Combined Chlorine

Combined Chlorine is a generic chemical term for chemical compounds formed from the reactions of chlorine with organic and biological contaminants in pool water.
Chlorine, and specifically the active chlorine compound hypochlorous acid (HOCl), is a very powerful oxidizer. Oxidizers, in chemical terms, take electrons away from the molecules that they oxidize and, in the process, break those molecules down into different compounds. The most common forms of combined chlorine found in swimming pools are those compounds where chlorine has reacted with nitrogen-containing chemicals in human bather waste (sweat, urine, etc).
Combined chlorines are responsible for that “gross chlorine pool smell” and that smell is a good sign that the pool water it is coming from is not being properly maintained.