Water Chemistry
The unreacted chlorine available in pool water to kill bacteria and oxidize organic contaminants. Illinois IDPH requires 1.0–5.0 ppm at all times in semi-public pools.
Free chlorine is the working sanitizer — every measurable ppm is chlorine that hasn't yet bonded with an organic compound. Below 1.0 ppm, the pool isn't legally sanitizing and should not remain open. Above 5.0 ppm, water is out of code range and causes irritation. Free chlorine is measured alongside total chlorine; the difference between the two gives combined chlorine, a separate and more operationally important reading. Illinois requires twice-daily free chlorine testing during operating hours and continuous logging.
High free chlorine isn't a safety backup — it pushes the pool out of code and irritates swimmers.
`Chlorine smell` is almost never a free chlorine problem. That's combined chlorine (chloramines).
A pool with normal free chlorine but high combined chlorine is still sanitizing-impaired.
Read more on the blog
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.
Combined Chlorine (Chloramines)
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.
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.
Aqua-Guard runs certified commercial pool operations for 200+ Chicagoland HOAs, condos, and clubs. We handle the credentials so your board doesn't have to.
Request a Written Proposal