Why Your Indoor Pool Smells Like "Chlorine" in Winter (and Why It's Usually Not Enough Chlorine)

When the doors close for winter, a lot of indoor aquatic facilities start hearing the same complaints: a strong "chlorine smell," red eyes, and swimmers coughing. Most people assume there's too much chlorine in the water. Usually, it's the opposite.

What's really causing that smell

That smell comes from chloramines (also called combined chlorine). They form when free chlorine reacts with sweat, body oils, urine and other contaminants swimmers bring into the pool. Chloramines are weak sanitizers, they irritate eyes and lungs, and they gas off into the air. In the winter, when buildings bring in less fresh outside air, they build up in the natatorium.

What operators can do

• Test combined chlorine, not just free chlorine. Subtract free chlorine from total chlorine. Many operators aim to keep combined chlorine at or below 0.4 ppm. Check your local health code for your requirements.

• Encourage pre-swim showers. Less contamination going in means fewer chloramines forming.

• Check your air handling. Make sure your HVAC is bringing in enough fresh air and the exhaust near the water surface is working. Winter energy savings often come at the cost of air quality.

• Consider supplemental treatment. Secondary systems like UV can break down chloramines as water circulates, which can make a noticeable difference in indoor facilities.

• Keep your controller calibrated. A well-maintained chemical controller holds steady pH and sanitizer levels, which keeps chloramine problems from snowballing.

  
  

Want to dig deeper?

The CDC's Model Aquatic Health Code covers how chloramines affect indoor air quality and what helps reduce them: CDC Model Aquatic Health Code Annex (PDF).

If your facility is fighting that winter pool smell, give our team a call at 303-762-9470 or reach out here. We're happy to help you track down the cause.