What are Trihalomethanes (THMs)?

Disinfectants, like chlorine, are used to kill harmful bacteria, viruses, and microorganisms in your water. When chemical disinfectants react with organic and inorganic material present in source water new compounds known as disinfection byproducts (DBPs) are formed. Trihalomethanes are one such DBP.

Action Level for Treatment Technique = 0.08 milligrams per Liter (mg/L) or 0.08 parts per million (ppm) or 80 parts per billion (ppb)

Sources of Contamination

Byproduct of drinking water disinfection

Health Effects

If consumed in excess of EPA’s standard over many years, some people who drink water containing trihalomethanes in excess of the MCL could experience liver, kidney, or central nervous system problems and increased risk of cancer. The US EPA has developed the Stage 2 DBP rule to protect public health by limiting exposure to these disinfectant byproducts.

What Does the Stage 2 Disinfectants and Disinfectant Byproducts (DBP) Rule Apply To?

The Stage 2 DBP rule applies to community and nontransient noncommunity water systems that add and/or deliver water that is treated with a primary or residual disinfectant other than ultraviolet light.

What Are the Requirements for the Final Rule?

Under the Stage 2 DBP rule, systems conduct an evaluation of their distribution systems, known as an Initial Distribution System Evaluation (IDSE), to identify the locations for potentially high disinfection byproduct concentrations. These locations will then be used by the systems as the sampling sites for Stage 2 DBP rule compliance monitoring.

Compliance with the maximum contaminant levels for two groups of disinfection byproducts (TTHM and HAA5) will be calculated for each monitoring location in the distribution system. This approach, referred to as the locational running annual average (LRAA), differs from current requirements, which determine compliance by calculating the running annual average of samples from all monitoring locations across the system.


Table Showing Locational Running Annual Average of Contaminant Levels for TTHM & HAA5 Disinfection Byproducts

Location Q3 Q4 Q1 Q2 LRAA (ppb)
101 Water Street 79.1 58.6 30.2 48.3 54.05

The Stage 2 DBP rule also requires each system to determine if they have exceeded an operational evaluation level, which is identified using their compliance monitoring results. The operational evaluation level provides an early warning of possible future MCL violations, which allows the system to take proactive steps to remain in compliance. A system that exceeds an operational evaluation level is required to review their operational practices and submit a report to their state that identifies actions that may be taken to mitigate future high DBP levels, particularly those that may jeopardize their compliance with the DBP MCLs.

More information about drinking water standards can be found online through the United States Environmental Protection Agency (USEPA) or Ohio Environmental Protection Agency (EPA).