Disinfection Byproducts (DBPs)

How It Works

Disinfection byproducts are an unavoidable consequence of treating water to kill harmful bacteria and viruses. When chlorine reacts with naturally occurring organic matter (leaves, soil, algae), it creates compounds including trihalomethanes (THMs) and haloacetic acids (HAAs). Long-term exposure to elevated DBP levels has been linked to bladder cancer, colon cancer, and adverse reproductive outcomes. The EPA sets MCLs for total trihalomethanes (80 ppb) and five haloacetic acids (60 ppb). Water systems with high organic matter (surface water sources) are most likely to have DBP challenges. Many systems have switched from chlorine to chloramine (which produces fewer THMs) or added advanced treatment to reduce DBP formation.

Related Terms

• Maximum Contaminant Level (MCL) — The highest level of a contaminant allowed in drinking water — set by the EPA and enforceable by law. Exceeding the MCL triggers a health-based violation.
• Chlorination — The most common method of disinfecting drinking water — adding chlorine or chloramine to kill bacteria, viruses, and parasites before water reaches your tap.
• Surface Water — Water from rivers, lakes, and reservoirs — the drinking water source for the majority of large cities. Surface water requires more intensive treatment than groundwater because it is more exposed to contamination.