Urine in a chlorinated pool is genuinely harmful, though not in the way most people assume. The problem is not the urine itself but what it becomes when it meets chlorine. Nitrogen-rich compounds in pee react with the disinfectant to produce a cocktail of chemicals called disinfection byproducts, or DBPs, some of which irritate the eyes and lungs and a few of which are classified as potentially carcinogenic. The familiar “pool smell” that people associate with too much chlorine is actually a sign of too many of these byproducts, driven largely by the organic material swimmers bring into the water.
What Happens When Urine Meets Chlorine
Chlorine is added to pool water to kill bacteria and other pathogens. It does that job well, but it also reacts aggressively with organic nitrogen, the kind found in urine, sweat, and skin oils. Urine is a particularly potent ingredient in this chemistry because it contains urea, uric acid, creatinine, and amino acids, all of which are rich in nitrogen and highly reactive with chlorine.
One of the most studied reactions involves uric acid, a waste product your kidneys filter into urine. When uric acid meets chlorine, it efficiently produces cyanogen chloride and trichloramine. In controlled experiments, the yield of cyanogen chloride from uric acid was as high as 44 percent under certain conditions, and the yield of trichloramine reached over 100 percent of the available nitrogen at neutral pH. Researchers analyzing actual pool water samples found that uric acid chlorination likely accounts for a large fraction of cyanogen chloride formation in swimming pools.
1Environmental Science & Technology. Volatile Disinfection Byproducts Resulting from Chlorination of Uric Acid: Implications for Swimming PoolsUrea, the most abundant organic compound in urine, is another prolific precursor. At a neutral pH typical of swimming pools, urea converts roughly three-quarters of its nitrogen into trichloramine, the pungent gas that stings your eyes and nose above the waterline.
2PubMed. Trichloramine in swimming pools–formation and mass transferComprehensive analyses of pool water have identified over 100 distinct disinfection byproducts, many of them nitrogen-containing compounds that were likely formed from human inputs like urine, sweat, and skin cells.
3PubMed Central. What’s in the pool? A comprehensive identification of disinfection by-products and assessment of mutagenicity of chlorinated and brominated swimming pool waterSweat contributes many of the same precursors, but urine packs them in at much higher concentrations. One bench-scale study simulating pool conditions found that continuous input of body fluids led to rapid accumulation of organic matter and rising concentrations of harmful byproducts. Urine solutions generated more trihalomethanes, while sweat solutions produced more haloacetic acids, but both kept climbing the longer the fluids stayed in the system.
4PubMed. Bench-scale assessment of the formation and control of disinfection byproducts from human endogenous organic precursors in swimming poolsThe Chemicals You Actually Breathe and Absorb
The byproducts that form from urine and chlorine do not just stay dissolved in the water. Many of them are volatile, meaning they escape into the air right above the pool surface, exactly where swimmers breathe. Trichloramine is the biggest offender. It is the compound primarily responsible for the harsh chemical smell at indoor pools, and it is a powerful irritant to mucous membranes in the eyes, nose, throat, and lungs.
Cyanogen chloride, another volatile product of uric acid chlorination, is more toxic molecule-for-molecule, though it appears at lower concentrations. It affects the central nervous system and respiratory system at high doses. The concentrations typically found in pool air are well below acutely dangerous levels, but long-term, repeated exposure is harder to dismiss as harmless.
Some byproducts are not volatile but still reach swimmers through skin absorption. Research on newly identified halogenated DBPs in pool water showed that several of these compounds, mostly halo(nitro)phenols formed from chlorination of urine in the presence of bromide, can permeate across the skin. The authors recommended that swimmers avoid urinating in pools and limit prolonged swimming to reduce chlorine contact and skin absorption.
5PubMed. New halogenated disinfection byproducts in swimming pool water and their permeability across skinRespiratory Problems in Swimmers and Pool Workers
The strongest evidence linking pool chemistry to health effects comes from the lungs. Indoor pool workers, who spend hours in the trichloramine-rich air above the water, show elevated rates of upper respiratory symptoms compared to the general population. A study of Dutch swimming pool employees found that higher cumulative chloramine exposure was associated with increased odds of hoarseness, voice loss, and sinusitis, and that the overall risk of respiratory symptoms ranged from roughly 1.4 to over 7 times that of the general population, depending on the symptom.
6European Respiratory Journal. Exposure to trichloramine and respiratory symptoms in indoor swimming pool workersA Swedish study of pool workers found a roughly two-and-a-half-fold increase in asthma risk among those with higher trichloramine exposure, though the confidence intervals were wide enough that the result did not reach conventional statistical significance.
