(I) Some research in Belgium has shown that lung damage may occur in children and adults as a result of swimming In an indoor pool where the level of chlorine in the water exceeds 490 micrograms per cubic meter. (II) This is because a toxic gas builds up when chlorine reacts with organic chemicals in sweat and urine.(III) A survey of 1881 children in Brussels also revealed a strong link between asthma and pool attendance. (IV) The gas, nitrogen trichloride, is what gives indoor pools their distinctive smell. (V) And the younger the children were when they started swimming the more likely they were to have asthma.