leaving water out overnight to decrease TTHM levels
• Taking shorter baths or showers and ventilating the bathroom
• Opening windows or using fans when boiling water or washing dishes
• Using bottled water for infant formula
• Spending less time in chlorinated pools

The MCL is an enforceable standard set by the EPA that limits the amount of a contaminant that can be present in a public water system. These guidelines are used to keep consumers safe. The EPA has set an MCL for TTHM.

1. Bureau of Water Resources, Drinking Water Program. Total Trihalomethane In Drinking Water Information for Consumers. Massachusetts Department of Environmental Protection. 2022. https://www.mass.gov/doc/factsheet-total-trihalomethane-in-drinking-water-information-for-consumers/download
2. Dedham-Westwood Water District. Total Trihalomethanes (TTHM). 2022. https://www.dwwd.org/tthm/
3. Westminster MA Water Quality. Frequently Asked Questions About Total Trihalomethanes (TTHM). 2022. https://www.westminster-ma.gov/sites/g/files/vyhlif1431/f/news/westminster_faq_-_tthm.pdf
4. Environmental Working Group. Total trihalomethanes (TTHMs). EWG’s Tap Water Database. 2023. https://www.ewg.org/tapwater/contaminant.php?contamcode=2950
5. US EPA. How EPA Regulates Drinking Water Contaminants. 2015. https://www.epa.gov/sdwa/how-epa-regulates-drinking-water-contaminants