Bob Lowry on the Dangers of High CYA Levels

Cyanuric acid (CYA), often called pool stabilizer or pool conditioner, is usually introduced as a simple shield that protects chlorine from sunlight. But the deeper story is what makes or breaks clear water: CYA changes chlorine effectiveness by binding most of the chlorine in the water and controlling how much active sanitizer is available at any moment. In this conversation, Bob Lowry explains why “normal” chlorine readings can be misleading, how CYA acts like a buffer in pool water, and why understanding the stabilizer to chlorine relationship is a cornerstone of reliable pool maintenance for service pros and homeowners alike.  

The key insight is the equilibrium between free chlorine and chlorine bound to cyanuric acid. Once CYA rises above about 30 ppm, a very large percentage of chlorine becomes bound, leaving only a small fraction immediately active. That does not mean the chlorine disappears, it means it releases more slowly as the active portion gets consumed. The practical impact is simple: the higher the cyanuric acid level, the more chlorine you must maintain to keep enough active hypochlorous acid (HOCl) present to prevent algae and bacteria. This is why a pool can test “fine” one week and then suddenly struggle as stabilizer drifts upward.  

Bob walks through the logic behind a rule of thumb that replaces constant chemistry calculations: maintain free chlorine at about 7.5% of your CYA level when you are not using borates, and about 5% of CYA when you are using borates. The reasoning ties back to how much HOCl is present at a typical pH like 7.5, and what minimum HOCl level is needed to kill algae. The math highlights the danger of very high stabilizer. At 100 ppm CYA, you may need around 7.5 ppm free chlorine to stay protected without borates, a level many pool owners do not want to maintain.  

This is also where recommended CYA targets become practical. For many outdoor pools, keeping cyanuric acid roughly in the 30 to 50 ppm range makes sanitation easier while still slowing sunlight degradation of chlorine. Bob notes an important exception for a saltwater chlorine generator (SWG): a higher stabilizer level, around 70 ppm, can help protect freshly generated chlorine returning to the pool where sunlight can destroy it quickly at the surface. The theme is not “low CYA at all costs,” but “right-sized CYA so your chlorine can work without forcing extreme dosing.”  

High CYA problems often trace back to stabilized chlorine products, especially trichlor tabs. Trichlor adds cyanuric acid as it delivers chlorine, so stabilizer can rise steadily during summer demand. Bob gives a memorable rule: for each 10 ppm of chlorine added via trichlor, about 6 ppm of CYA is added, turning pool care into a vicious cycle where you must keep raising chlorine just to match the rising stabilizer. A practical exit ramp is to use trichlor only until CYA reaches a reasonable target, then switch to liquid chlorine or a dosing method that does not keep adding stabilizer, such as a liquid chlorine dispenser or a peristaltic pump that feeds smaller daily amounts.