7PubMed Central. Lung function in volunteers before and after exposure to trichloramine in indoor pool environments and asthma in a cohort of pool workersFor recreational swimmers, the exposure is shorter but still measurable. Children may be especially vulnerable. A study of over 340 schoolchildren in Brussels who attended the same indoor pool at varying rates found that cumulative pool attendance was one of the strongest predictors of asthma diagnosis, ranking behind only allergic sensitization and family history. The association was strongest in children who were already prone to allergies, and it was dose-related: more hours in chlorinated pool air meant higher risk. Attendance before age six or seven appeared to matter most.
8PubMed Central. Chlorinated pool attendance, atopy, and the risk of asthma during childhoodThe trichloramine concentrations measured in that Brussels pool’s air were between 0.3 and 0.5 milligrams per cubic meter, a range common in many indoor facilities. These are not extreme outlier conditions. They represent what a typical busy indoor pool produces when ventilation is ordinary and swimmers are not showering beforehand.
Eye Irritation, Skin Rashes, and Acute Exposures
Anyone who has opened their eyes underwater in a chlorinated pool knows the sting. That irritation is not caused by chlorine alone. Trichloramine and other DBPs are far more irritating to the delicate tissues of the eyes than free chlorine at normal disinfection levels. The same goes for skin: prolonged contact with DBP-laden water can cause rashes in sensitive individuals.
A case investigation at an indoor pool in the United States documented what happens when chloramine levels spike well above recommended levels. Among people who entered the pool area during the incident, swimming was strongly associated with illness, with the odds of symptoms being dramatically higher in swimmers than in bystanders who stayed on the deck. The most common complaints were cough (affecting about 84 percent of those affected), eye irritation (about 78 percent), and rash (about 34 percent). Pool chloramine levels measured after the incident were above 0.7 parts per million, well past the state’s maximum of 0.5 ppm.
9ScienceDirect (Elsevier). Health effects associated with indoor swimming pools: A suspected toxic chloramine exposureThat was an acute event, not typical pool conditions. But it illustrates the continuum: the same chemicals that make your eyes red after a casual swim are the same ones that, in higher concentrations, produce coughing fits and dermatitis. Every swimmer who urinates in the pool nudges the chemistry further along that continuum.
Could Pool Water Cause Cancer or Genetic Damage
This is where the evidence gets more tentative but also more unsettling. Laboratory analysis has shown that all disinfected pool water samples induce more genomic DNA damage than the source tap water used to fill the pools. Researchers have noted that behavior modification by swimmers, including not urinating in the pool, could reduce the precursors of the most toxic byproducts.
10PubMed. Genotoxicity of water concentrates from recreational pools after various disinfection methodsOne study that examined swimmers before and after a 40-minute swim in an indoor chlorinated pool found biomarker changes suggesting potential genotoxic effects from exposure to DBPs.
11PubMed Central. Genotoxic effects in swimmers exposed to disinfection by-products in indoor swimming poolsHowever, a later study looking at similar biomarkers found that most measures of genotoxicity and lung damage remained unchanged after a swim. Some moderate associations appeared in one specific marker, but the authors cautioned that the sample size was small and the result needed careful interpretation.
12PubMed. Exposure to disinfection by-products in swimming pools and biomarkers of genotoxicity and respiratory damage – The PISCINA2 StudyThe honest reading of the science is that many individual DBPs are mutagenic or carcinogenic in laboratory settings, and pool water as a whole shows more genotoxic potential than the tap water used to fill it. Whether the actual exposure levels experienced by a recreational swimmer who visits a pool a few times a week translate into meaningful cancer risk over a lifetime is still unresolved. The concern is plausible enough that researchers consistently recommend reducing the organic precursors, urine chief among them, rather than waiting for definitive long-term epidemiological data.
How Much Urine Is Actually in Pools
People understandably want to know the scale of the problem. Surveys provide a lower bound, since admitting to peeing in a pool is not something most people volunteer easily. An Italian survey of pool users found that about 13.5 percent of respondents said they had urinated at least once in a swimming pool.
13Microchemical Journal. What about behaviours in swimming pools? Results of an Italian multicentre studyResearchers at the University of Alberta took a more creative approach, measuring acesulfame potassium (ACE), an artificial sweetener that passes through the body unchanged and ends up entirely in urine. Because ACE is stable in chlorinated water, it works as a reliable chemical tracer for urine. Testing pools and hot tubs confirmed widespread urine presence.
14Environmental Science & Technology Letters. Sweetened Swimming Pools and Hot TubsThe takeaway from these measurement studies is that urine in pools is not a rare event driven by a few rude swimmers. It is a consistent, pervasive input into pool chemistry, and its volume is large enough to meaningfully contribute to the DBP burden.
