Cyanuric acid protects chlorine from sunlight. Target 30-50 ppm during spring through fall when UV is strongest. In winter, CYA can run lower. Cooler temperatures and reduced sunlight mean chlorine doesn't need as much protection. Trichlor tablets steadily raise CYA over time. The only way to lower it is to drain and refill with fresh water.
What Is Cyanuric Acid (CYA) in a Pool?
Cyanuric acid (CYA), also called stabilizer or conditioner, protects chlorine from ultraviolet (UV) degradation. Without it, sunlight destroys about 90% of free chlorine within two hours. With proper CYA levels, chlorine lasts much longer.
Think of CYA as sunscreen for your chlorine. It shields chlorine molecules from UV rays, allowing them to remain active longer. The CDC's Model Aquatic Health Code caps CYA at 100 ppm for public pools, recognizing that excessive stabilizer impairs chlorine's ability to kill pathogens.
What Is the Ideal CYA Level for a Pool?
The same CYA target applies to all pool types, including salt chlorine generators, liquid chlorine, and traditional chlorine. CYA protects chlorine from UV, and that need is driven by sunlight and temperature, not by how chlorine is generated.
| Season | CYA Target | Why |
|---|---|---|
| Spring through Fall | 30-50 ppm | High UV intensity and warm temps degrade chlorine fast. Full protection needed. |
| Winter | 5-30 ppm acceptable | Lower UV intensity and cooler water reduce chlorine consumption. Less protection needed. |
CYA also affects the LSI calculation through carbonate alkalinity. Higher CYA means you need slightly higher total alkalinity to maintain the same LSI.
What Happens When CYA Is Too High?
Trichlor tablets each contain roughly 58% CYA by weight. A pool using one 3-inch tablet per week will typically see CYA rise 25-30 ppm per month until it exceeds safe limits, with no way to lower it except draining and refilling.
While CYA protects chlorine, too much stabilizer actually reduces chlorine's sanitizing power, a condition called "chlorine lock" or "over-stabilization." At high CYA levels, chlorine bonds so tightly with stabilizer that it becomes slow to release and kill pathogens. Your test kit may show adequate chlorine, but it's not working effectively.
- 50 ppm CYA: Chlorine works normally
- 100 ppm CYA: Chlorine effectiveness reduced ~50%
- 150+ ppm CYA: Chlorine significantly impaired, and algae risk increases
What Causes High CYA?
- Trichlor tablets: 55% cyanuric acid by weight
- Dichlor granular: Also contains stabilizer
- Accumulation over time: CYA doesn't evaporate or break down
- Low water replacement: Not enough fresh water added
Many pool owners don't realize their "convenient" tablets are steadily raising CYA with every use. By mid-season, levels can exceed 100 ppm.
How to Lower CYA
Unlike most pool chemicals, CYA cannot be chemically neutralized. The only reliable solution is dilution:
- 1Test your current level
- 2Calculate water to drain. Typically 30-50% for significantly elevated CYA
- 3Drain and refill with fresh water
- 4Rebalance all other chemistry
- 5Switch chlorination method. Stop using stabilized chlorine
Preventing High CYA
- Use liquid chlorine: No stabilizer added
- Install a salt system: Generates pure chlorine
- Limit tablet use: Only for vacation periods or emergencies
- Monitor regularly: Test CYA monthly during swim season
Frequently Asked Questions
What is the ideal CYA level for a pool?
Target 30-50 ppm during spring through fall when UV and temperatures are highest. In winter, CYA can run lower because UV intensity drops and cooler water consumes less chlorine. The same target applies to all pool types: salt, liquid chlorine, and traditional. Per the CDC Model Aquatic Health Code, CYA above 100 ppm requires corrective dilution because chlorine effectiveness is so impaired the pool cannot be safely managed without it.
How do I lower my CYA level?
There is only one reliable method: dilution. Run your filter on the waste or backwash setting to lower the water level, then refill with fresh water. Stay within 30-50% replacement per cycle. Replacing half the water in a 100 ppm pool brings CYA to approximately 50 ppm. On the Oak Island coast, a full drain is not recommended due to the high water table. An empty pool can float out of the ground. Reach out to us before removing significant volume. Products marketed as CYA reducers have not demonstrated measurable results.
Why does my CYA keep rising?
Every time you add trichlor tablets or dichlor granular chlorine, you are also adding cyanuric acid (trichlor is 55% CYA by weight). As CYA accumulates above 80-100 ppm, it suppresses chlorine's oxidation potential to the point where adequate sanitation becomes impossible, a condition called chlorine lock. The CDC Model Aquatic Health Code sets 100 ppm as the maximum safe CYA level for managed pools. Switching to liquid chlorine or a salt system stops CYA accumulation entirely.