CYA: Too Much of a Good Thing?
Risks With Over-Stabilization of Chlorine
By Stanley R. Pickens, Ph.D.
You've heard the phrase before: "Too much of a good thing." When it comes to cyanuric acid (CYA)—or chlorine stabilizer—which is used to protect sanitizing forms of chlorine from degradation by sunlight, this phrase certainly applies. CYA can be an important tool in maintaining an effective sanitizer residual in outdoor bodies of water, such as swimming pools, on bright summer days. In fact, stabilizer is an appropriate name for CYA. It stabilizes chlorine, which means that it makes chlorine last longer.
Unfortunately, it also makes chlorine more sluggish in killing germs and oxidizing contaminants in the water. In essence, too much CYA shackles free chlorine, reducing its ability to do its job effectively. As a result, there can be greater buildup of oxidizable contaminants in the water. These contaminants can serve as food for microbes in the water. The buildup of these contaminants could, in turn, lead to an increased risk of an algae bloom, bacterial infestation or transmission of water-borne illnesses.
How much stabilizer is too much, and how big is the impact on sanitizing and oxidizing reactions? The graph in Figure 1 illustrates how the concentration of hypochlorous acid (HOCl)—the active, sanitizing fraction of measured free chlorine—drops as CYA concentration increases. Note the steep drop-off. Without any CYA, and at pH 7.5, about half of the free chlorine would be in the active, HOCl , form. However, with CYA at 10 parts per million (ppm), the HOCl fraction of free chlorine drops to 5 percent; with 20 ppm CYA, only 2 percent of the measured free chlorine is in the more active, HOCl , form. What's more, if the CYA concentration is allowed to rise to 50 or 100 PPM the HOCl fraction drops still further, to 0.8 percent and 0.4 percent, respectively.
A comparison to pH might be instructive on this issue. Trained pool operators generally try to prevent the pH of the water from exceeding about 8, since they recognize that less of the free chlorine is in the active HOCl form at higher pH. However, the impact of pH is relatively minor when compared to the impact of even just 20 PPM of CYA in the water. See Figure 2.
The free chlorine is not gone altogether; it's simply shackled to the CYA, and thus less free than it otherwise would be. In this fettered state, the chlorine still tests as free with most standard test kits or test strips, but it is hindered in its ability to kill germs and burn off oxidizable contaminants in a timely fashion. This cyanurate-bonded chlorine can eventually be freed from the cyanurate, but only as existing hypochlorous acid is consumed. With too much protection of the free chlorine, there can be inadequate protection of bathers from germs in the pool.
Since so much of what happens in recreational water is tied to the concentration of HOCl , the inverse relationship between HOCl concentration and CYA concentration is reflected in several other phenomena:
- The destruction of free chlorine by sunlight proceeds at a rate inversely related to CYA concentration.
- Oxidation potential (ORP in millivolts), a common indicator of chlorine activity or sanitizing ability, is inversely related to CYA concentration in a fashion resembling the curve in Figure 1.
- The killing of bacteria and other disease-causing microbes in water happens at a rate inversely related to CYA concentration for any fixed, measured free chlorine concentration.
The slowing impact of CYA can complicate the cleanup of a pool after a diarrheal fecal release or a Cryptosporidium outbreak. The U.S. Centers for Disease Control and Prevention recommends "hyperchlorination" with a 20 PPM free chlorine residual maintained for about 13 hours at pH 7.5 in the absence of CYA. However, they have observed that with CYA at 50 PPM, hyperchlorination is far less effective. Even if the pH is lowered to 6.5 and the free chlorine residual is doubled to 40 PPM, the same 99.9 percent kill could not be achieved even in 24 hours.
It is for such reasons that health codes, such as the proposed Model Aquatic Health Code, commonly call for one or more of the following: higher chlorine residuals when CYA is present; banning of CYA in increased-risk venues (such as kiddy/wading pools, spray pads or therapy pools and spas) as well as at indoor venues; and limiting CYA to no more than 50 PPM, when the use of CYA is allowed.