MARTIN MITTELSTAEDT
ENVIRONMENT REPORTER

The elevated incidence of asthma in wealthy Western countries has long puzzled researchers seeking to explain why affluence would be associated with a debilitating respiratory ailment. Now, a handful of studies suggests at least part of the answer may be the exposure of young children to the heavily polluted air emanating from chlorinated indoor swimming pools.

This so-called "pool chlorine hypothesis" has received a boost from these papers, including one study that found the incidence of the disease among European children closely tracks the number of pools in a country, and another based on a survey in Belgium that found the most consistent predictor of whether a child would develop the disease, after a family history of allergies or asthma, was high pool use.

The reason why indoor pools might cause asthma is being attributed to chlorine, which is added to the water to keep swimmers safe from infectious diseases.

While chlorine is an effective disinfectant, it has a drawback. When it destroys the bacteria and viruses found in water, and does the same to the sweat, urine, and saliva from swimmers, it produces trichloramine, the gas that is the main cause of the pungent odour associated with swimming pools.

Trichloramine, also known as nitrogen trichloride, is a powerful oxidant that, when inhaled, is able to damage the surface of lung tissues, making them more permeable to substances that might ultimately trigger asthma, such as air pollution, pet dander and other allergy-causing substances, according to the researchers investigating the possible link between pools and asthma.

"In these countries where you have a very high prevalence of asthma, you also have a lot of swimming pools, and because of the weather, the swimming pools are indoors," said Alfred Bernard, a toxicologist at the department of public health at the Catholic University of Louvain, in Brussels, and co-author of two studies issued earlier this month.

"We suspect this chemical affects the lungs, making the lungs more permeable to these allergens," he said.

The finding may also explain why competitive swimmers and lifeguards at indoor pools also have elevated asthma risks.

A third study, published last year in the New England Journal of Medicine, concluded that heavily chlorinated whirlpool baths might be harmful to those suffering asthma.

There are currently no standards for the amount of trichloramine allowed in the air around pools, except in Brussels, and Dr. Bernard said concentrations can attain such high levels that the gas often is the most intense indoor air pollutant to which young children are exposed.

He is particularly worried about the recent popularity of baby swimming programs, which expose the very young to high levels of pollutant and may contribute to asthma later in life.

The rise in the incidence of asthma and its peculiar international distribution has long perplexed researchers. The respiratory ailment is often 10 times more prevalent in wealthy industrialized countries than elsewhere, making it a disease of affluence.

Canada has one of the world's highest rates -- asthma strikes more than one in 10 children -- a rate that has made it the most common chronic disease of childhood in this country. Although asthma was once rare, the prevalence has soared in recent decades. In the United States, where good statistics are kept, the incidence rate has doubled in only the past two decades.

Genetic changes wouldn't happen quickly enough to be the reason for such a rise.

This has led investigators to look at what has changed in the environment that might be linked to asthma, such as exposure to secondhand smoke, pesticides, and the fumes given off by some plastics -- pollutants that are all able to irritate the lungs and are found in indoor air.

Researchers have also speculated that industrialized countries might be too hygienic, producing such a clean and sterile environment that children don't come into contact with enough germs early in life to strengthen their developing immune systems, leading to allergies, and to asthma attacks when youngsters are exposed to pollutants later in life.

There are disputes about whether excessive cleanliness is the real cause because of the timing of the increase in asthma incidence. The big advances in hygiene occurred before the 1940s, while the asthma incidence rate began rising in the 1960s.

Mr. Bernard said that one intriguing factor in the asthma incidence is that English speaking countries, including the U.K., Ireland, Australia and Canada have the highest rates of the disease, something he says may be linked to the earlier development in these jurisdictions of indoor swimming pools.

Pools can avoid the chlorine problem by increasing the amount of ventilation in their buildings, something that can be costly in cold countries, such as Canada.

However, it is not clear how much extra ventilation would be needed to get pollutants to low levels. The American Society of Heating, Refrigerating and Air-Conditioning Engineers, a professional body based in Atlanta, is currently funding research to find out how much fresh air would be needed to remove or dilute swimming pool pollutants.

Currently, it is recommended that pool areas have six to eight complete changes of the air each hour to maintain adequate ventilation.

Mr. Bernard said pools can also be disinfected using better filtration, ozone, and ultraviolet light, among other ways, avoiding the off-gassing problem from trichloramine.

WORLDVIEW

Asthma rates vary widely around the world, but the highest incidence is usually found in affluent Western countries.

Percentage of population living with asthma

SOURCE: GLOBAL INITIATIVE FOR ASTHMA REPORT