Chlorine and chloramines are added to municipal water systems to kill disease-causing bacteria, viruses, and parasites before the water reaches your tap. They remain the most widely used disinfectants in public water treatment because they’re inexpensive, effective against a broad range of pathogens, and, critically, they leave a residual in the water that continues protecting it as it travels through miles of pipes to your home. That residual protection is something most alternative technologies simply can’t provide.
What Chlorine Actually Does to Pathogens
When chlorine is added to water, it forms a highly reactive compound called hypochlorous acid. This molecule is small enough to penetrate the cell walls of bacteria and the outer proteins of viruses, disrupting their ability to function and reproduce. The process is fast: at the concentrations used in water treatment, chlorine can neutralize most common bacteria within minutes. Viruses take somewhat longer, and certain parasites with tough outer shells are more resistant, which is one reason treatment plants use multiple steps (like filtration) before disinfection.
The result of widespread chlorination has been dramatic. Disinfection of drinking water virtually eliminated waterborne epidemics of typhoid, cholera, and hepatitis in the United States. Before municipalities began chlorinating water in the early 1900s, these diseases killed thousands of Americans every year. The EPA considers drinking water disinfection one of the most significant public health achievements of the modern era.
Why a Residual Disinfectant Matters
Treating water at the plant is only half the challenge. That water then has to travel through a vast network of pipes, some of which may be decades old, before it arrives at your faucet. During that journey, bacteria can regrow. Biofilms, which are thin layers of microbial communities, can develop on pipe walls and harbor harmful organisms, including opportunistic pathogens like Legionella.
This is where chlorine and chloramines have a unique advantage over alternatives like UV light or ozone. UV and ozone are powerful disinfectants at the point of treatment, but they leave no lasting residual in the water. Ozone, for instance, decomposes rapidly after application. Once it’s gone, there’s nothing to prevent bacteria from multiplying as water sits in pipes or storage tanks. Chlorine and chloramines, by contrast, persist in the water and continue killing microorganisms throughout the distribution system.
The EPA tracks a concept called “water age,” which refers to how long water sits in the distribution system before someone uses it. In low-demand areas or at the far ends of a system, water can be days old by the time it reaches a tap. Without a chemical residual, that aging water becomes a breeding ground. Low residual levels allow microbial growth to spike, and in some cases, microbial activity can even corrode pipes and leach metals from pipe surfaces.
Why Some Cities Use Chloramines Instead of Chlorine
Chloramines are formed by combining chlorine with ammonia. Many municipalities have switched to chloramines for one primary reason: stability. Chlorine gets used up quickly in water. In large or complex distribution systems, there sometimes isn’t enough chlorine left to keep killing germs by the time the water reaches the tap. Chloramines break down much more slowly, maintaining effective disinfectant levels over longer distances and longer periods of time.
The other major reason for the switch involves byproducts. When chlorine reacts with naturally occurring organic matter in water (things like decomposing leaves or soil compounds), it creates disinfection byproducts. The most well-known groups are trihalomethanes and haloacetic acids. Long-term exposure to trihalomethanes has been associated with an increased risk of bladder cancer, and several haloacetic acids are classified as reasonably anticipated to be human carcinogens. Chloramines produce significantly lower levels of these specific byproducts, which makes them an attractive option for systems that struggle to meet regulatory limits.
That said, chloramines aren’t without their own complications. In systems with high water age and warm temperatures, chloramines can break down and fuel a process called nitrification, where ammonia-oxidizing bacteria proliferate in pipes. This can further deplete the disinfectant residual and degrade water quality. Managing chloramine systems requires careful monitoring.
Taste, Odor, and What You Notice at Home
Most people associate a “pool smell” with chlorinated tap water, but many of those odor complaints are actually caused by byproducts of incomplete chlorine reactions rather than the chlorine itself. In water drawn from surface sources that contain certain organic compounds, chlorine can produce chlorophenol, a chemical with a strong, unpleasant taste and smell. Chloramines don’t form this particular byproduct.
When municipalities switch from chlorine to chloramines, most consumers don’t notice a change in taste. The difference is subtle enough that it rarely registers in everyday drinking or cooking. Where the change does matter is for specific uses: chloramines are harmful to fish in aquariums and need to be removed from water used in kidney dialysis. They also require different filtration if you want to remove them at home.
Removing Chlorine and Chloramines at Home
If you prefer to filter out disinfectants before drinking, the type of filter you need depends on which chemical your utility uses. Standard activated carbon filters (the kind found in most pitcher filters and under-sink systems) work well for removing chlorine, along with volatile organic compounds and unpleasant tastes. Chloramines, however, are more chemically stable and resist standard carbon filtration.
For chloramine removal, catalytic carbon is the more effective option. This is a specially treated form of activated carbon that breaks chloramines down into harmless chloride. It’s commonly used as a pre-filter in reverse osmosis systems to protect the RO membranes from oxidation. For best results, catalytic carbon is often combined with granular activated carbon to address a wider range of contaminants at once. If your water utility uses chloramines (you can check their annual water quality report), it’s worth confirming that any filter you buy is rated specifically for chloramine reduction.
How Much Is in Your Water
The EPA sets a maximum residual disinfectant level of 4.0 milligrams per liter for both chlorine and chloramines. At that concentration, you could drink the water daily for a lifetime without expected health effects beyond minor eye or nose irritation and possible stomach discomfort at the upper limit. In practice, most utilities maintain residual levels well below the maximum, typically between 0.5 and 2.0 mg/L, balancing pathogen protection against taste concerns and byproduct formation.
Your water utility is required to publish an annual Consumer Confidence Report listing the disinfectant used, the average residual level, and the levels of regulated byproducts like trihalomethanes and haloacetic acids. These reports are usually available on your utility’s website and give you a clear picture of exactly what’s in your water and how it compares to federal limits.