In the United States, virtually all public tap water is disinfected before it reaches your faucet. Chlorination has been the primary method since 1908, when it was first used in a US public water supply — and the CDC considers it one of the greatest public health advances of the 20th century, alongside penicillin and the polio vaccine.
Today, water utilities add low levels of chlorine or chloramine to tap water to kill bacteria, viruses, and parasites — and to maintain a residual disinfectant that continues protecting the water as it travels through miles of pipes to your home
Two key regulatory bodies set the framework:
In practice, most US municipal systems maintain chlorine residuals between 0.2 and 2.0 mg/L — well within safe limits. The EPA's 4.0 mg/L ceiling ensures that even at the maximum allowable level, chlorine poses no significant health risk while effectively neutralizing harmful microorganisms.
Not all US tap water is treated the same way. Utilities use either chlorine or chloramine (a combination of chlorine and ammonia) as their residual disinfectant:
More than one in five Americans drinks water treated with chloramine. In fact, more than half of the largest US cities now use chloramine rather than chlorine as their residual disinfectant.
For example: New York, Atlanta, and Chicago use chlorine, while Los Angeles, Philadelphia, and Boston use chloramine. Some utilities switch between the two seasonally — temporarily reverting to chlorine to flush biofilm buildup in pipes
Chlorination is the final barrier — but safe tap water starts much earlier. Learn how every stage of treatment works together
In some applications, residual chlorine must be removed entirely — to protect aquatic life, meet discharge permits, or safeguard sensitive equipment.
Chlorine, chloramine, chlorine dioxide — each disinfectant plays a different role. Understand the full chemical toolkit.
Whether your source is a mountain reservoir or a deep aquifer, producing safe drinking water requires precision at every step.
Actual chlorine concentrations vary depending on source water quality, distribution network length, climate, and local regulations.
New York City's water — sourced from protected upstate reservoirs up to 125 miles away — is among the highest quality in the country. The city reports a chlorine residual of approximately 0.8 mg/L, benefiting from naturally soft, low-turbidity source water that requires minimal treatment.
Florida's warm climate accelerates microbial growth and chlorine decay, which means utilities often need to dose higher. Tampa Bay water, for example, typically contains 1 to 4 mg/L of chlorine or chloramine. The state's heavy reliance on groundwater (92% of the supply) and challenges like agricultural runoff and saltwater intrusion add to the complexity of disinfection.
California's vast and diverse water system means chlorine practices vary significantly from north to south:
Water quality in California is regulated by the California State Water Resources Control Board and nine Regional Water Quality Control Boards, ensuring consistent monitoring and compliance statewide
A small amount of chlorine always remains in treated water — that's the residual doing its job, continuing to neutralize any bacteria the water encounters on its journey through the distribution network. But sometimes, this residual becomes more noticeable.
When chlorine reacts with natural organic matter (NOM) found in rivers, lakes, and reservoirs, it can form chemical compounds called disinfection byproducts — including trihalomethanes (THMs) and haloacetic acids (HAAs). The EPA closely monitors and regulates these byproducts to ensure they remain at safe levels.
