Free chlorine vs combined chlorine

Free chlorine is the unreacted chlorine actively available to sanitize the water. Combined chlorine (chloramines) is chlorine that has reacted with ammonia, sweat, urine, sunscreen, and other organic contaminants. Combined chlorine is irritating to eyes and skin, produces the characteristic 'pool smell,' and provides minimal sanitation.

Total chlorine (the reading on most basic test kits) is the sum of free + combined. The relevant numbers for water quality are the breakdown: free chlorine should be 1-3 ppm; combined chlorine should stay under 0.5 ppm. When combined chlorine exceeds 0.5 ppm, the pool needs shock treatment (a high dose of chlorine) to oxidize the chloramines and break the chlorine-contaminant bonds.

For pool service operators, the free/combined breakdown is the most important diagnostic distinction. Pools with high combined chlorine but adequate total chlorine appear to be 'fine' on basic tests but actually need treatment. Better test kits (DPD reagent) distinguish free from combined; cheaper kits don't. Most customers assume their pool 'smells like chlorine' because there's too much chlorine — explaining that the smell is actually combined chlorine (and the fix is shock treatment, not less chlorine) reframes a recurring service call as expected maintenance rather than a complaint.