Division of Environmental and Community Health

Maine Center for Disease Control & Prevention

A Division of the Maine Department of Health and Human Services

DHHSMeCDCEnvironmental and Community HealthDrinking WaterCompliance & EnforcementRegulated Contaminants → Disinfection Byproducts

Disinfection By-products

Page Index

Maximum Contaminant Level

Health Effects


Additional Resources

Disinfection by-products (DBPs), also called trihalomethanes, are formed when chlorine and bromine interact with natural organic materials in water, such as in chlorinated drinking water. To protect drinking water from disease-causing organisms, or pathogens, water suppliers often add a disinfectant, such as chlorine, to drinking water. However, disinfection practices can be complicated because certain microbial pathogens are highly resistant to traditional disinfection practices. Also, disinfectants themselves can react with bromide or naturally-occurring organic matter (i.e., decaying vegetation) in the water to form by-products, which may pose health risks.

Different disinfectants produce different types or amounts of disinfection by-products. Disinfection by-products identified in drinking water for which regulations have been established include trihalomethanes, haloacetic acids, bromate, and chlorite.

A major challenge for water suppliers is how to control and limit the risks from pathogens and disinfection by-products. It is important to provide protection from pathogens while simultaneously minimizing health risks to the population from disinfection by-products.

Community water systems and Non-Community, Non-Transient (NTNC) systems that treat water with a chemical disinfectant for primary or residual treatment must monitor for – and are subject to – regulations for disinfection by-products.


Maximum Contaminant Level (MCL)

The Environmental Protection Agency (EPA) has established the following standards for disinfection by-products in drinking water:

  • Bromate:  10 ppb (micrograms per liter, µg/L)
  • Chlorite:  1 ppm (milligrams per liter, mg/L)
  • Haloacetic acids (HAA5):  60 ppb (micrograms per liter, µg/L)
  • Total Trihalomethanes (TTHMs):  80 ppb (micrograms per liter, µg/L)

Health Effects

  • Bromate:  Increased risk of cancer
  • Chlorite:  Some infants and young children who drink water containing chlorite in excess of the MCL could experience nervous system effects. Similar effects may occur in fetuses of pregnant women who drink water containing chlorite in excess of the MCL. Some people may experience anemia.
  • Haloacetic acids (HAA5):  Increased risk of cancer
  • Total Trihalomethanes (TTHMs):  Liver, kidney, or central nervous system problems and increased risk of cancer


Water systems that use surface water or ground water under the direct influence of surface water and use conventional filtration treatment are required to remove specified percentages of organic materials that may react with disinfectants to form disinfection by-products, prior to disinfection. Other control strategies include modification of disinfection practices in a manner that still provides adequate protection against pathogens.


Additional Resources

Updated 2/10/2023