Introduction
You walk into a hotel's indoor pool hall and you notice it immediately: that strong smell everybody calls "the chlorine smell". The glazing is fogged, there is condensation on the metalwork, and a guest at reception mentions their eyes were stinging.
Almost everyone reaches the same conclusion: there is too much chlorine in the water. It is exactly the other way round. The strong smell does not come from free chlorine — it comes from chlorine that has already been used up. Which is why the usual response, cutting the chlorine, makes the problem worse instead of fixing it.
This article explains the mechanism, why treating only the ventilation never works, and in what order the problem gets attacked on an indoor hotel pool.
What Actually Causes the Smell
Disinfectant in a pool exists in two forms, and the distinction is everything.
Free chlorine is the available disinfectant, the part that can still do work. At normal operating levels it is practically odourless.
Combined chlorine, also called chloramines, is chlorine that has already reacted with organic matter brought in by bathers: sweat, urine, sun cream, skin cells, cosmetics. It no longer disinfects. It is volatile, irritating to eyes and airways, and it is the source of the smell.
The practical consequence is counter-intuitive:
| Situation | Strong smell? | Why |
|---|---|---|
| Free chlorine high, combined low | No | Available disinfectant is practically odourless |
| Free chlorine low, combined high | Yes, badly | Chloramines accumulated, no disinfectant left to destroy them |
| Free and combined both high | Yes | Heavy organic load with dosing keeping up |
In other words: a pool that "smells of chlorine" typically has too little free chlorine and too much combined chlorine. Cutting the dose because it smells bad is precisely the wrong decision.
Why Ventilation Alone Never Fixes It
Here is the mistake that costs hotels the most money.
The smell is noticed, the HVAC company is called, the air-change rate is increased. The smell drops — for a week. Then it comes back, now with a bigger energy bill, because every cubic metre of fresh air in a pool hall is air that had to be heated and dehumidified.
The reason is simple: ventilation removes chloramines from the air, but does nothing to stop them being produced in the water. It is extracting smoke without putting out the fire.
The reverse is also true, and it is the symmetrical mistake. Treating only the water, in a space with insufficient air renewal, lets chloramines accumulate in the layer of air immediately above the surface — precisely where a swimmer's face is. Immaculate water with poor ventilation still gives you an unbreathable hall.
Both sides have to be treated, and the water first, because the water is what produces the problem.
Diagnosis: What to Measure
Before touching any equipment, measure five things and write them down:
| Measurement | Working value | What it reveals when out of range |
|---|---|---|
| Free chlorine | 1 to 3 mg/L, upper third under heavy use | Below: no capacity to destroy chloramines |
| Combined chlorine | As low as possible; watch above 0.5 mg/L | Above: the direct source of the smell |
| pH | 7.2 to 7.6 | Above 7.6, the disinfectant loses much of its effectiveness |
| Relative humidity | Typically 50 to 65% in pool halls | Above: condensation, corrosion, a stifling feel |
| Air vs water temperature | Air 1 to 2 °C above the water | Air cooler than the water sharply increases evaporation |
That last row is the one nobody checks and it explains half of all condensation cases. If the air is cooler than the water, evaporation accelerates, humidity climbs, and the whole dehumidification system is working against physics.
The Water Side: Five Actions, in This Order
1. Correct the pH first. Above 7.6, free chlorine loses much of its oxidising power. Without this, everything that follows returns less.
2. Bring free chlorine back into the upper third of the range. Counter-intuitive, but this is what destroys chloramines. Cutting the dose because it smells bad feeds the cycle.
3. Shock treatment to break down accumulated chloramines. Done outside bathing hours, with continuous filtration. It is the action that brings combined chlorine down measurably.
4. Increase water renewal. Fresh water dilutes what has built up — dissolved organic matter, by-products, cyanuric acid if you use stabilised chlorine. On an indoor pool in heavy use, it is the most neglected parameter of all.
5. Cut the organic load at the door. Compulsory showers before entering the water, signed and actually enforced. On a hotel pool this is the single best cost-benefit item on the list, and it costs nothing beyond two notices and staff insisting.
The Air Side: Four Checks
Air-change rate. A pool hall needs continuous renewal, not intermittent. Systems set to run on a timer let chloramines build up during the hours they are off.
