Combined chlorine
Definition
Chlorine that has already reacted with ammonia, urine or sweat to form chloramines — the compounds responsible for the strong 'pool smell' and red-eye irritation, indicating the water needs a shock treatment.
Combined chlorine is the bad chlorine. It has already done its job binding to contaminants and is no longer disinfecting. The chemical formula is monochloramine, dichloramine, or trichloramine — the latter is the eye-stinging culprit.
State codes typically require combined chlorine to stay below 0.4 ppm. When it climbs higher, the operator performs a 'shock' or 'breakpoint chlorination' — adding a large dose of free chlorine to oxidize the chloramines back to free chlorine plus inert byproducts. A pad that smells strongly of chlorine has too much combined chlorine, not too much free chlorine.