Free chlorine (hypochlorous acid) is extremely sensitive to UV light. Without stabilizer, the sun can destroy more than half your free chlorine in less than 20 minutes. This means an outdoor pool would need continuous chlorine additions throughout the day just to maintain safe levels.
Cyanuric acid solves this by forming a weak bond with chlorine molecules. The bond shields chlorine from UV while the chlorine is still available to sanitize when it contacts pathogens or algae. When chlorine is used, it is released from CYA and new chlorine must be added.
| CYA Level | Effect | Action Needed |
|---|---|---|
| Below 20 ppm | Chlorine burns off rapidly in sun | Add stabilizer |
| 30–50 ppm | Optimal protection, chlorine effective | Maintain this range |
| 50–80 ppm | Acceptable — slightly reduced effectiveness | Monitor; use higher FC target |
| 80–100 ppm | Chlorine significantly impaired | Partial drain and refill recommended |
| Above 100 ppm | Chlorine lock — pool not effectively sanitized | Drain 50% and refill required |
High CYA is the most underdiagnosed cause of persistent algae problems. A pool with 150 ppm CYA and "3 ppm free chlorine" may have the effective sanitizing power equivalent to less than 0.1 ppm. The chlorine tests as present but is so tightly bound to CYA that it cannot kill algae or bacteria efficiently.
Signs of CYA-induced chlorine lock:
CYA accumulates from two main sources:
If you use trichlor tablets exclusively, expect CYA to rise 5–8 ppm per week in summer. After 3–4 months, CYA can reach 80–120 ppm without a partial drain. Test CYA monthly and plan one partial drain per season if tablets are your primary sanitizer.
There is no reliable chemical method to reduce CYA. The only proven approach is dilution — partially drain and refill with fresh water. A 25% drain reduces CYA by 25%; a 50% drain cuts it in half. Switch to calcium hypochlorite or liquid chlorine after the refill to avoid rapid CYA accumulation.
Log CYA readings and get alerted when levels are approaching the danger zone. PoolLens tracks every parameter so you can plan a partial drain before chlorine lock sets in.
Open PoolLens Free →CYA above 80–100 ppm significantly reduces chlorine's sanitizing power. The pool may test at 3 ppm free chlorine but have near-zero effective sanitation — algae blooms despite apparent chlorine presence. The only fix is partial drainage and dilution with fresh water.
Drain 25–50% of the pool and refill with fresh water. Draining 50% cuts CYA in half. There is no reliable chemical method — products marketed to reduce CYA are not consistently effective.
No. Indoor pools are not exposed to UV sunlight, so CYA provides no benefit. It only accumulates and reduces chlorine effectiveness. Use unstabilized chlorine products for indoor pools.
Yes, CYA is a weak acid and lowers pH slightly when added. In normal concentrations the effect is minor. Always retest pH and alkalinity after adding stabilizer to your pool.
Pool stabilizer (cyanuric acid) is a long-term conditioner that protects chlorine from UV — it stays in the pool for months. Pool shock is a concentrated one-time chlorine boost to kill pathogens and chloramines. Dichlor shock combines both, adding CYA with each use.