There is a growing literature on health problems related to swimming in indoor chlorinated pools. Chlorine levels in the air even an inch or 2 above the water level are exceedingly low and often are unrecordable even in very smelly indoor pools. The smell in the air is not of chlorine but of chloramines, particularly nitrogen trichloride.
Most pool managers are more concerned with the sterility of the water than the air that swimmers breath. At least in cold climates there are substantial problems in keeping the air warm above the pool if a large volume of air is extracted by the air conditioning systems. There are technical ways of overcoming this with heat exchangers. Many pools do not have such sophisticated systems.
Much of the chloramines come from swimmers who urinate in the pool and who fail to wash before getting in the pool. This seems to be a particularly UK problem. In some countries, washing before going into the pool is mandatory and urinating in the pool leaves a blue stream behind the swimmer who can then be extracted (by adding a pH indicator to the water). I believe this happens in a number of pools in Germany.
Nearly all pools have free chlorine (or sometimes free bromine) in the water. The faster the water is turned over, the less residual chlorine is needed in the water. With pools with relatively slow water turnover 1 -2 % free chlorine in the water is common. For pools with rapid turnover 0.5% is often enough.
it is possible to sterilise water outside the pool, for instance with ozone or hydrogen peroxide. However, the water then returning to the pool has no biocidal activity and some chlorine is usually added for this.
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