Aqua Magazine: CYA and Chlorine in Plain Language[1]
The most important quote from that article is With a CYA:FC ratio of 20:1, the concentration of HOCl stays pretty constant at 0.02 ppm. Which means that 20 ppm CYA and 1 ppm FC is virtually the same as 200 ppm CYA and 10 ppm FC. With 3 ppm FC and zero CYA (perfectly safe and ideal according to the CDC) a body of water with pH of 7.5 at 80 degrees F would have an HOCl of 1.451. A pool with 2.5:1 CYA:FC concentration (SLAM Level) would have roughly 0.3 ppm HOCl. So a pool at SLAM level has only 1/5th the active chlorine (HOCl) as a pool that is within CDC guidance for safe and ideal. More chlorine can be less harsh, one of the most difficult things for anybody new to TFP to buy in to until it is experienced.
