Chlorine, hair and skin: what it does and what actually helps

Almost everything written about chlorine and swimmers is written by someone selling a shampoo. That is why the same four tips circulate unchanged, why nobody explains the mechanism behind them, and why the single most repeated claim in the genre — that chlorine turns blond hair green — is wrong.

This page is built from primary sources: the disinfectant chemistry, what a study of an entire national swimming team found in their hair, what a measured change in skin barrier looks like after a two-hour session, and which protective steps have evidence rather than a plausible story behind them. Where the evidence runs out, it says so.

What is actually in the water

The chlorine in a pool exists in two forms, and confusing them is the root of most of the bad advice. Free chlorine is the part still available to kill germs. Combined chlorine is what is left after the chlorine has already reacted with something — and in a pool, that something is almost always the swimmers.

The Centers for Disease Control and Prevention recommends at least 1 part per million of free chlorine in pools, at least 3 ppm in hot tubs, and a pH between 7.0 and 7.8. The World Health Organization approaches the same problem from the other end and recommends that free chlorine in public and semi-public pools should not exceed 3 mg per litre, with hot tubs allowed up to 5 mg per litre. For scale, WHO's health-based guideline value for chlorine in drinking water is 5 mg per litre. A correctly run pool is not a chemical bath; it holds less chlorine than the water authorities will let you drink.

The chlorine smell is not chlorine

If you can smell "chlorine" at a pool, you are mostly smelling chloramines. The CDC states this directly: chloramines form when chlorine combines with pee, poop, sweat, dirt, skin cells, deodorant and makeup washed off swimmers' bodies. A strong smell is therefore a sign of a pool carrying a heavy load of body waste, not a sign of a pool that has been over-chlorinated.

The chemistry is unglamorous. Urine averages about 10,240 mg of urea and 560 mg of ammonia per litre, and chlorine reacts with ammonia extremely rapidly to produce monochloramine, dichloramine and nitrogen trichloride.

Nitrogen trichloride is the one that causes trouble, because it is volatile: it leaves the water and sits in the air just above the surface, which is exactly where your face is. WHO notes it has an intense and unpleasant odour at concentrations in water as low as 0.02 mg per litre, and sets a provisional guideline value of 0.5 mg per cubic metre for chlorine species in the air of indoor pools — the level at which complaints from non-swimming pool users began in the study behind the figure.

Why your eyes sting, and why goggles beat every other fix

Chloramines irritate skin and eyes in the water, and once airborne they irritate the nose and airways too. Anyone at the pool — swimmers, spectators, staff — can get nasal irritation, coughing and wheezing.

No home remedy changes the water. What you can change is contact time and the barrier between the irritant and your cornea: goggles that seal, worn for the whole session rather than pushed up onto your forehead between sets. If yours fog so badly that you keep taking them off, that is not a cosmetic problem — it is the reason your eyes are exposed, and it has a physical fix covered in the guide on goggle fogging.

What chlorinated water does to hair

The most useful study on this subject examined 67 elite swimmers from the Japan National Swimming Team against 54 age-matched controls. Hair discoloration was present in 61% of the swimmers and 0% of the controls. Surface damage to the nail plates showed up in the same swimmers — a detail that quietly tells you the mechanism is not specific to hair.

Under the electron microscope, the swimmers' discoloured hair had lost its cuticle completely. In the cortex underneath, the melanosomes — the packets that hold hair pigment — were fewer, smaller and irregularly shaped, many of them with pale haloes. Elemental chlorine was detected inside the melanosomes of every discoloured swimmer's hair and in none of the controls. A test of pigment production at the hair root found no difference between swimmers and controls, which rules out the hair simply growing in lighter.

The authors' conclusion is the part almost nobody repeats: the discoloration was mainly due to cuticle damage by friction with water, after which hypochlorous acid could penetrate to the cortex and oxidise the pigment already there. The chemistry does the bleaching, but the mechanical wear of moving through water at speed, hour after hour, is what opens the door.

It also explains the pattern swimmers notice: the damage concentrates at the ends, the oldest and most-worn part of the fibre, and it is far worse in hair that was already chemically treated.

