Smog aggravates asthma in children born prematurely or of low birthweight

By Catherine Zandonella, Public Affairs

29 NOV 2000 | Smog-induced asthma symptoms are more severe in children born prematurely or of low birthweight, according to a study of 846 inner-city children conducted by researchers at Berkeley.

"The lower the birthweight and the more premature the child was as an infant, the greater the effect of pollution in triggering asthma attacks," said Kathleen Mortimer, a School of Public Health epidemiologist and lead author of the study, which appears in the Nov. 16 issue of American Journal of Respiratory and Critical Care Medicine.

While researchers have known that asthmatics often suffer more wheezing and shortness of breath on days when air pollution levels soar, they have been puzzled to see that the severity of symptoms varies greatly from patient to patient. Mortimer and Professor of Epidemiology Ira Tager, working with colleagues at Harvard Medical School and Case Western Reserve University, went looking for reasons why some children react more severely to air pollution than others.

The team found that asthmatic children born more than three weeks prematurely or weighing less than 5.5 pounds had a six-fold decrease in breathing capacity on smoggy days as compared to full-weight, full-term children. The children also reported a five-fold greater incidence of symptoms like wheezing, coughing and tightness in the chest.

Previous studies have found that low birthweight and premature birth are risk factors for asthma, but this study is the first to show that air pollution aggravates asthma more severely in such children.

"It is a piece of evidence that has to be taken seriously," said Tager. "This study demonstrates the short-term adverse consequences of air pollution at relatively low levels."

Indeed, it did not take a large jump in air pollution to provoke asthma symptoms in the children. Less than five percent of the days in the study, which spanned from the beginning of June to the end of August, had air pollution levels that exceeded federal standards.

"Our results raise the question of whether the federal standards are sufficient to protect sensitive subgroups like children born prematurely," said Mortimer. The study could help regulators establish air quality standards targeted towards sensitive subgroups, rather than blanket standards.

Approximately five million, or seven percent, of children under age 17 suffer from asthma nationally. Increased asthma symptoms translate to more missed school days, more physician visits and more hospitalizations, all of which can have significant financial impacts.

The study was part of a larger, multi-center study of inner-city children aged four to nine who suffer from asthma. The children lived in neighborhoods where at least 30 percent of resident had incomes below the U.S. poverty line, and that were located in eight urban areas across the nation: The Bronx and East Harlem, N.Y.; Baltimore, Md.; Washington, D.C.; Detroit, Mich.; Cleveland, Ohio; Chicago, Ill.; and St. Louis, Mo.

Because all the children were from low-income neighborhoods, the study could not examine whether socioeconomic status influenced the children's response to air pollution. "There is no reason to think this wouldn't hold up for all socioeconomic classes," said Tager, "but a study to confirm that should be done."

During the study, Mortimer and her colleagues asked children to keep daily records of symptoms such as wheezing, coughing and tightness in the chest. Each child also measured his or her peak exhalation flow rate by breathing into a measuring device each morning and evening.

The researchers interviewed parents to determine which children were born prematurely or of low birthweight. The researchers compared the asthmatic symptoms to the average air quality data gathered in each city by the U.S. Environmental Protection Agency. Of the four pollutants measured by EPA (sulfur dioxides, nitrogen dioxide, ozone and particulate matter of less than 10 micrometers in size), only ozone measurements were available for all days in all eight urban areas, so ozone levels were used as a marker for summer air pollution.

Only morning symptoms were elevated during smoggy periods. Symptoms that occur in the morning may be a more sensitive indicator of a child's response to air pollution, said Mortimer, because the children had not taken any asthma medication yet or engaged in any strenuous activity that might have exaggerated the effect.

The study did not see any evidence that exposure to environmental tobacco smoke influenced how the children responded to outdoor air pollution.