What Causes Red Eyes After Swimming in a Pool?

Red, stinging eyes after swimming are caused by chloramines and by pH sitting outside the 7.2 to 7.8 range. They are not caused by too much chlorine, which is the assumption almost everyone starts with.

That distinction matters because the instinct is to add less chlorine, and that makes the problem worse. A pool that burns your eyes usually needs more chlorine, not less.

What chloramines actually are

Chloramines form when free chlorine combines with nitrogen compounds that swimmers bring into the water. Sweat, body oils, cosmetics, sunscreen and urine are the main sources. Once chlorine binds to those compounds it becomes a combined chlorine (a chloramine), which no longer sanitizes effectively and which irritates eyes, skin and airways.

That sharp smell people call a “chlorine smell” is chloramines, not chlorine. Properly maintained water has almost no odor. A strong pool smell and burning eyes both signal too much spent chlorine and not enough free chlorine to break it down.

The pH half of the problem

Human tear fluid sits at roughly pH 7.4, which is why balanced pool water does not sting. When pool pH drifts below 7.2 or above 7.8, the water is chemically different enough from your tears to cause irritation on its own, even with chloramines under control.

High pH is the more common drift in our area, and it compounds the issue. As pH rises, chlorine loses much of its killing power, so chloramines build up faster at the same time the water is becoming more irritating. Fixing pH often solves half the complaint immediately.

How to fix a pool that stings

Test first. You want free chlorine at 1 to 4 ppm, pH at 7.2 to 7.8, and combined chlorine (total minus free) as close to zero as possible. Combined chlorine above about 0.5 ppm is the signature of a chloramine problem.

To clear chloramines, shock the pool. Breaking chloramines apart requires pushing free chlorine well above the combined chlorine level, which is what a proper shock dose does. Shock in the evening so the sun does not burn it off, run the pump overnight, and wait until free chlorine falls back below 4 ppm before swimming, which typically takes 8 to 24 hours.

Correct pH separately, and let the pump circulate between doses. Never add two chemicals at once. Never mix chlorine with acid, and never mix different chlorine types with each other, because either can release toxic chlorine gas. Always add chemicals to water, never water to chemicals.

Also check cyanuric acid (stabilizer). Very high stabilizer slows chlorine enough that chloramines accumulate even when free chlorine looks acceptable.

Preventing it from coming back

• Ask everyone to shower briefly before swimming. This removes much of the sweat, oils and product that feed chloramines.
• Never let anyone urinate in the pool. It is a leading contributor.
• Take small children on scheduled bathroom breaks and use swim diapers.
• Maintain free chlorine consistently rather than letting it crash and then overcorrecting.
• Keep the filter clean and run the pump long enough for full turnover.
• Goggles are comfort, not a fix. If eyes sting, the water needs attention.

If eyes stay red, painful or blurry for more than a short while after swimming, or if there is discharge or light sensitivity, see a doctor. Persistent irritation can be conjunctivitis rather than chemistry.

Why this shows up so often in Key West

Sunscreen is the local complication. People here apply it heavily and reapply all day, and every bit of it washes into the pool and feeds chloramine formation. Combine that with year round swimming, warm water that accelerates the reactions, and no screen cages to keep out organic debris, and Keys pools carry a heavier oxidizer demand than most.

Vacation rental pools see it worst. Bather loads are high, guests rarely shower first, and the pool may go days between service visits while being used hard every afternoon. Testing combined chlorine between turnovers catches this before guests complain.

If your pool smells strong and stings, read that as a pool asking to be shocked rather than a pool with too much chlorine in it.