Cyanuric Acid

Cyanuric acid is the one pool chemical that does nothing you can see, cannot be removed, and quietly decides whether the chlorine you are paying for does any work.

What it does

Unstabilized chlorine in an outdoor pool has a half-life of roughly 45 minutes in direct summer sun. CYA binds to hypochlorite and shields it from UV, pushing that half-life out to hours.

That is the whole benefit, and it is a large one. An outdoor pool with zero CYA burns through chlorine faster than any feeder can supply it.

What it also does

The same bond that protects chlorine from sunlight makes it slower to kill anything. The measured free chlorine reading does not distinguish between chlorine that is available and chlorine that is locked to CYA — so a pool can read 3 ppm free chlorine, be genuinely at 3 ppm by the test, and still be losing to algae.

The relationship is proportional, which is the useful part: the chlorine a pool needs is a fraction of its CYA level, not a fixed number.

Read that table twice. At 100 ppm CYA, shocking the pool means holding 39 ppm — five times the chlorine, five times the cost, for the same result.

Where it comes from

Nobody adds much of it deliberately. It arrives in stabilized chlorine: trichlor tablets add about 6 ppm of CYA for every 10 ppm of free chlorine delivered; dichlor adds about 9.

A season of tablets in a 20,000 gallon pool puts on 40 to 60 ppm without a single bag of stabilizer being opened. This is the standard route to a pool that reads plenty of chlorine and still turns green in August.

Getting it out

Dilution. That is the list.

CYA does not evaporate, does not break down in sunlight, and is not consumed. Draining a third of the pool lowers it by a third. Reverse-osmosis service exists in hard-water regions and costs several hundred dollars.

There is a partial exception — a small amount degrades anaerobically over a winter in some pools — but it is neither reliable nor plannable.

The rule that avoids all of it

Test CYA in spring and mid-season. Once it is above 50, switch to liquid chlorine or a salt system and stop adding tablets. Liquid adds nothing but salt.