Swimming pools are synonymous with summer relaxation and fun, but the strong odor often associated with them can be off-putting. Many people assume that this unmistakable “pool smell” is caused by chlorine. However, the truth is more complex — and deeply rooted in chemistry.
In this article, we’ll explore what chemical causes pool smell, how it forms, and why understanding it is key to improving pool hygiene and swimmer comfort.
The Misconception: Chlorine as the Culprit
For decades, the sharp, pungent smell of a pool has been blamed on chlorine. After all, chlorine is the primary disinfectant used in most swimming pools to kill harmful bacteria, algae, and other contaminants. But the reality is that pure chlorine itself has a relatively mild scent. The overwhelming odor is actually the result of a chemical reaction involving chlorine and organic substances in the pool water.
Chlorine doesn’t cause the strong pool smell — it reacts with contaminants to form disinfection byproducts (DBPs), and these are the real culprits behind the unpleasant odor.
Let’s delve into which chemical compounds are responsible and how they form.
The Real Smell: Chloramines Are the True Offenders
The primary chemical responsible for the “chlorine smell” in pools is a group of compounds known as chloramines.
Chloramines form when chlorine (specifically, hypochlorous acid, the active form of chlorine) reacts with ammonia and other nitrogen-containing compounds introduced into the water by swimmers. These compounds primarily come from sweat, urine, skin cells, cosmetics, and even lotions.
There are three main types of chloramines:
- Monochloramine: Most stable and often used in municipal water treatment.
- Dichloramine: More volatile and irritating, contributing more to the strong smell.
- Trichloramine: Highly volatile and known to be a leading cause of respiratory irritation and strong pool odor.
Out of these, trichloramine (also known as nitrogen trichloride) is the most responsible for the sharp, pungent smell associated with indoor swimming pools. It’s also linked to eye irritation, respiratory issues, and even asthma symptoms among frequent swimmers.
How Chloramines Form in Pool Water
To understand why chloramines develop and cause that well-known odor, it’s essential to look at the chemistry of pool disinfection.
When chlorine is added to water, it undergoes a chemical reaction and forms hypochlorous acid (HOCl), which is the active disinfecting agent:
Hypochlorous acid is highly effective at sanitizing pool water by breaking down bacteria and viruses. However, it also reacts with organic compounds introduced by swimmers.
This is where the problem begins. When HOCl reacts with ammonia (NH₃), which is a component of urine and sweat, chloramines begin to form:
Further reactions with additional HOCl can produce dichloramines and trichloramines:
Trichloramine is not only the most irritating, but it also evaporates easily into the air, especially in indoor pools with poor ventilation. As a result, it accumulates in the air just above the water, where swimmers inhale it — leading to discomfort and the unmistakable “chlorine” smell.
Organic Contaminants: The Main Sources of Chloramines
To reduce chloramine formation, it’s crucial to understand their origin. The primary source is not the chlorine itself but swimmers’ bodies. The more contaminants swimmers introduce, the more likely chloramines are to form.
Here’s a breakdown of typical organic contaminants found in pool water:
| Contaminant | Source | Contribution to Pool Chemistry |
|---|---|---|
| Urine | Swimmers, especially children | High in urea and ammonia — reacts with chlorine to produce chloramines. |
| Sweat | Sun exposure, physical activity | Contains amino acids and ammonia, which contribute to chloramine formation. |
| Cosmetics & Personal Care Products | Shampoos, conditioners, lotions | Contain organic compounds that react with chlorine, increasing DBPs. |
| Skin Flakes | Natural shedding of skin | Introduce organic matter into the pool water, promoting chloramine formation. |
The more contaminants present, the more the chlorine becomes consumed in forming chloramines, which reduces the amount of free chlorine available to sanitize the pool — leading to a cycle of poor water quality.
Why Indoor Pools Smell Stronger than Outdoor Pools
If you’ve ever noticed that indoor pools tend to have a stronger odor than outdoor ones, there’s a scientific reason for that.
Outdoor pools benefit from natural ventilation and sunlight. UV rays from the sun help break down some of the chloramines, and the open air disperses the volatile compounds more effectively. In contrast, indoor pools have limited airflow, allowing the chloramines to accumulate in the air.