Alongside this, the American Academy of Dermatology describes two direct chemical effects: chlorine dissolves the hair lipids that coat and protect the shaft, and it can break the chemical bonds of the hair protein, which leads to more breakage. The cuticle is also the layer that manages the fibre's response to moisture — it swells as it absorbs water and does most of the fibre's moisture handling at high humidity — so losing it changes how hair takes up and holds water. That is the honest answer to the common question about hair feeling curlier or coarser after a swimming block: the fibre's outer layer is thinner and handles water differently. It is a mechanism, not a measured outcome.

The green tint is copper, not chlorine

Chlorine does not turn hair green. Copper does. In the dermatology literature the condition is called chlorotrichosis, and it is defined as the accumulation of insoluble copper deposits fixed to the hair cortex in people with light-coloured hair and pre-existing hair damage. The usual source is a pool treated repeatedly with copper-based algaecides without adequate replacement of the water — or, less often, domestic water with a high copper content.

A 2022 case in the International Journal of Trichology puts numbers on it. A woman developed green strands after swimming in a community pool whose pH and chlorine were both in range, but whose copper measured 3 ppm. For reference, the United States Environmental Protection Agency sets its action level and health goal for copper in drinking water at 1.3 mg per litre. The pool had not had a full water change in seven years; after replacing a third of the volume, copper fell to 1.1 ppm. Where the case occurred, chlorine and pH testing is mandatory for pools while copper testing is only suggested — which is a good part of why this keeps happening.

Two further details are worth having. First, prior damage to the cuticle appears to be essential: the copper binds to the exposed cortex, and the hair that greens is overwhelmingly hair that has been bleached or highlighted. The 2022 case had self-applied highlights days earlier, and a 2024 case in Skin Appendage Disorders describes the same pattern in a teenager who had just started swimming regularly. Second, fewer than ten cases involving pools with clearly raised copper have been published at all, so this is a real but uncommon event — not the default outcome of swimming with blond hair.

The treatment in the 2024 case was a shampoo containing EDTA, a chelating agent that binds metal ions so they can be rinsed away; the pigmentation regressed significantly over eight weeks. The prevention was a conditioner enriched with quaternary ammonium compounds applied before exposure — the evidence-backed version of "put conditioner on before you swim", working by occupying the binding sites on damaged hair rather than forming a waterproof coat.

What actually protects hair, ranked by how well it is evidenced

To protect hair from chlorine, soak it in clean water before you get in, coat it with a conditioner or light oil, cover it with a silicone cap, and rinse and wash it within minutes of getting out. That is the sequence the dermatology bodies recommend. How well each step is supported varies, and the differences matter.

Two things to stop doing matter as much as anything on that list. Do not towel-dry by rubbing: friction is the mechanism the national-team study identified. And if you swim five or six times a week, bleaching or highlighting puts your hair in exactly the condition the literature identifies as susceptible — to breakage, to hypochlorous acid, and to copper.

What chlorinated water does to skin

Skin has a measurable version of the same story. A 2021 study compared 58 elite athletes — swimmers against footballers — measuring transepidermal water loss, the standard proxy for how well the skin barrier holds water in, before a two-hour session, immediately after, and thirty minutes later.

The swimmers rose significantly at the forearm, the inner elbow and the back of the hand. The footballers changed only on the back of the hand, and the swimmers' change was the larger of the two at forearm and inner elbow. In plain terms: one two-hour pool session measurably loosens the skin barrier, and outdoor training of the same length does not do it the same way.

That gives the familiar advice a reason. Shower with fresh water and a mild cleanser as soon as you are out, pat rather than rub, and moisturise while the skin is still damp — you are replacing a barrier just measured to be more permeable than it was two hours ago. If you have eczema or atopic skin, ask a dermatologist for a routine built for you rather than assembling one from swimming forums.

The rash that is not a chlorine rash

An itchy, bumpy rash that appears a day or two after swimming, with pus-filled spots around hair follicles and worst where your swimsuit sat, is probably not a reaction to chlorine. That is the CDC's description of hot tub rash, caused by Pseudomonas aeruginosa. It appears days after using a poorly maintained hot tub, lake or pool, and it is worse in areas where a swimsuit held water against the skin.

The significance is the direction of the error. Pseudomonas is readily inactivated by chlorine, and CDC guidance is that maintaining at least 1 ppm of free chlorine in public venues prevents waterborne transmission of most pathogens, this one included. So the rash people call a chlorine rash is, more often than not, evidence of too little effective disinfectant, or of a hot tub whose chemistry has drifted. Mild cases usually clear up in a few days without treatment, and the CDC's advice is to see a healthcare provider if a rash gets worse or if you get it more than once. The prevention is mundane: get out of the wet swimsuit, shower with soap, and wash the suit.

