Public indoor pools have a lot of chlorine in the water. And the chlorine smell in the pool area can be overpowering. I think chlorine is a major health hazard. It was first used around the turn of the century to purify public water supplies, dramatically reducing the incidence of cholera and typhoid fever.
Chlorine works because it is a powerful and toxic oxidizing agent.
During World War I, German use of chlorine gas was the first significant instance of a chemical weapon in warfare. In addition to chlorine's caustic properties and the harm it does to lung tissues, it can also promote the development of cancer. Some studies have associated long-term consumption of chlorinated water with bladder and rectal cancer. Others suggest that it might also increase risk of coronary heart disease. I think it is not healthy to breathe the chlorine fumes that accumulate in and around the swimming pool.