A pool is over-chlorinated when free chlorine exceeds safe levels for swimmers. The safety thresholds:
| Free Chlorine Level | Safety Status |
|---|---|
| 1–3 ppm | Safe — ideal range for swimming |
| 3–5 ppm | Marginally safe — short exposure OK |
| 5–10 ppm | Not safe for swimming — irritation risk |
| Above 10 ppm | Dangerous — corrosive to eyes and airways |
Stop adding chlorine and remove the pool cover. UV light degrades free chlorine rapidly — typically 1–2 ppm per hour in direct sunlight. A pool shocked to 10 ppm will usually drop to below 5 ppm within 6–8 hours on a sunny day.
Sodium thiosulfate is a chlorine neutralizer. It reacts with free chlorine to reduce it immediately. Add in small increments (1–2 oz per 10,000 gallons per 1 ppm you want to reduce) with the pump running, and retest after 15 minutes. It is easy to overshoot and end up with zero chlorine, so add conservatively.
Drain 20–30% of the pool water and replace with fresh water. This dilutes all chemical levels proportionally. Useful when you want to lower chlorine and also reduce CYA or TDS at the same time.
Never try to "swim it off." Exposing swimmers to high chlorine to dilute it faster is not safe. Wait for levels to drop to below 5 ppm — confirmed by testing — before allowing anyone in the pool.
SplashLens calculates the exact dose to reach your target chlorine level — no guesswork, no overdosing. Enter your volume, current reading, and target; get the precise amount to add.
Open SplashLens Free →Excess chlorine causes eye and skin irritation, bleaches swimsuits and hair, irritates airways, and can damage vinyl liners and rubber seals. Equipment like heaters and pump seals degrade with prolonged high-chlorine exposure.
Three options: let sunlight naturally reduce it (1–2 ppm per hour in direct sun), add sodium thiosulfate chlorine neutralizer to reduce levels immediately, or partially drain and refill with fresh water to dilute.
On a sunny day, free chlorine drops 1–2 ppm per hour from UV exposure alone. A pool at 10 ppm typically drops below 5 ppm in 6–8 hours in full sun, or 12–16 hours on a cloudy day.
Yes — 10 ppm is not safe for swimming. This is the shock treatment level. Wait until free chlorine drops below 5 ppm (ideally 1–3 ppm) and test before allowing swimmers in after any shock treatment.
Exposure to high free chlorine levels causes irritation of the eyes, skin, and respiratory tract. In extreme cases (primarily with chlorine gas in indoor pools), it can cause nausea and respiratory distress. Properly shocking outdoor pools and waiting for levels to drop prevents health risks.