Beyond chlorine: Why your eyes turn red after swimming

As Nairobi shivers through its chilly July days, many turn to heated indoor pools for exercise or relaxation. But for some, a few laps in the water come with an unwelcome side effect: red, irritated eyes.

While many swimmers blame chlorine, health experts say the real culprit is something far more unsettling, a chemical reaction between chlorine and human waste.

When chlorine reacts with substances like urine, sweat, saliva, mucus, skin cells, and even spit, it forms compounds called chloramines. These are the real eye-burners.

“It’s not the chlorine alone that causes red eyes, it’s what it reacts with,” explains Dr Pauline Kimani, a public health expert.

“When someone urinates, spits, or removes mucus in the pool, they’re not just being unhygienic, they’re creating toxic irritants.”

Chlorine is widely used in pools to disinfect and kill harmful microbes and bacteria.

However, according to the Centers for Disease Control and Prevention (CDC), the burning sensation and redness swimmers experience is not primarily caused by chlorine itself, but by chloramines, chemical compounds that form when chlorine mixes with organic matter from the human body.

What causes the burning sensation and the red eyes

“It’s not the chlorine that is causing red eyes, it’s what happens when chlorine mixes with things people bring into the pool,” says Dr Lena Harris, a researcher at the National Institute for Water Safety. “Those chloramines are what irritate your eyes, your skin, and even your lungs.”

That is why lifeguards and pool attendants constantly urge swimmers to shower before entering the pool. A quick rinse helps remove sweat, oils, dead skin, and personal care products that contribute to the chemical load in the water.

“The more contaminants on your body, the harder the chlorine has to work, and the more chloramines get produced,” adds Dr Kimani.

Despite posted warnings, hygiene violations remain widespread. Many swimmers skip the pre-swim shower, seeing it as unnecessary, while others spit or clear their throats into the water without a second thought.

Very few people realize that substances like sputum and mucus contain proteins and bacteria that react with chlorine in much the same way as urine.

“You can’t keep telling a person to stop spitting in the water, at some point, that’s just common sense, people are having fun and all, without realising these small things, ” says Peter Njoroge, a lifeguard at a popular Nairobi gym. “But you'd be surprised how many adults ignore the rules. It’s not just disrespectful, it affects everyone’s health.”

Pool staff say they are constantly reminding swimmers not to spit, urinate, or blow their noses into the water, but many continue to do so out of habit or carelessness. Such behaviour degrades water quality and makes maintaining safe chemical levels more challenging.

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Does peeing change the colour?

Adding to the confusion is a long-standing myth: that urine in pool water will instantly change colour, turning blue or orange to expose the culprit.

While it is a popular scare tactic, especially among parents, to teach children to be mindful, experts say it's just that: a myth.

“It would be great if that were real,” laughs Dr Kimani. “But unfortunately, pool staff have to rely on signs of contamination and regular water testing, not glowing pee.”

Still, the impact of urinating in pools is real. Urine contains nitrogen and ammonia, which bind with chlorine to form chloramines.

These cause not only red eyes and skin irritation but also the strong ‘chlorine’ smell many people mistakenly associate with cleanliness. A well-maintained pool should barely smell at all.

Swimming and menses

Another often overlooked issue is swimming during menstruation, a topic still surrounded by stigma and misinformation.

Many women fear embarrassment or health concerns, but medical experts emphasize that swimming during your period is completely safe as long as appropriate menstrual hygiene products like tampons or menstrual cups are used.

Menstrual blood, if released directly into pool water, can also react with chlorine and contribute to chloramine formation. While a small amount is not likely to cause a major issue, experts advise using protection not just for personal comfort, but out of respect for other swimmers and for overall water hygiene.

“There’s still a lot of stigma around periods, especially in public spaces like pools,” says Dr Kimani. “But with the right products and knowledge, women can swim confidently and hygienically at any time of the month.”

PH level of the pool also matters

Another hidden but important factor is the pH level of the pool. Ideally, pool water should be maintained at a pH between 7.4 and 7.6.

When pH levels are off, the protective tear layer of the eye becomes disrupted, making eyes dry and more prone to irritation.

"Unbalanced pH doesn't just affect your comfort, it impacts the pool's chemistry,” explains Dr Harris. “Even properly chlorinated water can still irritate if the pH is wrong.”

Overcrowded pools and hot weather can throw off these delicate balances. This is why pool managers are required to regularly test and adjust both chlorine and pH levels.

According to the CDC, nearly half of swimmers do not shower before getting into the water, and 1 in 5 admit to peeing in pools. While some may dismiss this as harmless or humorous, experts warn that these habits directly impact pool safety.

“They also reduce the effectiveness of chlorine,” says Dr Harris. “So not only do chloramines form, but the pool becomes less sanitary overall.”

To protect your eyes and your health, experts recommend wearing snug, high-quality swimming goggles, avoiding pools with strong chemical odours, and rinsing your eyes with clean water after a swim.

In cases of severe irritation, lubricating eye drops, or a visit to an eye specialist, may be necessary.

A clean pool, they say, is easy to spot. Look for clear (not cloudy) water, smooth non-slimy tiles, and minimal or no strong “chemical” smell.

Always ask to check out the pool before paying for it to swim. That sharp odour is usually a sign of too many chloramines, not high chlorine levels.

Also, look for staff actively monitoring water quality and hygiene compliance. “Clean pool water shouldn’t smell harsh,” says Njoroge. “If your eyes are burning before you even get in, that’s a red flag.”

Red eyes are your body’s way of saying something’s off in the water, and often, the solution starts with the swimmers themselves.