What Chlorine Kills, and What It Does Not

Chlorine is extremely good at killing most of what gets into a swimming pool, and almost useless against one particular parasite. Knowing which is which changes how you respond to an incident, and it is the difference between a pool that is genuinely safe and one that merely looks it.

What chlorine does

Free chlorine attacks the cell walls and internal chemistry of micro-organisms. At the levels a properly maintained pool holds, most bacteria are killed within a minute or less, and many viruses within a few minutes.

E. coli, the bacterium most associated with fecal contamination, is destroyed in under a minute at ordinary chlorine levels. So is most of what swimmers carry in on their skin. This is why a well-maintained pool is a genuinely low-risk place to swim, and it is worth saying plainly because articles on this subject tend to be alarming without being useful.

The word “properly” is doing a great deal of work in that sentence, and the rest of this article is largely about what it means.

What chlorine struggles with

Some organisms have protective outer shells that chlorine penetrates slowly or not at all in practical timeframes.

Cryptosporidium — “crypto” — is the significant one. Its oocysts have a tough outer wall, and it survives at ordinary pool chlorine levels for days rather than minutes. It is the leading cause of outbreaks linked to treated recreational water in the US, and it is why a diarrhoeal incident is treated completely differently from an ordinary contamination.

Giardia is more resistant than bacteria but far less resistant than crypto, typically requiring tens of minutes rather than days.

The practical consequence is that a pool with correct chlorine is not automatically free of everything. The response to a diarrhoeal incident is a hyperchlorination procedure with a specified concentration and contact time — the CDC publishes the figures in its prevention guidance, and it is worth having read before you need it.

Why pH matters as much as chlorine level

This is the part that is most often missed, and it undermines everything else when it is wrong.

Chlorine exists in water in two forms. Hypochlorous acid does the sanitising. Hypochlorite ion is far weaker. The balance between them is decided by pH alone: at 7.2 roughly two-thirds is in the active form, at 8.0 only about a quarter.

So a pool at pH 8.2 holding 3 ppm of free chlorine has roughly the disinfecting power of a pool at pH 7.4 holding under 1 ppm. The test kit reads 3 either way. Nothing on the strip tells you that three-quarters of it is inert.

This is why pH control is a health measure rather than a cosmetic one, and why “we keep the chlorine up” is not by itself a statement about safety.

And why stabiliser matters too

Cyanuric acid protects chlorine from sunlight by binding to it. Bound chlorine is held in reserve, which is useful — but it is not available to disinfect while it is bound.

The practical effect is that the free chlorine level you need rises with your stabiliser level. The working rule is a minimum of about 7.5% of the cyanuric acid reading. At 30 ppm CYA that is 2.3 ppm; at 90 ppm it is nearly 7 ppm.

A pool at 90 ppm CYA holding 2 ppm of free chlorine reads as “chlorinated” on any test strip and is, in disinfection terms, substantially under-sanitised. This is the most common way a home pool ends up unsafe while its owner is doing what they believe is the right thing, and it is covered in cyanuric acid explained.

The smell tells you the opposite of what people think

A pool that smells strongly of chlorine almost always has too little free chlorine, not too much.

What you are smelling is chloramines — chlorine that has already combined with sweat, urine, skin cells and other nitrogen compounds. Chloramines are what irritate eyes and airways, and they form precisely because there was not enough free chlorine to complete the reaction and destroy them.

A properly sanitised pool smells of almost nothing. The strong “chlorine smell” people associate with swimming pools is a sign that the pool needs shocking, not that it has been over-treated. See why your pool smells of chlorine.

Indoor pools are where this becomes a genuine health issue, because chloramines accumulate in the air above the water with nowhere to go. Respiratory irritation at indoor pools is a ventilation and chloramine problem, not an excess of chlorine.

What people actually bring into the water

Not a pleasant subject, but the numbers explain the recommendations.

Every swimmer carries a measurable quantity of fecal matter on their body into the water. It is a trace amount per person, and chlorine handles it comfortably — but it is why a rinse shower before swimming makes a genuine difference, and why it is standard practice at public pools.

Sweat and urine are the larger contributors to chloramines. They contain nitrogen compounds that bind chlorine, consuming sanitiser and producing exactly the irritation people blame on chlorine itself.