Where the air is extracted. Chloramines are denser than air and concentrate just above the water surface. Extraction mounted at the highest point of the ceiling removes clean air and leaves the loaded air where the people are. It is a common design error and it explains halls with powerful HVAC and unbreathable air.
Supply air across the glazing. Dry supply air run along glazed surfaces is what stops condensation on the glass. If the glazing is permanently fogged, this is where to look.
Air temperature above water temperature. One to two degrees above. It is a setpoint adjustment, not a building project, and it reduces evaporation, humidity and dehumidification load all at once.
The Hidden Cost: Corrosion
The smell is the symptom guests notice. Corrosion is what actually costs money.
Chloramines in a humid environment attack metal, and attack it fastest wherever there is condensation. In a badly balanced indoor pool, the wear concentrates in predictable places:
- Ladder, handrail and starting-block fixings
- Roof steelwork and lighting supports
- Ducting, grilles and components of the air-handling unit itself
- Door ironmongery and window frames
- Electrical panels in the plant room
A hall running constantly above 70% humidity with high combined chlorine degrades steelwork in years, not decades. It is the argument that convinces management when the smell on its own does not: this is not a comfort question, it is an asset question.
The Economics of the Decision
The usual objection to increasing air renewal is the energy cost, and it is a legitimate objection. But the right comparison is not "ventilate more" against "ventilate less". It is:
- Treating the water properly reduces chloramine production, so it reduces the air renewal needed. That is a saving, not a cost.
- Setting the air one or two degrees above the water reduces evaporation, so it reduces dehumidification work.
- Compulsory showers cut the organic load at the door, at zero energy cost.
Only after those three does it make sense to discuss increasing the air-change rate — and by then the increase needed is usually smaller than what would have been proposed at the start.
We cover the residential case in Indoor Pools: Ventilation and Humidity and the smell itself in Strong Chlorine Smell in the Pool.
Frequently Asked Questions
Does a chlorine smell mean there is too much chlorine?
No, it typically means the opposite. The smell comes from combined chlorine — chlorine already spent reacting with organic matter — and it builds up precisely when there is not enough free chlorine left to destroy it. Cutting the dose because of the smell makes it worse.
Does more ventilation fix the problem?
It relieves the symptom without treating the source, because chloramines keep being produced in the water. On top of that, every cubic metre of fresh air carries an energy cost in a heated hall. The right sequence is to treat the water first and adjust the air afterwards.
What relative humidity should an indoor pool hall have?
Typically between 50 and 65%. Below that, evaporation and heating cost climb; above it, you get condensation, corrosion and discomfort. The exact figure for your installation depends on the design and should be set with whoever sized the air-handling unit.
Why is the glazing always fogged?
Two usual causes, and often both together: relative humidity too high in the hall, and no dry supply air run along the glazed surfaces. The second is a question of air distribution, not of equipment capacity.
How often should combined chlorine be measured?
On an indoor pool in public use, every day it is open. It is the parameter that gives the earliest warning: it rises before guests notice the smell, and long before it turns into a complaint.
Guests say their eyes sting. Is that the chlorine?
It is the chloramines, and frequently a pH out of range as well. Eyes are a reasonably reliable sensor of unbalanced water — but they cannot tell you the cause, which is why you measure instead of guessing.
Can you work without closing the pool to guests?
The measurements and dosing adjustments, yes. The shock treatment that breaks down accumulated chloramines needs the pool out of service and is done outside bathing hours, in a window agreed with management.
Conclusion
An indoor pool's smell is a water problem that shows up in the air. Treating only the air is extracting smoke without putting out the fire.
Key points to remember:
- The smell comes from combined chlorine, not free chlorine. Cutting the dose makes it worse.
- Correct the pH, restore free chlorine, break the chloramines with a shock, renew water.
- Compulsory showers before the water are the most profitable measure of all.
- Air should sit one to two degrees above water temperature.
- Corrosion, not the smell, is the real cost of a badly balanced hall.
If you run an indoor pool with a strong smell, condensation or guest complaints, the on-site technical diagnosis measures the five parameters in the table above and tells you in writing what is causing what. It costs €45, deductible from your first month under contract, in Lisbon and on the South Bank.