Chlorine-related skin complaints, in contrast, tend to be dryness, tightness and itch spread over the skin that was in the water, arriving the same day rather than two days later, and easing with a moisturiser.

Is it a chlorine allergy?

Usually not, in the strict sense. Most reactions to pool water are irritant rather than allergic: chloramines and the loosened skin barrier above make plenty of people itchy with no immune sensitisation involved. That is not a distinction to settle from a web page, and it matters if the reaction is severe, involves hives or swelling, or affects your breathing. Those are reasons to see a doctor.

The airway side deserves the same caution. WHO links symptoms in swimmers and pool staff to the atmosphere of indoor pools, notes the studies could not confirm which chemical was responsible, and says symptoms are likely to be particularly pronounced in people with asthma. If an indoor pool consistently sets off your asthma, tell your clinician exactly that, including whether the ventilation is poor and the smell strong.

Children, and the mistake adults make on their behalf

Children spend more time in the water per unit of body surface, swallow more of it, and are the group least likely to shower first, so both halves of the CDC's guidance apply more sharply to them.

The recurring adult mistake is scrubbing. A child comes out smelling of the pool and the instinct is a vigorous wash with something strong. That smell is chloramine residue on skin whose barrier has just been loosened; a quick rinse, a mild cleanser and a moisturiser do more than a scrub. The same goes for hair: rinsing immediately beats washing aggressively later.

If you are an adult swimmer yourself, the other half of this is the pre-swim shower, which is the one thing every source on this page agrees about. The CDC says rinsing off for one minute removes most of the dirt on your body, and WHO lists pre-swim showering as a primary control on the formation of disinfection by-products. It is the rare piece of advice that protects your own hair and skin and makes the water better for everyone else. If you are still getting comfortable with pool routines generally, the guide to learning as an adult covers the rest of the etiquette.

The lever nobody mentions: fewer hours for the same fitness

Every effect on this page scales with time in the water: cuticle wear is cumulative friction, barrier disruption was measured after a two-hour session, chloramine exposure is a dose over minutes. So the largest single change available to most adult swimmers is not a product — it is getting the same training out of less pool time.

That is a solvable problem, and it is the part of this site built for it. Work out your critical swim speed with the CSS calculator and you get a pace to anchor sets to, instead of swimming an undifferentiated middle speed for an hour. The workout generator builds sessions around that pace, and the interval calculator sets send-offs that keep the rest honest, so a focused 45-minute session does the work of a vague 75-minute one. If you want to see what your current times imply per 100, the pace calculator does that conversion.

The same reasoning sits behind how many laps actually make a good workout and the case for not swimming every single day. Less time in chlorinated water for more training effect is a straight win, and it is also how people avoid the other slow-accumulating pool problems, such as swimmer's ear and shoulder pain from volume.

Indoor pools, outdoor pools, and why one smells worse

Nitrogen trichloride is volatile, so what matters is whether it can leave. Outdoors it disperses; indoors it accumulates in the layer of air just above the water unless ventilation removes it, which is why WHO's provisional air guideline is written specifically for indoor pools.

So an indoor pool that smells strongly as you walk in is telling you about its ventilation and its bather load before you have swum a length, and an outdoor pool with identical water chemistry will feel milder. Pool length matters far less: a 50-metre pool means fewer turns and less churned water at the wall than the same distance short course, but that is minor next to how long you are in. And swimming open water trades chloramines for a different set of considerations rather than escaping to something with no trade-offs.

What to do this week

Shower for a minute before you get in, hair soaked through with conditioner or a light oil in it, under a cap. Wear goggles that seal for the whole session. When you get out, rinse straight away, wash with a mild cleanser, pat dry, and moisturise while your skin is damp. Add a chelating shampoo once a week if you swim often. That is the whole routine, and every step traces back to something above.

If your pool consistently smells strong and stings, say so to the operator in those words — that it smells of chloramines and that the air just above the water is the problem. That is a legitimate complaint about ventilation and turnover, and far more likely to get somewhere than asking for less chlorine. The rest of the technique section covers what to do with your time in the water once skin and hair are no longer what you are thinking about.