Hey pool owners! Is your chlorine “dead” in the water? When chlorine combines with contaminants, it becomes combined chlorine. And that chlorine isn’t as effective at killing off contaminants. Combined chlorine is also known as chloramines, which gives off that nasty chlorine-like smell in your water. So if you smell chlorine, it actually means you don’t have enough active chlorine in the water. You can get rid of that used-up chlorine by shocking your water with a chlorine or non-chlorine shock

Chlorine... - Health, Safety & Environment - HSE Education