CYA, or cyanuric acid, is a pool stabilizer that protects chlorine from being destroyed by UV sunlight. The right level is 30 to 50 ppm for most outdoor pools and 60 to 80 ppm for saltwater pools. The problem is that CYA only goes up over time – it never burns off – so many pools quietly accumulate dangerously high levels that make chlorine nearly useless, even when test results look normal. If your chlorine keeps disappearing or your pool keeps going green despite regular dosing, high CYA is often the reason.
What Does CYA Actually Do?
Chlorine is extremely sensitive to UV light. Without protection, sunlight can destroy up to 90 percent of your pool’s free chlorine within two hours on a sunny day. CYA works by forming a temporary bond with chlorine molecules, shielding them from UV radiation. Think of it as sunscreen for chlorine. This is why stabilized chlorine products were a game-changer for outdoor pools – a pool with 30 to 50 ppm CYA holds onto chlorine dramatically longer than one with no stabilizer at all.
For a deeper look at how CYA interacts with your chlorine residual, Cyanuric Acid in Pools: What It Does and How Much You Need covers the chemistry in plain terms.
Why Does CYA Keep Climbing in Most Pools?
The most common culprit is stabilized chlorine – specifically trichlor tablets (the standard 3-inch pucks) and dichlor granules. Both products contain CYA as part of their chemical formula. Every pound of trichlor added to 10,000 gallons of water raises CYA by roughly 6 ppm. Use a puck feeder all season long and it is completely normal to see CYA climb from 30 ppm in May to 120 ppm or higher by August.
Many pool owners do not realize their chlorine tabs are loaded with CYA. They buy a bucket of pucks, keep the dispenser full all summer, and wonder why their pool looks hazy by July despite constant chlorination. The tab feeder is doing its job – it is just adding CYA along for the ride every single time.
What Happens When CYA Gets Too High?
At high CYA levels, the bond between CYA and chlorine becomes so strong that it seriously limits how much “active” chlorine is available to kill bacteria and algae. This is what people commonly call chlorine lock. At 100 ppm CYA, you need free chlorine levels of 7 to 10 ppm just to match the sanitizing power that 3 ppm of chlorine provides at 30 ppm CYA. In practice, most pool owners are running 1 to 3 ppm of free chlorine – which at high CYA levels is almost nothing.
The result looks like a chlorine problem, but it is actually a CYA problem. You add shock, the numbers spike on a test strip, and within 24 to 48 hours the chlorine is gone again. Algae start forming. Water goes hazy. You add more chlorine, and the cycle repeats. Meanwhile, the real issue – your CYA sitting at 150 ppm – is not going anywhere.
How Do You Test CYA Accurately?
Standard test strips often measure CYA poorly, especially at elevated levels. The most reliable method is the turbidity test (also called the dot test or Taylor-style drop test), where you dilute your pool water into a tube and add a reagent until a black dot on the bottom of the tube disappears. This gives you a solid reading from 0 to 100 ppm. For levels above 100 ppm, dilute your pool water 50/50 with fresh tap water before testing and multiply the result by two.
If you suspect your CYA is high but want confirmation before taking action, sending a water sample to a local pool store for a full panel test is worth the few dollars it costs. Ask them specifically for a CYA reading – some basic water tests skip it.
How Do You Lower CYA That’s Too High?
CYA does not degrade on its own. It does not burn off in sunlight, get consumed by bacteria, or disappear from backwashing. Your options are:
- Partial drain and refill: The most reliable method. Drain 25 to 50 percent of your pool water and refill with fresh water. If your CYA is 120 ppm and you drain half the pool, you will come back to roughly 60 ppm after refilling. This works, but it wastes water and takes time to reheat and rebalance.
- Enzyme-based CYA reducers: A newer category of products uses bio-degradation to break down cyanuric acid. Results take a couple of weeks and work best for moderate overages. This is a good option if a drain-and-refill is not practical – if you are in a drought area or have a heater that makes partial drains expensive.
- Switch your chlorine source: Once CYA is corrected, switching from puck-based chlorine to unstabilized liquid chlorine (sodium hypochlorite) or calcium hypochlorite granules will stop CYA from climbing further. These forms of chlorine add zero CYA to your water.
AquaDoc makes an unstabilized pool chlorine for exactly this reason – it gives you full sanitizing power without adding CYA, which is a real advantage once you have dialed in your stabilizer level.
Preventing CYA Buildup Going Forward
The cleanest long-term approach is to use trichlor pucks only when your CYA is in the lower half of the target range (30 to 40 ppm), and shift to unstabilized chlorine once you are in the upper half. You can also run your pool at the lower end of the CYA range in spring so you have room to absorb some CYA from pucks before levels get too high. Test CYA monthly during swim season, not just once at opening – this is one number that can sneak up fast, especially during high-use summer weeks.
If you are managing chemistry during a stretch when the pool is getting hammered with swimmers, this pairs closely with the broader approach covered in Summer Pool Chemistry When Everyone’s Jumping In Every Day.
Frequently Asked Questions
What is the ideal CYA level for a pool?
For outdoor pools using chlorine, target 30 to 50 ppm. If you use a saltwater chlorine generator, aim for 60 to 80 ppm. Anything above 100 ppm starts to significantly reduce chlorine effectiveness and should be addressed before the water gets worse.
Does CYA ever go down on its own?
No. CYA does not break down from sunlight, heat, or normal chemical use. The only reliable ways to lower it are partial draining and refilling with fresh water, or using a CYA-reducing enzyme product.
Can high CYA make my pool cloudy?
High CYA does not directly cloud the water, but it weakens chlorine enough that algae and bacteria can grow unchecked, which does cause cloudiness. If your pool is hazy and your chlorine reads fine, check your CYA level – it is often the missing piece.
How much CYA does stabilized chlorine add over time?
Every pound of trichlor added to 10,000 gallons of water raises CYA by roughly 6 ppm. Use puck-style chlorine all season and CYA can easily hit 100 to 150 ppm by midsummer without ever doing anything “wrong.”
What is chlorine lock and is it real?
Chlorine lock is the reduced effectiveness of chlorine at high CYA levels, and it is a real phenomenon. At 100 ppm CYA, you need 3 to 5 times more free chlorine to achieve the same sanitizing power as you would at 30 ppm CYA. Most pool owners are not running nearly enough chlorine to compensate, which is why algae wins.
CYA is one of those numbers that is easy to ignore right up until it explains every problem you have been chasing all summer. Test it monthly, keep it between 30 and 50 ppm, and choose your chlorine source accordingly. Fix the stabilizer level and the rest of your chemistry will behave a lot better. For professional perspective on pool water chemistry management, River Pools and Spas has useful resources from experienced pool builders and service professionals.