Moreover, the temperature and humidity in indoor pools promote swimmers to perspire more and use skincare products, further increasing contaminant load.
Highly concentrated chloramines in indoor pools can lead to respiratory irritation, especially for lifeguards, competitive swimmers, and children who may not be able to avoid long-term exposure.
Organizations such as the Centers for Disease Control and Prevention (CDC) and the Occupational Safety and Health Administration (OSHA) have set guidelines to control chlorine and chloramine levels in indoor pool environments to reduce health risks.
The Health Effects of Chloramines and Poor Air Quality
While chloramines are less effective disinfectants than free chlorine, their accumulation can pose significant health risks. Exposure, particularly in indoor pool environments, has been linked to:
- Eye irritation (red, itchy eyes)
- Respiratory discomfort (coughing, wheezing)
- Increased asthma risk, especially among children and lifeguards
- Skin irritation or rashes
- Allergic reactions in sensitive individuals
Studies have shown that individuals who spend long hours in indoor aquatic environments, such as competitive swimmers or pool staff, are more likely to develop respiratory symptoms consistent with asthma, partly due to chronic exposure to chloramines.
How Pool Operators Combat Chloramine Build-Up
To maintain safe and enjoyable swimming conditions, both public and private pool operators use several strategies to reduce chloramine levels:
- Increasing free chlorine levels: By super-chlorinating or “shocking” the pool with a high dose of chlorine, chloramines can be effectively oxidized and broken down.
- Using secondary disinfectants: Systems like ultraviolet (UV) light or ozone generators can help destroy chloramines and reduce the need for high chlorine doses.
- Implementing air circulation systems: Especially in indoor pools, proper ventilation controls the build-up of harmful vapors and improves air quality.
- Encouraging bather hygiene: By enforcing showering before entering the pool and using restroom facilities, pool operators can reduce the introduction of organic contaminants.
- Regular water testing and filtration: Monitoring chlorine levels, pH, and total dissolved solids helps in maintaining a balanced and healthy swimming environment.
Debunking Other Common Pool Smell Myths
There are several popular myths about pool chemistry and odor that deserve to be clarified:
Myth 1: “Heavily chlorinated pools are the healthiest.”
In reality, the most effective disinfectants are free chlorine (Cl₂) or hypochlorous acid (HOCl). When pools have a strong odor, it’s a sign of combined chlorine (chloramines), not high free chlorine levels. A properly chlorinated pool should have little to no smell.
Myth 2: “Green pool water means high chlorine.”
Green pool water usually indicates an algae bloom — an insufficient or unbalanced disinfection process. Algae can thrive when combined chlorine levels are high and free chlorine levels are low.
Myth 3: “If a pool doesn’t smell, then it’s clean.”
While a lack of strong odor is reassuring, pools with poorly maintained chemistry might not smell but could still harbor harmful bacteria and viruses. Regular testing is necessary for accurate pool safety.
How Swimmers Can Reduce Pool Odors
As a swimmer, you play a crucial role in maintaining pool hygiene and minimizing unpleasant odors.
Here are some best practices:
Shower Before Entering the Pool
Showering removes sweat, oils, and cosmetics from your skin, reducing the amount of organic compounds that react with chlorine to form chloramines.
Use the Restroom Before Swimming
Avoid urinating in the pool. While it may seem harmless, a small amount of urine from many swimmers can dramatically increase ammonia levels.
Stay Out of the Pool if Ill or Injured
Contaminants from wounds, open sores, diarrhea, and other illnesses can introduce harmful pathogens as well as reactive compounds into the pool water.
Dress Appropriately
Swimwear should be clean and free from lotions and oils. Using waterproof sunscreens or avoiding heavy skincare products before swimming can also help reduce contamination.
Alternatives to Chlorine: Reducing Chloramine Concerns
For those concerned about chloramines and seeking a “no-chemical” or reduced-chemical pool experience, alternatives to chlorine are available:
Saltwater Pools
Saltwater pools use salt chlorinators to generate chlorine through electrolysis. While they still produce chlorine, the process is gentler and can lead to fewer chloramines compared to manually added chlorine. However, they are not “chemical-free” pools, and chloramines can still form.
