What is that pool 'chlorine smell' actually a sign of?
The sharp 'chlorine smell' and eye irritation in a pool come not, as most people assume, from too much chlorine, but from chloramines (combined chlorine). Chloramine forms when chlorine combines with nitrogen compounds from sweat, urine and skin residues in the water. The solution is not to reduce chlorine but to disinfect with a more effective form of chlorine, lowering chloramine formation at the source.
In this article
When you enter an indoor pool, the sharp smell that hits you registers in most people’s minds as “there’s plenty of chlorine in this pool.” The truth is almost the exact opposite.
Free chlorine and combined chlorine
Chlorine in pool water exists in two states:
- Free chlorine: the active chlorine that does the disinfecting work. This is what you want.
- Combined chlorine (chloramine): compounds formed when chlorine combines with nitrogen contaminants in the water (sweat, urine, skin shedding, cosmetics). Their disinfecting power is low, but they are responsible for odour and irritation.
That familiar “pool smell” is not the smell of free chlorine but of chloramines. It is also what stings the eyes, dries the skin and degrades air quality in indoor pools.
What is the smell a sign of?
Chloramine build-up is usually a sign of two things: a high contaminant load and/or free chlorine being insufficient to meet that load. So the sharp smell usually means not “too much chlorine” but an insufficiency of effective chlorine. Reducing chlorine further makes the problem worse; the real solution is to make the disinfection more effective.
Why does hypochlorous acid make a difference?
One way to reduce chloramine formation is to be able to deliver the same disinfecting power at a lower total chlorine. Here the form of chlorine comes into play.
Hypochlorous acid is the most effective disinfecting form of chlorine; at the same free-chlorine level it shows far higher biocidal efficacy than hypochlorite. In practice this means being able to disinfect the pool effectively at a lower total-chlorine level — and less total chlorine means less potential for chloramine formation.
A hypochlorous acid-dominant solution from on-site generation is therefore preferred in indoor pool operations to reduce odour and irritation complaints.
Getting rid of chemical transport
Alongside the chloramine problem, pool operations carry a second burden: transporting hypochlorite drums, keeping an acid-chlorine store, and the risk of expired chemicals. On-site generation removes this burden too; the operating input becomes nothing more than salt and electricity.
Summary
The sharp smell in a pool is not a sign of “plenty of disinfection” but usually a sign of imbalance. The right approach is not to flee from chlorine but to disinfect with a more effective form of chlorine, at the right dose and with continuous monitoring.
The pool chemistry in numbers
There is a measured reason why pH comes up so often in pool water.
The equilibrium chlorine establishes in water between hypochlorous acid and the hypochlorite ion is described by a dissociation constant: pKa ≈ 7.5.1 The 7.2–7.6 range targeted in pools sits precisely on the steep part of that curve — meaning a small pH deviation shifts the proportion of chlorine in the active form appreciably.
Concretely: at pH 7.2 roughly two-thirds of free chlorine is in the active hypochlorous acid form. At pH 8.0 that falls to about one quarter. Your test kit can read the same “free chlorine” figure in both cases.
The same relationship shows up in official disinfection tables. The US Environmental Protection Agency’s CT values show that at 10 °C and 1.0 mg/L free chlorine, 3-log inactivation requires 79 mg·min/L at pH 6 and 162 at pH 8 — pH alone doubles the required dose.2
The by-product side
Chloramines are not alone; the by-products forming in chlorinated water are a broader family. The reference review in this field establishes that by-product formation tracks the disinfectant’s dose and its contact time with organic matter.3
For a pool operator the practical consequence is this: the way to reduce the load is not more chlorine but less of the organic material chlorine reacts with — shower discipline, fresh water make-up, filtration and correct pH — together with keeping the chlorine you do dose in the active form as far as possible.
References
The peer-reviewed studies and official guidance documents cited in this article. Every citation has been verified against the primary record.
(1948). The Behavior of Chlorine as a Water Disinfectant. Journal AWWA, 40(10), 1051–1061.
What this source establishes: The foundational study describing the HOCl ⇌ OCl⁻ equilibrium in water and how pH shifts it. Its dissociation constant (pKa ≈ 7.5) is what makes it possible to calculate how much of a free-chlorine residual is actually in the active form.
Open source ↗ DOI 10.1002/j.1551-8833.1948.tb15055.x
(2003). LT1ESWTR Disinfection Profiling and Benchmarking Technical Guidance Manual — Appendix B: CT Tables. EPA Office of Water, EPA 816-R-03-004.
What this source establishes: The official CT (concentration × contact time) tables for free chlorine. At 10 °C and 1.0 mg/L, 3-log Giardia inactivation requires 79 mg·min/L at pH 6, 162 at pH 8 and 234 at pH 9 — the required dose doubles to triples on pH alone.
(2007). Occurrence, Genotoxicity, and Carcinogenicity of Regulated and Emerging Disinfection By-Products in Drinking Water: A Review and Roadmap for Research. Mutation Research — Reviews in Mutation Research, 636(1–3), 178–242.
What this source establishes: The reference review of drinking-water disinfection by-products. It establishes that by-product formation tracks the disinfectant’s dose and its contact time with organic matter — which is why placing the right dose at the right point is a water-quality question, not just an efficiency one.
Open source ↗ DOI 10.1016/j.mrrev.2007.09.001PMID 17980649