Chlorinated water is tap water that has been treated with a small amount of chlorine to kill harmful bacteria, viruses, and parasites. Nearly every public water system in the United States uses either chlorine or a closely related chemical called chloramine to keep drinking water safe as it travels from the treatment plant to your faucet. The chlorine levels in your tap water are low enough to be safe for drinking but high enough to prevent waterborne diseases like cholera and typhoid.
How Chlorine Disinfects Water
When chlorine is added to water, it doesn’t simply float around waiting to bump into germs. It reacts chemically with the water to form hypochlorous acid, a powerful oxidizer that destroys the cell walls and internal structures of bacteria and other microorganisms. This reaction also produces a small amount of hydrochloric acid, but the water’s natural buffering capacity neutralizes it easily.
Hypochlorous acid is the real workhorse of water disinfection. It’s effective against a wide range of pathogens, including the bacteria that cause typhoid fever and cholera, and it works quickly at the concentrations used in municipal water systems. The chlorine that remains in the water after initial treatment, called residual chlorine, continues killing any germs the water encounters on its way through miles of pipes to your home.
Why It Became Standard Practice
Jersey City, New Jersey became the first U.S. city to routinely chlorinate its drinking water in 1908. Within a decade, thousands of cities and towns followed. The results were dramatic: in 1900, typhoid fever struck roughly 100 out of every 100,000 Americans. By 1920, that number had dropped to 33.8 per 100,000. Cities from Baltimore to Chicago saw rapid declines in waterborne illness as they adopted disinfection. Chlorination is widely considered one of the most significant public health achievements of the 20th century.
Chlorine vs. Chloramine
Some water utilities use chloramine instead of chlorine. Chloramine is made by combining chlorine with ammonia, and it has a few practical advantages: it lasts longer in the water distribution system, so it can keep killing germs in pipes that stretch far from the treatment plant. It also produces fewer disinfection byproducts, which is a growing concern (more on that below).
The tradeoff is that chloramine is less aggressive as a disinfectant, and some utilities will temporarily switch back to chlorine when they need to clear out bacterial buildup inside pipes. Water treated with chloramine also tends to taste and smell different than chlorine-treated water, though opinions vary on which is more noticeable.
The Taste and Smell of Chlorinated Water
That swimming-pool taste in your tap water comes from residual chlorine, and sensitivity to it varies widely. According to World Health Organization guidelines, most people can detect chlorine at concentrations well below 5 parts per million, and some can taste or smell it at levels as low as 0.3 ppm. Since U.S. drinking water typically contains around 0.2 to 4 ppm, many people do notice it. The taste is harmless but understandably off-putting for some.
Disinfection Byproducts and Health Concerns
Chlorine doesn’t just react with germs. It also reacts with naturally occurring organic matter in the water (things like decaying leaves and soil particles) to form compounds called trihalomethanes. These byproducts are the main health concern associated with chlorinated water, and they’ve been studied extensively.
A systematic review examining 29 studies and tens of thousands of participants found that people exposed to higher levels of trihalomethanes in drinking water had a 33% higher risk of bladder cancer and a 15% higher risk of colorectal cancer compared to those with the lowest exposure. The dose-response analysis found that bladder cancer risk became statistically significant at trihalomethane concentrations above 41 micrograms per liter. The researchers classified the evidence as “limited-suggestive,” noting that the risk appears to exist at levels below current U.S. and EU regulatory limits.
To put this in perspective, the risk from disinfection byproducts is real but modest, and it needs to be weighed against the catastrophic disease burden that existed before water chlorination. The goal for water treatment regulators is minimizing byproducts while still killing pathogens effectively.
Chlorine Levels in Pools vs. Tap Water
Swimming pools contain significantly more chlorine than your drinking water. The CDC recommends a minimum free chlorine level of 1 ppm for public pools, while hot tubs need at least 3 ppm because warm water breeds bacteria faster. Splash pads require 1 to 2 ppm depending on whether stabilizers are used. These levels are higher than typical tap water concentrations because pools face constant contamination from swimmers’ bodies, and the chlorine needs to work quickly in a shared environment.
This is why your eyes and skin can feel irritated after swimming but not after drinking a glass of water. The chlorine concentration is simply much higher, and you’re soaking in it rather than passing a small amount through your digestive system.
How to Remove Chlorine From Tap Water
If the taste or smell bothers you, the simplest solution is an activated carbon filter. Pitcher filters and refrigerator filters typically use activated carbon, and their primary purpose is improving taste and smell rather than making water safer. They’re effective and inexpensive for this purpose.
Letting water sit in an open container for several hours will also allow chlorine to dissipate on its own, since it’s a volatile gas that naturally escapes into the air. This is why aquarium owners often let tap water sit before adding fish, as chlorine is toxic to aquatic life at concentrations that are safe for humans.
One important consideration: if you install a whole-home filtration system that removes chlorine, you lose the residual protection it provides inside your plumbing. Without that residual disinfectant, bacteria can grow more easily in your home’s pipes. For most people, a point-of-use filter at the kitchen faucet or a simple pitcher is a better approach, since it removes chlorine right before you drink it while leaving the rest of your plumbing protected.