Chlorine keeps drinking water and swimming pools free of dangerous bacteria, but it comes with real downsides. When chlorine reacts with organic matter in water, it creates hundreds of chemical byproducts linked to cancer, respiratory problems, and reproductive harm. It also strips protective oils from your skin, irritates your lungs, and may alter the balance of bacteria in your gut. The risks depend heavily on how much chlorine you’re exposed to, how long you’re exposed, and whether you’re drinking it, breathing it, or soaking in it.
What Happens When Chlorine Meets Organic Matter
Chlorine itself isn’t the main concern. The bigger issue is what chlorine creates. When it reacts with naturally occurring organic material in water (leaves, soil, algae, even skin cells), it produces a class of chemicals called disinfection byproducts, or DBPs. Researchers have identified hundreds, possibly thousands, of these compounds in treated water.
The two most common families are trihalomethanes (THMs), first identified in drinking water back in 1974, and haloacetic acids, which make up roughly 25% of the total byproducts in chlorinated water. Both have shown high levels of cell toxicity and the ability to cause genetic mutations in laboratory studies. These aren’t theoretical risks: long-term exposure to these byproducts has been linked to bladder, liver, and colon cancers, as well as cardiovascular disease including stroke.
Chlorine and Cancer Risk
Bladder cancer has received the most research attention. A meta-analysis published in the Journal of Epidemiology and Community Health combined data from multiple studies and found that long-term consumption of chlorinated drinking water raised bladder cancer risk by about 40% compared to people drinking unchlorinated water. The risk climbed steadily with duration: 20 years of exposure was associated with a 13% increase, 40 years with a 27% increase, and 60 years with a 43% increase.
To be clear, a 40% relative increase doesn’t mean 40% of people will get bladder cancer. Bladder cancer is relatively uncommon to begin with, so the absolute risk remains small. But across entire populations drinking chlorinated water for decades, even a modest relative increase translates to a meaningful number of additional cases.
Reproductive and Developmental Concerns
Studies have also flagged risks during pregnancy. Exposure to high levels of trihalomethanes has been associated with increased rates of spontaneous abortion, low birth weight, and neural tube defects in newborns. These findings are based on population-level studies rather than controlled experiments, so the exact threshold of dangerous exposure isn’t nailed down. But the pattern across multiple studies is consistent enough to be taken seriously.
Respiratory Effects, Especially at Pools
If you’ve ever walked into an indoor pool and immediately noticed that sharp chemical smell, you’re breathing chloramines. These form when chlorine in pool water reacts with sweat, urine, and body oils from swimmers. The smell most people associate with “too much chlorine” is actually chloramines, not chlorine itself.
Indoor pools concentrate these irritants in enclosed, humid air. Pool workers who spend hours in this environment show higher rates of respiratory symptoms. For people with existing asthma or allergies, the interaction between chloramine exposure and allergens is the most likely explanation for symptom flare-ups. Even in drinking water, chloramine byproducts have been linked to increased rates of asthma and bronchitis with sustained exposure.
For context, most people can smell chlorine at concentrations between 0.2 and 0.4 parts per million in air. The EPA’s maximum allowable level in drinking water is 4 parts per million, though most municipal systems aim well below that.
Skin and Hair Damage
Chlorine is an oxidizer, which means it breaks down organic compounds on contact. Your skin’s outer layer is coated in a thin film of natural oils (sebum) that keeps moisture in and irritants out. Research on swimmers found that a single pool session substantially removes this protective layer from the skin surface.
With that barrier stripped, skin dries out faster and becomes more vulnerable to irritation. Chlorinated pool water can trigger irritant contact dermatitis, sometimes called “pool dermatitis,” even in people without sensitive skin. For people with eczema, the drying and irritating effects of chlorine on already compromised skin are a well-recognized concern, though high-quality studies on exactly how much chlorine worsens flare-ups are still limited.
Hair gets a similar treatment. Chlorine attacks the protein structure that gives hair its strength and flexibility. Regular swimmers often notice their hair becoming dry, brittle, and discolored over time, particularly if they have lighter or chemically treated hair.
Effects on Gut Bacteria
A more recently studied concern is what residual chlorine in drinking water does to the microbes living in your digestive system. A study of infants in Haiti compared gut bacteria in children from households with detectable chlorine in their drinking water to those with no detectable chlorine. The differences were striking.
Infants exposed to chlorinated water had significantly fewer bacterial species in their guts and lower overall microbial diversity. Their gut communities were also more uniform, clustering tightly around a similar composition, while infants drinking unchlorinated water had more varied and individualized microbiomes. Specifically, chlorine-exposed infants had higher levels of Bifidobacterium but lower levels of several bacterial families (Streptococcaceae, Lachnospiraceae, Ruminococcaceae) that play important roles in digestion and immune function.
Microbial diversity in the gut is generally considered a marker of health. A less diverse microbiome has been associated with increased susceptibility to infections, allergies, and chronic inflammatory conditions. This research is still in its early stages for humans, but it suggests that the same germ-killing property that makes chlorine useful in water treatment doesn’t stop working once it reaches your intestines.
Reducing Your Exposure
The simplest way to lower chlorine exposure from drinking water is a standard activated carbon filter, the kind found in most pitcher filters and faucet-mount systems. According to the Water Quality Association, a basic 10-inch carbon cartridge running at half a gallon per minute removes about 95% of incoming chlorine and can treat roughly 2,500 gallons before needing replacement. These filters also reduce trihalomethanes and other byproducts, though effectiveness varies by product.
For swimming, rinsing off immediately after leaving the pool removes chlorine and chloramines from your skin before they do more damage. Applying a moisturizer right after showering helps restore the oil barrier that chlorine stripped. Wetting your hair with clean water before swimming can also reduce how much chlorinated water it absorbs, since hair takes in less liquid when it’s already saturated.
If you have a home with well water or another unchlorinated source, keep in mind that chlorine exists in public water systems for a reason. Waterborne diseases like cholera, typhoid, and dysentery killed enormous numbers of people before water disinfection became standard. The health risks of chlorine byproducts are real, but they’re measured in small percentage increases over decades. The risks of drinking untreated water contaminated with bacteria or parasites are immediate and potentially fatal. For most people, filtering tap water at the point of use offers a practical middle ground: removing the chlorine while relying on the municipal system to do the heavy lifting of killing pathogens before water reaches your home.