Chlorine is still the most popular means of sanitizing swimming pool water. It does an excellent job sanitizing water, but it does so at a huge price.
Chlorine is a very caustic, corrosive chemical. It is effective because it attacks anything organic. Unfortunately, it also attacks the skin, hair, and eyes of the humans swimming in it.
Chlorine is toxic and hazardous to handle, ingest or breathe. It is also dangerous to store. Chlorine can be extremely flammable.
Chloramines, a by-product of chlorine and organic waste, are known carcinogens. Unfortunately there is no practical way of preventing chloramines from forming in chlorinated swimming pools.
Maintaining chlorine levels at minimal acceptable levels can be very difficult to do at times. Residual chlorine is highly affected by the water temperature, rain, pH, and number of swimmers. Chlorine is absorbed in the skin of swimmers and therefore the more people that swim in the pool, the more chlorine is absorbed by their skin and the more chlorine that needs to be added to the pool water to properly sanitize the water.