Ozone Systems
Ozone (O₃) is a powerful oxidizer that kills pathogens and oxidizes contaminants. While ozone can reduce the need for chlorine, it’s typically used as a supplement rather than a standalone system.
Ultraviolet (UV) Light Systems
UV systems use short-wave UV light to break down chloramines and other byproducts, improving water quality and reducing odor. Many indoor pools use UV light technology to enhance air quality and reduce the risk of respiratory irritation.
Halogen-Free Systems (Bromine and others)
Bromine is another halogen used in pools, especially in spas. While it doesn’t produce chloramines, it has its own odor and byproducts. In some cases, bromamines (similar to chloramines) can still cause irritation.
The Future of Pool Chemistry
Pool maintenance technology is evolving, with increasing emphasis on swimmer health and environmental sustainability. Some of the latest trends include:
- Smart pool systems that automatically monitor and adjust pH and chlorine levels in real-time.
- Nano-filtration systems that reduce chemical reliance through advanced particle removal.
- Enzyme-based pool cleaners that break down organic contaminants before they react with chlorine.
These innovations are shaping the future of pool management and may help reduce the formation of chloramines, improve air quality, and enhance the overall swimming experience.
Conclusion: Clearing the Air – Understanding and Addressing Pool Smell
To answer the original question: trichloramine is the main chemical responsible for the strong pool smell. It forms when chlorine reacts with ammonia-containing substances — primarily sweat and urine — from swimmers. While chlorine itself is essential for sanitizing pool water, its byproducts create the odor and health concerns many people associate with pools.
By improving bather hygiene, using supplemental disinfection methods, optimizing ventilation (especially indoors), and regularly monitoring water quality, pool operators and swimmers can work together to minimize chloramine formation.
Understanding what chemical causes pool smell isn’t just about eliminating an odor — it’s about promoting a healthier, more enjoyable aquatic environment for everyone.
Whether you’re a casual swimmer, a competitive athlete, or a pool operator, staying informed is the first step toward ensuring a clean, comfortable, and odor-free swim.
What chemical is responsible for the strong smell in pools?
The strong chemical smell commonly associated with swimming pools is primarily caused by chloramines, which are chemical compounds formed when chlorine-based disinfectants react with organic substances such as sweat, urine, and body oils introduced into the water by swimmers. Contrary to popular belief, the odor is not due to chlorine itself but to these byproducts created when chlorine binds with nitrogen-containing compounds. These chloramines are less effective at disinfecting than free chlorine and are known for their pungent smell that can oftentimes be mistaken for “too much chlorine.”
Proper pool maintenance plays a critical role in managing chloramine levels. Pool operators must regularly test the water to ensure free chlorine levels are adequate to effectively sanitize and break down contaminants. Additionally, super chlorination or “shock treatment” can be performed to oxidize and remove these odor-causing compounds. Ensuring swimmers practice good hygiene—like showering before entering the pool and avoiding urination in the water—also helps minimize the formation of chloramines and, in turn, reduces unpleasant pool smells.
Why does chlorine in a pool not cause the smell itself?
Chlorine, in its active or “free” form, typically has a mild scent and is primarily used in pools to disinfect water and kill harmful bacteria and pathogens. The misconception that chlorine is solely responsible for the strong pool odor comes from the chemical reactions chlorine undergoes when it interacts with organic materials in the pool. When free chlorine combines with substances such as ammonia and urea, which are introduced through sweat and urine, it forms byproducts known as chloramines. These chloramines are responsible for the sharp, irritating odor typically noticed at pool facilities.
The type of chloramines present also matters. For example, trichloramines are a particularly volatile type of chloramine that can vaporize into the air and contribute significantly to the noticeable smell near pool water surfaces. This can cause discomfort, including burning eyes and respiratory irritation, especially for individuals with asthma or heightened sensitivity to chemicals. Therefore, identifying the source of the smell as chloramines rather than pure chlorine is essential to properly manage the water quality and reduce any negative effects on swimmer comfort.
How do chloramines affect water quality and swimmer comfort?
Chloramines not only cause unpleasant odors but also reduce the effectiveness of chlorine in killing bacteria and viruses, leading to a decline in overall water sanitation quality. When chlorine becomes bound up in chloramines, it is no longer available to serve its primary disinfecting purpose. This creates a situation where harmful microorganisms may survive in the water, increasing the risk of recreational water illnesses. Poor water quality can also lead to algae growth, cloudy water, and a general feeling of uncleanliness.
