Water Chemistry
Chlorine that has already reacted with organic contaminants and lost most of its sanitizing value. The primary cause of the strong `chlorine smell` in pools.
Combined chlorine is chlorine that has bonded with ammonia, urea, sweat, sunscreen, and other organic nitrogen compounds. It no longer effectively sanitizes water, and it produces the sharp odor swimmers commonly (mis)attribute to `too much chlorine.` The corrective action is usually to shock the pool (raise free chlorine to ~10x combined chlorine to break the bond), not to reduce chlorine feed. IDPH code caps combined chlorine at 0.4ppm; Aqua-Guard targets anything above 0.2ppm as a trend worth investigating.
The `chlorine smell` in a public pool is typically NOT too much chlorine — it's not enough effective chlorine.
Combined chlorine does not dissipate on its own. Shock is the remediation.
Heavy bather load is the most common cause — not equipment failure.
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Related terms · Water Chemistry
Cyanuric Acid (CYA / Stabilizer)
A pool additive that protects chlorine from UV degradation in outdoor pools. Essential in moderation, counterproductive above 70ppm.
Chlorine Lock
A state in which chlorine test readings appear normal but the chlorine is ineffective at actually sanitizing the pool. Usually caused by excessive cyanuric acid.
Langelier Saturation Index (LSI)
A composite calculation (pH + temperature + calcium hardness + alkalinity + CYA adjustment) that measures whether pool water is corrosive, balanced, or scale-forming.
pH (Pool Water)
A measure of how acidic or basic pool water is on a 0–14 scale. Illinois IDPH requires semi-public pools to hold pH between 7.2 and 7.8, with 7.4–7.6 as the practical operating target.
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