The culprit is not Free Chlorine (FC), but Combined Chlorine (CC), commonly known as Chloramines.
1, Formation: Chloramines are created when Free Chlorine binds with nitrogen-based contaminants introduced by swimmers (e.g., sweat, urine, oils, sunscreen, cosmetics).
2, Properties: Chloramines have very little sanitizing power (<3% of FC) but are highly volatile and irritating. They evaporate from the water into the air, causing the classic chlorine odor and mucous membrane irritation.
3, Root Cause: Inadequate chlorine levels or oxidation power to quickly and completely destroy these contaminants, allowing chloramines to accumulate.

1, Shock Treatment ("Breakpoint Chlorination")
Chemical: Calcium Hypochlorite (Cal-Hypo) or Potassium Monopersulfate (MPS).
Purpose: The primary goal of shocking is to achieve "breakpoint chlorination." The massive oxidizer dose destroys the chloramine molecules, breaking the chemical bonds and liberating Free Chlorine.

Application Method:
Superchlorination: Raise the Free Chlorine level to 10 times the Combined Chlorine level (or to 10 ppm) and maintain it until the CC reading is <0.5 ppm. This is the most definitive solution.
Using MPS: This is an excellent option for indoor pools or chlorine-sensitive users as it oxidizes chloramines without adding more chlorine.

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