From a swimmer’s perspective, chloramines can severely affect comfort and health. They are volatile compounds that evaporate into the air, especially in indoor pools with poor ventilation, causing a strong odor and irritating the respiratory system and eyes. Symptoms such as coughing, wheezing, and red eyes can resemble allergies or asthma attacks. Reducing chloramine concentrations through regular oxidation, proper ventilation, and diligent water testing helps ensure a safer, more pleasant swimming environment for everyone.
Can pool smell indicate improper water chemistry?
Yes, a strong pool smell can be a direct indicator that the water chemistry is out of balance, particularly in terms of chlorine and chloramine levels. When there’s a buildup of chloramines due to insufficient free chlorine or poor oxidation practices, swimmers notice the odor as a sign that contaminants are not being properly removed. This issue often occurs in busy pools or when the water turnover rate is low, allowing organic compounds to accumulate.
Maintaining proper water chemistry means regularly measuring and adjusting the levels of free chlorine, pH, and other water balance parameters. A well-maintained pool should have a slight chlorine scent at most, not an overpowering stench. If the strong smell persists despite regular chlorination, this might indicate that more aggressive treatments—such as shock dosing or water dilution—are necessary to reset the chemical balance. Monitoring these factors ensures cleaner water and a more enjoyable swim for guests.
What steps can pool operators take to reduce the smell caused by chloramines?
Pool operators can take several proactive steps to minimize chloramine-related odors. One of the most effective methods is properly shocking the pool with a high dose of chlorine or using non-chlorine shock to oxidize contaminants and break down chloramine bonds. Regular shock treatments help restore the level of free chlorine, allowing it to resume efficient disinfection. Additionally, maintaining adequate water circulation and filtration ensures even distribution of chemicals and removal of organic materials.
Another important step is to ensure proper ventilation, especially in indoor pool facilities. Good air circulation helps dissipate volatile chloramines from the poolside air, reducing swimmer discomfort and airborne irritation. Encouraging swimmers to shower before entering the pool and providing facilities for hand washing can greatly reduce the introduction of urea, sweat, and oils into the water. Pool staff should also educate patrons about hygiene practices and the negative impact that improper behavior, like urinating in pool water, can have on chemical balance and air quality.
Can swimmers contribute to the pool smell, and how?
Swimmers are a primary source of the organic compounds that contribute to chloramine formation and, consequently, the strong pool odor. Materials such as sweat, urine, body oils, lotions, and even traces of fecal matter can introduce nitrogenous compounds like ammonia and urea into the water. These substances react with chlorine, forming the chloramines that cause irritation and the characteristic pool smell. The higher the swimmer load and the lower their hygiene level, the greater the potential for chloramine buildup.
Encouraging pre-swim hygiene significantly reduces these contributions. Swimmers should be required or strongly encouraged to shower thoroughly before entering the pool, especially removing lotions, cosmetics, sweat, or any residues from skincare products. Urinating in the pool should be actively discouraged, as even small amounts of urine can react with chlorine to form strong-smelling trichloramines. Pool operators can enforce hygiene rules and educate guests on their impact to help maintain water cleanliness, reduce chemical odor, and improve air quality.
Are there alternatives to chlorine that can help reduce the pool smell?
While chlorine remains the most widely used disinfectant for pools due to its effectiveness in killing germs and ease of application, several alternative systems can be used in conjunction with chlorine to reduce chloramine formation and minimize odor. Saltwater pools, for example, generate chlorine through electrolysis but often allow for better control of chlorine levels, which can result in fewer chloramines if properly maintained. Other options include UV or ozone systems that help oxidize contaminants and reduce chlorine demand, allowing for lower chlorine concentrations and improved air quality.
However, it’s important to note that none of these alternatives fully replace the need for some chlorine in pool disinfection, as they alone cannot consistently ensure complete sanitation. Combining these systems with proper chlorination and water testing gives the best results. For indoor pools especially, a multi-barrier approach to disinfection—meaning incorporating one or more supplemental systems—can substantially reduce the presence of odor-causing chloramines while maintaining comfortable and safe swimming conditions.