The Unexpected Truth About Red Eyes and Pools

Girls swimming with their eyes open in the pool.

Many people are familiar with getting red eyes from swimming in a pool. However, many people are also under the assumption this is caused by the chlorine disinfectant.

Chlorine, the Go-To Disinfectant for Water

When it comes to maintaining pool hygiene, chlorine is an essential tool. Acceptable pool water quality is defined in the Swimming Pool Hygiene Standards set forth by the Ministry of Health, Labour and Welfare, and the Swimming Pool Environmental Hygiene Standards set forth by the Ministry of Education, Culture, Sports, Science and Technology.
These standards include information on residual chlorine concentrations, pH values, turbidity, and total trihalomethane.
The recommended residual chlorine in swimming pools is between 0.4 mg/L and 1.0 mg/L.

Chlorine can also be found in tap water. Tap water is required to have residual chlorine levels between 0.1 mg/L and 1.0 mg/L at the faucet. Higher amounts of residual chlorine give off a stronger chlorine odor. In general, chlorine odor is not noticeable at residual chlorine levels below 0.4 mg/L.

But if the upper residual chlorine limit for pools is the same as for tap water, why do pools have that distinctly strong odor of chlorine?

The Truth About Odor and Red Eyes

Despite popular belief, chlorine is not the direct cause of your eyes turning red in a pool. The real cause involves urine and sweat. The ammonia contained in urine and sweat reacts with chlorine to form chloramine. Chloramine is what causes red eyes and that distinctive odor when swimming.
This shocking fact was announced by the Centers for Disease Control and Prevention in the US in June 2015. The fact that the world only recently discovered this fact is also surprising.

The Importance of User Etiquette and Manager Oversight

The good manners of pool-goers are essential for ensuring hygienic pool use. This includes using the toilet and showering before entering the pool.

On the managerial side, swimming pool operators must use water quality measurement equipment to verify that the water quality meets established standards. The measurement results can be used to check the status of circulation filtration devices designed to remove chloramine and other impurities, and to review chemical dosage levels needed to maintain water quality.