![]() ![]() This results in a CYA:FC ratio of 90:2 or 45:1. The MAHC currently recommends a minimum of 2 ppm FC when CYA is present, and limits the CYA maximum to 90 ppm. With 100 ppm CYA, 49 ppm FC would be needed to reduce the risk of Giardia to approximately the same as 1 ppm FC with no CYA. If 10 ppm CYA is used with 1 ppm FC, people are three times as likely to get ill from Giardia, and if 100 ppm CYA is used, people are about 17 times more likely to get ill. The table shows that people are 357 times more likely to get ill from Giardia if no chlorine is present, compared to 1 ppm FC. coli with 1 ppm FC and 100 ppm CYA than they are with 1 ppm FC with no CYA.įollowing is a table showing the relative risks of illness from Giardia: But we can say that people are much more likely to get ill from E. coli each year with 1 ppm FC and 100 ppm CYA. So what does this mean? It means that we can’t really say that 1 in 41,000 people would actually get ill from E. The authors of the model caution that because there is so much variability in the data going into the model, the model cannot provide an accurate estimate of absolute risk, however it is believed to provide meaningful estimates of relative risk. ![]() coli infection is increased to about 1 in 41,000 people, indicating there may be the potential for multiple infections in a year. With 100 ppm CYA and 1 ppm FC, the risk of E. These values point to the absolute importance of maintaining a FC residual in the pool. coli illness about once every 290,000 years. So if there is always 1 ppm FC with no CYA, there would be an E. To put this into perspective, consider that the Centers for Disease Control and Prevention estimated that in 2009 there were approximately 301 million swimming visits each year by persons over the age of six. With just 1 ppm FC and no CYA, the risk shows a tremendous drop down to 1 in 87,000,000,000,000 people. coli, is about 1 in 2,200 people if there is no FC in the water. coli O157:H7, a potentially deadly form of E. Using calculations from the model published by the ad hoc committee, the risk of getting ill from E. With a CYA:FC ratio of 20:1, the concentration of HOCl stays pretty constant at 0.02 ppm. The following table shows the HOCl concentration with various levels of CYA and FC (pH 7.5, 77 ☏): Luckily, when the CYA:FC ratio is kept constant, the HOCl concentration is kept constant, so operators would only need to measure CYA and FC to determine if sufficient sanitizer is present. The HOCl concentration can be calculated using a complex series of equations, but pool operators need a way to control their sanitizer levels without having to pull out a computer every time they run an analysis. ![]() Unfortunately, there is no pool-side test for HOCl, so there is no way for pool operators to directly measure the concentration of active sanitizer in the water. Even with a CYA concentration as low as 10 ppm, the HOCl concentration from 1 ppm FC is reduced to less than 0.066 ppm (pH 7.5, 85 ☏). In other words, there is about 70 times more active sanitizer at 0 ppm CYA than at 100 ppm CYA. This means that at 1 ppm FC, HOCl concentrations can range from 0.47 ppm HOCl (0 ppm CYA) to 0.0067 ppm HOCl (100 ppm CYA) (pH 7.5, 85 ☏). Many state codes allow up to 100 ppm CYA. The following graph shows how HOCl decreases with increasing CYA.Īs seen in the graph, the amount of CYA in the water has a dramatic effect on the HOCl concentrations. However, pool care training programs have not usually focused on the effect that CYA has on HOCl concentrations. The committee recommends that the current Model Aquatic Health Code ratio of cyanuric acid: free chlorine be reduced from 45:1 to 20:1.įor decades, pool care training programs have taught that pH must be controlled to ensure that sufficient hypochlorous acid, the active sanitizer in pools and spas, is maintained. CYA decreases the concentration of active sanitizer in the water and so increases the risk of disease transmission.
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