Free chlorine is not the main aggressor in a spa. It is the chloramines, by-products of the reaction between chlorine and organic matter, that concentrate the majority of health and material nuisances. Understanding this mechanism radically changes the approach to water treatment in balneotherapy.
Chloramines in a spa: the mechanism that dosage instructions overlook
When chlorine comes into contact with sweat, dead skin cells, urine, or cosmetic residues, it forms chloramines (monochloramine, dichloramine, trichloramine). In a pool at room temperature, these compounds disperse in a large volume of water and partially evaporate at the surface.
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In a spa, three factors accelerate their accumulation. The volume of water is reduced, the temperature generally exceeds 35 °C, and the constant agitation from the jets projects the chloramines into the surrounding air. Trichloramine, volatile and irritating, is found in much higher concentrations in the respiratory zone of bathers than at the edge of an outdoor pool.
It is these chloramines that cause the characteristic odor wrongly attributed to “pure” chlorine. The stronger the odor, the higher the chloramine load, which paradoxically signals that the residual free chlorine is insufficient to oxidize the contaminants. We regularly observe this confusion among users who reduce their dosage thinking that “too much chlorine” causes irritation, while the problem stems from a lack of oxidizing power against the organic load.
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An article detailing the dangers of chlorine for a spa confirms this reality: most of the inconveniences attributed to chlorine actually result from a chemical imbalance amplified by the thermal conditions of the basin.
Skin and respiratory risks associated with chlorine in hot water
Dermatologists distinguish two types of reactions. Contact irritation, the most common, manifests as redness, itching, and dry skin after bathing. It particularly affects atopic or weakened skin. Chronic irritative dermatitis can develop in daily users of chlorinated spas.
The skin barrier weakens faster in chlorinated hot water than in cold water. Heat dilates the pores, increases the permeability of the epidermis, and facilitates the penetration of chemical agents. Cosmetics (sunscreens, body oils) left on the skin before immersion exacerbate the formation of chloramines upon direct contact with the skin.
From a respiratory standpoint, inhaling trichloramine in a closed room or under a spa shelter can cause a dry cough, chest discomfort, and irritation of the nasal mucous membranes. Children and asthmatic individuals are the most exposed. Ventilation of the spa area is often an underestimated preventive measure compared to the choice of disinfectant alone.
Chlorine and pH interaction in a small volume
Chlorine only acts effectively within a pH range of 7.2 to 7.6. Beyond that, hypochlorous acid (the active form) transforms into hypochlorite ion, which is significantly less biocidal. In a spa, the pH drifts faster than in a pool due to agitation, CO₂ degassing, and temperature.
Poorly managed pH forces an increase in chlorine dosage to maintain sufficient disinfecting power, which mechanically increases chloramine production. We recommend measuring pH before each addition of disinfectant, not after.
Alternatives to chlorine for spa treatment
No alternative is perfect, but several significantly reduce exposure to chlorinated by-products.
- Bromine: effective at a wider pH (7.0-7.6) and at high temperatures. The bromamines formed remain disinfectant, unlike chloramines. Bromine generates less odor but can degrade certain internal components (seals, pumps) in cases of repeated overdosing.
- Active oxygen (hydrogen peroxide or potassium persulfate): a powerful oxidizer without persistent residue, suitable for sensitive skin. Its limited duration of action requires more frequent dosing and a supplement of UV or ozone for stable results.
- Combined UV or ozone systems: these devices destroy microorganisms through irradiation or gas oxidation in the piping. They reduce the amount of chemical disinfectant needed without eliminating it entirely, as they do not ensure residual action in the basin.
- Salt electrolysis: generates chlorine in situ at low concentration and continuously, which limits peaks of chloramines. The installation cost is higher, and the generator requires regular maintenance of the cell.
Selection criteria based on spa type
For an inflatable spa used seasonally, active oxygen combined with frequent water renewal is sufficient in most cases. The small volume makes complete drainage easy every few weeks.
For a rigid spa used intensively, the combination of ozone or UV with a residual of bromine offers the best compromise between biocidal efficiency and skin comfort. Ozone alone does not guarantee disinfection in the basin, as its action ceases as soon as it exits the treatment cell.
Reducing chloramines without changing disinfectant
For installations already equipped with chlorine, three measures effectively reduce exposure to chloramines:
- Soapy shower before each immersion: it reduces the organic load (sweat, cosmetics, dead skin) and limits the formation of by-products.
- Active ventilation of the area: sufficient air renewal evacuates volatile trichloramine and protects the respiratory tract, especially in enclosed spaces.
- Regular shock treatment (superoxidation): it destroys accumulated chloramines and restores the disinfecting power of free chlorine. The frequency depends on the intensity of spa use.
Partial water renewal remains the most effective action. Regularly diluting the spa water lowers the concentration of chlorinated by-products and dissolved substances that filtration does not capture.
The choice of disinfectant matters less than the rigor of the maintenance protocol. A chlorine-treated spa with stable pH, proper ventilation, and bathers who shower before entering will produce fewer chloramines than a neglected bromine spa. The chemistry of the water in a small heated basin does not tolerate approximation.