What Actually Reduces the Risk
The chemistry is clear: reduce the organic inputs, and you reduce the harmful byproducts. The two most effective interventions are behavioral, not technological.
The first and most obvious is not urinating in the pool. This removes a major source of uric acid, urea, creatinine, and amino acids that fuel the most troublesome reactions. Every swimmer who uses the restroom instead of the pool directly lowers the concentration of trichloramine, cyanogen chloride, and dozens of other byproducts in the water and air.
The second is showering before getting in. Research on initial pollutant release shows that pre-swim showering reduces both chemical and microbiological contamination, very likely resulting in decreased DBP formation and lower chlorine demand.
15PubMed. Definition and quantification of initial anthropogenic pollutant release in swimming poolsDespite this, compliance is low. A Toronto study observed bather behavior at a public pool and found that showering rates fell well short of what health guidelines recommend.
16Environmental Health Review. Compliance of bathers to showering before swimming in a public pool in Toronto, OntarioOn the facility side, good ventilation is one of the most important factors for protecting indoor pool users. An investigation into air quality at an indoor pool found that trichloramine levels in the air were significantly lower when ventilation rates reached at least two full air changes per hour, and when fresh water was regularly cycled into the pool.
17PubMed. Investigation of Air Quality Problems in an Indoor Swimming Pool: A Case StudyAdvanced water treatment can also help. One promising approach combines UV treatment with ozone, applied in sequence. Standard UV treatment alone can break down chloramines, but it sometimes increases the concentration of other chloro-organic byproducts when the water is re-chlorinated afterward. Adding ozone directly after UV treatment counteracted this rebound effect, and repeated combined treatments predicted meaningful improvements in overall water quality.
18PubMed. Improved DBP elimination from swimming pool water by continuous combined UV and ozone treatmentIndoor Pools Versus Outdoor Pools
Nearly all of the concerning research involves indoor pools, and that is not a coincidence. Indoor facilities trap volatile byproducts in a closed airspace, allowing trichloramine and other irritant gases to build up right above the water where swimmers breathe. The same chemical reactions occur in an outdoor pool, but the byproducts disperse into the open air and rarely accumulate to the concentrations measured indoors.
If you swim outdoors, your exposure to airborne DBPs is dramatically lower. That does not eliminate skin and eye exposure to waterborne byproducts, which depends on pool chemistry and how much organic matter is in the water, but it does remove the respiratory dimension that accounts for the most robust health effects documented in the literature. For people who are concerned about the respiratory side of the equation, especially parents of young children with allergic tendencies, outdoor pools carry a meaningfully different risk profile than indoor ones.
Why the “Chlorine Kills Everything” Myth Matters
A common misconception is that chlorine in a pool neutralizes urine and makes it harmless. In reality, chlorine does react with urine, but the products of that reaction are the problem, not the solution. When people say “the chlorine takes care of it,” they are describing the exact process that generates trichloramine, cyanogen chloride, trihalomethanes, and the rest of the byproduct family. More urine in the water does not just mean dirtier water in some vague sense; it means the chlorine that should be disinfecting the pool is instead being consumed by reactions with urine compounds, simultaneously producing irritant and potentially toxic chemicals while leaving less free chlorine available to kill germs.
This double penalty is worth understanding: urine both creates harmful byproducts and undermines the pool’s germ-killing capacity. A heavily used pool where many swimmers skip the shower and some urinate in the water is simultaneously more chemically irritating and less effectively disinfected than a pool where swimmers take basic precautions.
Competitive and Elite Swimmers
Competitive swimmers spend far more time in indoor chlorinated water than the average recreational swimmer, sometimes logging 20 or more hours per week. This extended exposure means higher cumulative intake of volatile DBPs through inhalation and higher skin absorption of waterborne compounds. The respiratory issues documented in pool workers, who have similarly prolonged exposure, are likely relevant to elite swimmers as well.
Anecdotally, “swimmer’s cough” and chronic rhinitis are well-recognized complaints among competitive swimmers. The research on trichloramine exposure in occupational settings provides a plausible mechanism: repeated inhalation of chloramine-rich air damages the epithelial lining of the airways, promoting chronic inflammation. For parents of children in competitive swim programs, the evidence on early-childhood pool attendance and asthma risk noted above may be particularly relevant. Choosing a well-ventilated facility, or one that uses supplemental UV or ozone treatment, can reduce exposure substantially. Advocating for strict pre-swim shower policies and restroom breaks at a team level is arguably the single most cost-effective measure a swim club can adopt to protect its athletes.