Chlorine is the active chemical element most commonly used in the sanitization of swimming pools, though other chemicals are used as cleaners as well. Bromine is the next most common chemical used in cleaning pools.
Chlorine in its natural state is a heavy, green gas that is highly toxic to humans and other forms of life. As such, special chlorine compounds that are less dangerous have been formulated for pool sanitation. Bromine is similar to chlorine in that both are halogens, but bromine remains in a liquid state at room temperature. Pure bromine is also harmful to humans, so special bromine compounds have been made for use in pools.