Sunscreen, body oils, cosmetics and hair products all add organic load that chlorine must work through. A busy afternoon can measurably drop a pool’s free chlorine, which is why testing after heavy use is worthwhile.

The rules that actually reduce risk

• Rinse before swimming. A one-minute shower removes most of what would otherwise consume chlorine.
• Do not swim with diarrhoea, and do not let children who have had it swim for two weeks afterwards. Crypto can be shed for that long.
• Take children on bathroom breaks regularly rather than asking whether they need to go.
• Change diapers away from the pool, in a bathroom, not on the poolside.
• Do not swallow the water. Obvious, frequently ignored by children, and the main route of transmission.
• Test before anyone swims, particularly after heavy use, heavy rain or several days of neglect.

These are the CDC’s healthy swimming recommendations in short form, and they are unglamorous precisely because they work.

Responding to an incident

The response depends entirely on what happened, and treating everything the same is a mistake in both directions.

The diarrhoeal case is the one that requires the full procedure, because that is the one likely to carry crypto. The required concentration and contact time are published by the CDC and they are considerably more demanding than an ordinary shock — this is not a case for approximating.

Other water-related illness

Beyond the gastrointestinal, two others are worth knowing about.

Pseudomonas causes hot tub rash and swimmer’s ear. It thrives in warm water with low sanitiser, which makes spas and heated pools the usual setting. Proper chlorine or bromine levels prevent it; warm water with a neglected reading is where it appears.

Legionella is a risk in warm water systems that aerosolise — hot tubs and spas rather than pools. It is a respiratory infection contracted by breathing in contaminated droplets rather than by swallowing water. Maintaining sanitiser in a spa is a different order of importance from maintaining it in a pool, because spa water is warm, heavily aerated and holds far less water per bather.

What “properly maintained” means in practice

Reduced to the essentials, a pool is doing its job if four things are true.

1. Free chlorine is at or above 7.5% of the cyanuric acid reading, at all times rather than on average.
2. pH is between 7.2 and 7.6, so that the chlorine present is actually active.
3. Combined chlorine is under 0.5 ppm, so there is no backlog of chloramines.
4. The water turns over at least once a day, so sanitiser reaches every part of the pool.

All four are measurable and none of them is expensive. A pool meeting all four is a safe place to swim; a pool failing any one of them may look identical and not be.

Spas and hot tubs are a different problem

Everything about a spa works against sanitiser, and owners often apply pool habits to water that behaves nothing like pool water.

The water is warm, which speeds up biological growth and consumes chlorine faster. It is heavily aerated, which drives chlorine off and pushes pH up continuously. And there is vastly less water per person — a four-person spa may hold 400 gallons against a pool’s 20,000, so each bather represents fifty times the load.

The practical consequence is that spa water needs testing before each use rather than twice a week, and sanitiser demand after a single session can exceed a pool’s demand over a fortnight. Bromine is often preferred over chlorine in spas because it is more stable at high temperature, though it needs its own routine.

Two organisms find spas particularly congenial. Pseudomonas causes the itchy rash people call hot tub folliculitis, and it thrives in warm water with depleted sanitiser. Legionella is the serious one: a respiratory infection contracted by breathing aerosolised droplets rather than by swallowing water, which is exactly what a jetted spa produces in quantity.

Neither is a reason to avoid spas. Both are reasons to treat a spa as requiring more attention than a pool rather than less, which is the opposite of how most people treat them.

Why public pool rules exist, and which are worth copying

Public pools operate under health codes that private pools do not, and several of those requirements are worth adopting at home because they address real risks cheaply.

Testing frequency. Commercial pools test several times a day. A home pool does not need that, but testing before use after a period of neglect is a habit worth copying — particularly after heavy rain or a gap of several days.

Exclusion after illness. The two-week rule after diarrhoea is not caution for its own sake; it reflects how long cryptosporidium can be shed. It applies at home exactly as it does at a public pool, and it is the single most effective thing a household can do to prevent an outbreak among its own guests.

Rinse showers. Standard at public pools, almost unknown at home, and genuinely effective. A one-minute rinse removes most of the sweat, sunscreen and body oil that would otherwise consume chlorine and form chloramines.

Written procedures for incidents. Commercial operators have a documented response for a fecal incident. Having read the CDC procedure once, before you need it, is the home equivalent — and it takes ten minutes.