Weekly visits to indoor swimming pools are damaging children’s lungs to the same degree as seen in adult smokers, say Belgian researchers. The cause, they suggest, is chlorine.

Alfred Bernard and colleagues from the Catholic University of Louvain tested blood from 266 eight-to 12-year-olds for three surface proteins that indicate lung damage from exposure to oxidants.

Lung damage was found to be correlated with exposure to chlorinated water: children who swam weekly had more of the protein in their blood than children who swam less frequently. Children who never swam had the lowest levels of the three groups studied.

Disinfected water loses its chlorine, which is converted to gas that combines with nitrogen and carbon to form toxic compounds. Dr Bernard said:

“These effects are most probably caused by nitrogen trichloride, an irritant and highly volatile water-insoluble gas present at relatively high concentrations in the air of poorly-ventilated, indoor, chlorinated pools. This gas is responsible for the chlorine odour.”

Presenting their findings at the 2001 Conference of the European Respiratory Society in Berlin, the team called for improved air quality control in indoor pools. Other researchers had found that lifeguards who worked in indoor pools had higher incidence of asthma than lifeguards at outdoor sites. “Ventilation is the key aspect. Outdoor pools are not really concerned by these effects,” Dr Bernard said.

Chlorine reacts with nitrogen in sweat and urine to form powerful oxidants that damage cells. Tissue with oxidised cells is more permeable and allows harmful substances to penetrate deeper. Inhaled particles can trigger allergic responses.

Though weekly swimmers in the study had the same level of surfactant proteins as adult smokers, the mechanism of the damage is different. In smokers it is due to irritation of the lungs whereas in swimmers it is due to oxidative stress.

Some authorities say it is too early to blame children’s asthma on chlorine in pools. They point to the benefits of swimming as a treatment for asthma.

Dr Bernard and his team plan to study the relationship between pool attendance and asthma. In the meantime they have called for a reduction of the amount of chlorine used in pools or a switch to disinfectants such as bromine.