The most common chemical is calcium hypochlorite which is generally used every few days. If the treatment is done in sunlight a large part of the active hypochlorite is lost to the air as chlorine gas. The build up of chlorine gas may be an issue of concern in some indoor swimming pools. Most pool chemicals are strong oxidisers and many are corrosive. The majority of the chemicals are toxic when ingested.
Some people have or develop skin reactions after contact with pool chemicals.