See our page on Chemical Safety for more info. Being closed systems, pools just don't contact nature very much. There could be some hazard involved with waste water from the pool that has either extremely high chlorine levels or extremely low pH levels. It is unlikely that the amount of water expelled during backwashing your filter could pose much harm, but impacts have been seen on drain and clean jobs. Pool water is very similar in make up to the city water right out of the tap.
Many people water their lawns with higher chlorine and lower pH than is found in their pool.
There are certain industrial uses of chlorine chemistry which are detrimental to the environment, but in a swimming pool environment, the potential hazards are much lower, even non-existent. There has been much publicity and controversy over chlorine in recent years, with some organizations calling for bans on its use. You decide if chlorine chemistry is right for you and your pool. For more information on chlorides, contact the NSPI . In its elemental state, chlorine exists as a gas.