Almost everyone has walked into a pool hall, been hit by the smell, and concluded that the pool has a lot of chlorine in it. It is one of the most widely held beliefs about swimming pools and it is approximately backwards. Understanding why changes how you read a facility, and it changes what you ask for when something seems wrong.
Free chlorine and combined chlorine are different things
When a chlorine based disinfectant is dosed into water it exists, initially, in a form that is available to do disinfection work. Operators call this free chlorine. It is the useful fraction.
Free chlorine is chemically reactive, which is the entire point. When it meets nitrogen containing material in the water, it reacts with it. The products of those reactions are collectively called combined chlorine, and the individual compounds are chloramines. Combined chlorine is chlorine that has already been spent. It is a far weaker disinfectant, and several of the compounds in it are volatile, irritant and distinctly smelly.
So the total amount of chlorine in a pool tells you very little. The split between free and combined tells you a great deal, and it is exactly that split which operators test for and record. A pool can hold plenty of total chlorine while having very little free chlorine left to do any work.
The smell is not the disinfectant. The smell is the receipt for disinfectant already spent on contamination.
Where the nitrogen comes from
Nitrogen containing material enters a pool almost entirely on and in the people. Sweat. Urine. Skin cells and the natural oils on skin. Saliva. Personal care products including deodorants, moisturisers, hair products, make up and sunscreen. Some arrives on the supply water and on airborne dust, but the dominant source, by a wide margin, is bathers.
This is why bather load and combined chlorine track each other. A pool at the end of a heavily attended day, especially one with sessions where swimmers arrive straight from elsewhere without showering, will have more combined chlorine than the same pool at opening. It is also why the smell tends to be worst at exactly the times parents are most likely to encounter it, which is teaching and public swimming in the late afternoon.
Urine in pools is a genuine contributor and it is very common, particularly with small children. It is not a scandal and it is not worth moralising about. It is a design assumption that plant is specified around, and it is one of the reasons that toilet breaks before a session, and a genuine pre swim shower, are asked for. The point of saying it plainly is that "just go before you swim" is a chemistry instruction, not a politeness.
What combined chlorine actually does to people
Chloramines are irritants. In practice, the effects people report in halls with high combined chlorine and poor ventilation are stinging or reddened eyes, a scratchy throat, a cough, and irritation of the airways that can be more pronounced in people with asthma. Competitive swimmers, teachers and lifeguards notice it most because they are there for hours rather than for a forty minute session.
We are not going to tell you how to manage a respiratory symptom, and we will not characterise anybody's cough. If swimming reliably triggers breathing difficulty, chest tightness or a cough that persists after you leave, that is a conversation with a GP or a pharmacist, not with a website. If you have asthma and a particular pool consistently makes it worse, your asthma review is the right place to raise it. See NHS or speak to a pharmacist.
Indoors, this is as much an air problem as a water problem
Chloramines leave the water. That is the whole reason you can smell them. Outdoors they disperse into the sky and nobody notices. Indoors they accumulate in the layer of air immediately above the water surface, which is precisely where swimmers breathe, and stay there unless the ventilation system moves them out.
This produces a specific and useful diagnostic. A hall that smells strongly at the water surface but much less on the spectator balcony is telling you something about air movement at the water line. A hall that smells strongly everywhere is telling you something about the ventilation system as a whole. Neither is a swimmer's problem to solve, but both are reasonable things to raise, and both are recognised issues in Health and Safety Executive, HSG179 Health and safety in swimming pools and in the technical guidance published by Pool Water Treatment Advisory Group.
What does a strong smell tell me here?
It tells you two things at once and you cannot separate them from the poolside: that combined chlorine is high, and that the air handling is not removing it. Either could be the dominant problem.
It is a reasonable thing to raise with the operator, and it is a more serious signal indoors than outdoors because the exposure is sustained and concentrated at breathing height.
Outdoor pools rarely smell, because the compounds disperse. That is a genuine advantage and it is one reason lidos feel pleasanter to swim in than a badly ventilated hall.
But absence of smell outdoors does not mean absence of combined chlorine. The water chemistry problem can be identical. You simply cannot smell it, so the only signal is the operator's testing.
What actually reduces it
The fixes are all on the operator's side or on the swimmer's side, and they divide cleanly.
| Approach | Who does it | What it is |
|---|---|---|
| Reduce what comes in | Swimmers, prompted by the operator | A genuine pre swim shower, using the toilet first, not swimming with heavy cosmetic or hair product on |
| Remove it from the water | Operator | Additional treatment stages exist specifically to break down combined chlorine as water passes through the plant |
| Dilute it | Operator | Replacing a proportion of the pool volume with fresh water, which is a routine and planned operation |
| Get it out of the air | Operator | Ventilation designed to extract from the layer immediately above the water surface, and maintained |
| Maintain free chlorine and pH | Operator | Keeping the disinfection stage working properly so free chlorine is not exhausted |
Note the balance. Swimmers control one row. Operators control four. That proportion is typical of this subject and it is why the framing of this publication is what it is.
Doing something about a pool hall that smells and stings
Owns the input side: shower properly, use the toilet first, keep heavy product off. Can and should report the smell to the operator, in writing if it recurs. Can decide not to swim there. Cannot fix the plant or the ventilation and should not be made to feel it is their failing.
Owns the decision about whether to run the session at all. A teacher or coach who notices that a hall is stinging eyes across a whole squad has information the operator may not have, and passing it on is part of the job. Owns briefing swimmers about showering before the session.
Owns the water treatment, the additional treatment stages, the dilution regime, the ventilation system and its maintenance, the testing, and the decision to restrict or close. Owns the signage and facilities that make a pre swim shower realistic rather than theoretical.
Where a duty attachesHealth and Safety Executive, HSG179 Health and safety in swimming pools covers both water treatment and pool hall air quality. General duties to employees, including teachers, coaches and lifeguards working in that air, sit under the Health and Safety at Work etc. Act 1974.
Reading a pool as a swimmer
You cannot assess a pool from the poolside, and anyone who tells you they can read water chemistry by looking is overselling. But there are a few signals worth noticing, and they are worth noticing because they should prompt a question rather than a conclusion.
- A strong smell that hits you in the entrance rather than at the water line suggests the air handling is struggling, not that the pool is heavily dosed.
- Eyes stinging within a few minutes, across several swimmers rather than one, is a stronger signal than one person's discomfort.
- Absence of smell in an indoor hall is a good sign about the air. It is not a statement about the water, which you still cannot see.
- Showers that are cold, broken or bypassed by the layout will predictably produce higher contaminant input, whatever the signs on the wall say.
None of these are measurements and we will not dress them up as any. They are prompts for a question to a person who has the records. That question is set out in what gets tested and recorded.