Water chemistry
Combined chlorine
Combined chlorine is chlorine that has already bonded with sweat, oils, and other waste, so it no longer sanitizes well.
Combined chlorine is the used-up part of your chlorine. It forms when chlorine reacts with swimmer waste, leaves, and sunscreen. It causes the strong pool smell and red eyes.
Most health departments limit combined chlorine to 0.4 ppm or less, according to CDC. You find it by taking total chlorine and subtracting free chlorine. If the number is high, the fix is to raise free chlorine well above it, which is called breakpoint chlorination. Follow the product label.
Related terms
Sources
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