Chloramines and combined chlorine

What chloramines are

Chlorine added to water forms hypochlorous acid, which is the molecule that does the disinfecting. It is called free available chlorine, and it is what a test kit's "free chlorine" reading measures.

Swimmers bring nitrogen compounds into the water — perspiration, saliva, urine, skin cells, cosmetics. When hypochlorous acid meets ammonia and other nitrogen compounds, it combines with them to form chloramines, also called combined chlorine. Chloramines are chlorine that has already reacted. They are weak disinfectants, they are strong irritants, and they are what produces the harsh smell people call "too much chlorine".

It is worth stating plainly: when someone complains that a pool has too much chlorine, it is almost always the opposite. A properly maintained free chlorine residual has very little odour. The smell is combined chlorine, and combined chlorine forms because there was not enough free chlorine to burn the contaminants through completely.

Why chloramines are a real problem, not just a smell

Chloramines are stable and persistent. That stability is the whole difficulty. Once formed, they linger, and their presence promotes the formation of more of them. Meanwhile they consume free chlorine without providing disinfection, which is the phenomenon called chlorine demand.

The failure pattern is recognisable and extremely common:

1. The owner tests and finds little or no free chlorine.
2. They add chlorine. Within hours the reading is near zero again.
3. They add more. Same result.
4. Each addition, being below the level needed to break through, produces more chloramine rather than less.
5. The water goes cloudy, or algae appears.
6. The owner now treats the cloudiness or the algae — the symptoms — while the underlying chloramine load goes untouched.

By the second or third round of this the pool has consumed a great deal of chemical, the water is worse than when it started, and the owner has concluded that something is wrong with the chlorine. Nothing is wrong with the chlorine. It is being consumed by exactly the thing it should be destroying, at a rate that partial doses cannot overcome.

Breakpoint chlorination

There is a threshold. Add chlorine gradually to water carrying chloramines and, for a while, the combined chlorine level rises and free chlorine barely moves. Keep adding, and at a certain point the chlorine begins destroying the chloramines faster than it forms new ones — the chloramines are oxidised away to gases and simple compounds, combined chlorine collapses, and free chlorine finally starts to register. That point is breakpoint.

The classic figure is that reaching breakpoint requires roughly ten times the measured combined chlorine level, added as free chlorine, in one dose. This is the arithmetic that makes chloramine problems expensive to fix and cheap to prevent, and it is why the dose must be delivered at once rather than spread across an evening. Half of a breakpoint dose is not half a cure; it is a larger chloramine problem.

How to test for it

Most test kits and strips report free chlorine. Diagnosing chloramines needs two numbers:

Combined chlorine above roughly 0.2 to 0.5 ppm is generally taken as the point at which shocking is warranted. A pool that smells strongly and has combined chlorine at 1.0 ppm or more needs a full breakpoint dose, not a routine shock.

If a strip only reports "total chlorine", it cannot distinguish a healthy pool from a badly chloraminated one, and a pool showing "chlorine present" on such a strip can be almost entirely combined chlorine.

Non-chlorine shock

Potassium monopersulfate — sold as non-chlorine shock — oxidises organic contaminants without adding chlorine, and it does so quickly enough that swimming can usually resume within an hour rather than the following day. It is genuinely useful for routine oxidation and for keeping the chloramine load down between chlorine shocks.

Two limitations are worth knowing. It is an oxidiser, not a sanitiser, so it does not replace maintaining a chlorine residual. And because it is itself an oxidiser, it interferes with some combined-chlorine test methods for a period after dosing, which can produce a falsely alarming reading. Test before dosing, not immediately after.

Preventing chloramine build-up

• Maintain a consistent free chlorine residual. Chloramines form when contaminants arrive faster than free chlorine can oxidise them. A steady residual keeps you ahead; a residual that repeatedly falls to zero guarantees you fall behind.
• Shock on a schedule and after events — heavy bather load, a party, a storm, a hot spell — rather than waiting until the water declares a problem.
• Reduce what goes in. A rinse before swimming genuinely reduces the nitrogen load, and it is the single most effective thing a household can change.
• Watch cyanuric acid. Excessive stabiliser slows chlorine's reactions, which makes chloramine problems both more likely and slower to clear. See why chlorine will not stay in the water.
• Keep the pH in range. Chlorine at pH 8.0 is a fraction as active as at 7.4, and sluggish chlorine is exactly how chloramines accumulate.

Indoor pools are a special case

Outdoors, chloramines are volatile and much of the load leaves the water into open air. Indoors it does not: chloramines accumulate in the air above the water, concentrate at the surface where swimmers breathe, and are the main reason indoor pool halls develop the harsh atmosphere associated with them. Corrosion of metal fittings and building structure in indoor pool rooms is largely a chloramine phenomenon too.

The remedies are the same chemistry plus ventilation. Indoor air quality is a distinct discipline; the EPA's overview of indoor air quality is a reasonable general starting point on why air exchange matters in an enclosed space with a large evaporating surface.

Chloramines in drinking water are a different thing

Worth separating, because it causes confusion. Many municipal water systems deliberately use chloramine as a residual disinfectant, at low and regulated levels, precisely because it is stable and persists through the distribution network. That is a controlled treatment choice governed by the National Primary Drinking Water Regulations, and it has nothing to do with the uncontrolled chloramine formation described on this page.

The practical consequence for pool owners is that filling from a chloraminated supply introduces some combined chlorine at the outset, which is worth knowing when a freshly filled pool tests oddly.

Summary

A strong chlorine odour, eye and skin irritation, chlorine that disappears within hours, and a rising sense that the pool is fighting back — these are one problem, not four. Test for combined chlorine, deliver a full breakpoint dose in a single addition with the pump running continuously, and prevent the recurrence by keeping the residual steady rather than by adding more chlorine after the fact.