Haloacetic Acids (HAA5) in Drinking Water

Haloacetic acids (HAA5) are the second major group of disinfection byproducts, alongside trihalomethanes. The "5" refers to the five regulated species: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. Like TTHMs, they form when chlorine or other disinfectants react with natural organic matter in source water. HAA5 violations are among the most frequently reported EPA violations across U.S. water systems, often occurring alongside TTHM violations.

The primary health concern with haloacetic acids is cancer. Animal studies have shown that several individual haloacetic acids cause cancer, and human epidemiological studies suggest increased risks of bladder cancer and possibly other cancers with long-term exposure to disinfection byproducts. Some research also points to reproductive and developmental effects. The EPA set the MCLG at zero because there is no known safe level for carcinogenic compounds, but the enforceable MCL of 60 parts per billion reflects the practical tradeoff between managing cancer risk and maintaining effective disinfection.

Water systems serving smaller communities often struggle the most with HAA5 compliance because they may lack the resources for advanced treatment technologies. Systems using surface water and free chlorine disinfection tend to have the highest levels. Unlike TTHMs, which are volatile and can be partially removed by letting water sit, haloacetic acids are non-volatile and remain in the water. This means effective filtration is the primary home treatment option.