SMOG damages the lungs of growing infants and causes asthma, suggests
unpublished research carried out at the University of California at Davis. The
findings confirm a link that has been widely suspected, but never proven.
In the past 10 years, studies have found circumstantial evidence of a link
between asthma and childhood exposure to ozone, one of the more noxious
constituents of smog. However, researchers have not proved that ozone causes
asthma because they cannot measure how much ozone an individual has been exposed
to.
To settle the issue, the Davis team carried out a series of experiments with
young rhesus monkeys. One group of monkeys had ozone added to their air supply.
A second group breathed air containing the dust mite allergen, a component of
household dust that is a common trigger of asthma attacks. A third group
breathed air containing both the allergen and ozone, and a fourth group breathed
clean air.
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The amount of ozone in the contaminated air supply was varied to mimic
conditions in the real world, says Charles Plopper, one of the team鈥檚 leaders.
For five days the monkeys breathed air containing ozone, followed by nine
ozone-free days鈥攁 cycle based loosely on Environmental Protection Agency
records of ozone levels in Los Angeles, says Plopper. The concentration of ozone
was 0.5 parts per million. 鈥淭hat is high for California,鈥� he says, 鈥渂ut about
average for Mexico City.鈥�
After five months of exposure, monkeys that had been breathing ozone
developed symptoms similar to those of a child with borderline asthma, says
Plopper. Their lung capacity was reduced and they wheezed when briefly exposed
to the dust mite allergen. Monkeys that had continually breathed both ozone and
the allergen had more severe reactions, similar to full-blown asthma attacks:
rapid, shallow breathing and decreased blood oxygen levels.
Both groups of ozone-exposed monkeys had lung abnormalities typical of people
with asthma. The smooth muscle that controls the flow of air through the lungs
was hyperactive, constricting the airways. The lungs made more mucus than usual,
clogging up the airways. And the monkeys had unusually low levels of
glutathione, a chemical that protects the lungs from free radicals. 鈥淭hat puts
them at risk for other types of lung injury,鈥� Plopper says.
He and his colleagues had previously found that ozone does not cause the same
degree of lung damage in adult monkeys. They believe, therefore, that ozone
exposure is especially damaging early in life, when the lungs are still
developing.
Because lung development is similar in monkeys and humans, the research may
help explain why children who grow up in smoggy cities tend to have more
respiratory problems, says Ira Tager of the University of California, Berkeley,
School of Public Health. 鈥淚t fits well with what鈥檚 known in humans about
developmental effects,鈥� he says.