Asthma and ozone

Children face a greater risk

Published: Wednesday 31 January 2001

a recent set of experiments has proved that exposure to excessive airborne ozone, one of the more noxious constituents of smog, can cause asthma.

The experiments were carried out on young rhesus monkeys. One group of monkeys had ozone added to their air supply, while a second group breathed air containing the dust mite allergen, part of the household dust that is a common trigger of asthma. A third group was made to breathe air containing both the allergen and ozone. The last group breathed clean air. The amount of ozone added was the equivalent to that found in the ambient air.

The monkeys breathed ozone-replete air for five days, followed by nine 'ozone-free' days. The cycle was formulated on the basis of ozone levels recorded in Los Angeles by the us Environmental Protection Agency. After five months of exposure, monkeys breathing ozone developed symptoms observed among children with borderline asthma; their lung function capacity was low. Brief exposure to the dust mite allergen caused wheezing. Monkeys breathing both ozone and the allergen continually had more severe reactions. These resembled full-blown asthma attacks -- rapid, shallow breathing and decreased blood oxygen levels. Both groups of monkeys exposed to ozone experienced lung abnormalities typical of people with asthma. The muscles that control airflow through lungs were found constricting the airways. The lungs formed more mucus than usual, clogging the airways. The monkeys had unusually low levels of glutathione, a protective chemical.

The past 10 years have produced only circumstantial evidence of the link between asthma and children's exposure to ozone. However, researchers failed to prove conclusively that ozone causes asthma. They could not measure the amount of ozone exposure that an individual faced.

The study confirms fears raised by previous studies. It reveals that ozone does not cause the same degree of lung damage among adult monkeys. This implies that exposure to smog early in life, when the lungs are developing, could be more dangerous. As lung development in monkeys and humans is quite similar, the research may help explain why children who grow up in smoggy cities tend to have more respiratory problems, says Ira Tager of the school of public health at the University of California, Berkeley, usa .

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