Clues to Why Asthma Symptoms Are So Varied

Scientists have long known that asthmatics suffer more on dirty-air days but have been puzzled as to why symptoms vary greatly among individuals.

Now a study may help them understand this aspect of the illness--even as it raises questions about air quality standards.

Researchers at UC Berkeley, Harvard Medical School and Case Western University in Cleveland studied 846 asthmatic children from mostly poor, minority communities in eight cities across the United States.

They found that children born more than three weeks prematurely or weighing less than 5 1/2 pounds at birth had a sixfold decrease in breathing capacity on polluted days compared with kids who were born at full term or at full weight.

Furthermore, those children, ranging in age from 4 to 9 years, reported a fivefold increase in symptoms including wheezing, coughing and chest constriction. More than an inconvenience, those effects can lead to missed school days, hospitalization, even death.

The study, published Thursday in the American Journal of Respiratory and Critical Care Medicine, is the first to show air pollution aggravates asthma more severely in children who were born at low birth weight or prematurely. It also sheds light on the impact of federal air quality standards.

Severe asthma symptoms were reported on days when ozone, a key ingredient in smog, was below new national health limits established by the U.S. Environmental Protection Agency. Because there was wide agreement that existing smog standards did not adequately protect health, the EPA in 1997 issued more stringent limits for ozone and microscopic particles, though industry groups challenged the thresholds in court and they are not now in force.

Yet the study shows that even those standards may be insufficient to protect the most sensitive groups of the population, including many asthmatic children. The most affected kids reported symptoms even though air quality met the EPA’s new eight-hour ozone standard on 95% of the days during the study.

“The lower the birth weight and the more premature the child was as an infant, the greater the effect of pollution in triggering asthma attacks,” said Dr. Kathleen Mortimer, lead author of the study and an epidemiologist at the UC Berkeley School of Public Health. “Our results raise the question of whether the federal standards are sufficient to protect sensitive subgroups like children born prematurely.”

The research was conducted in the summer of 1993 in Baltimore; Washington, D.C.; Detroit; Cleveland; Chicago; St. Louis; and two locations in New York City. Ozone levels in the Los Angeles region are typically higher than in those cities.

David McKee, who heads the EPA program to set new smog standards, said the new ozone limit was not intended to protect every individual. To do so, he said, would require virtually eliminating the pollutant, which would be impractical and costly. He said that EPA recommends local air districts alert sensitive populations about rising ozone before it reaches concentrations considered threatening to the wider population.

Scientists are not sure why premature birth or low birth weight makes children appear more susceptible to smog, but they have some theories. It is possible that the airways in their lungs do not fully develop, which puts them at a disadvantage in coping with pollutants later in life, said Dr. Floyd Malveaux, an asthma expert and dean of the College of Medicine at Howard University in Washington, D.C.

Also, air pollution may enable other agents, such as dust or cat hair or flecks from cockroaches, to penetrate a mucous lining the airways and trigger an asthma reaction.

In the study, even full-term children and those of normal birth weight who were exposed to household antigens reported greater aggravation of asthma symptoms due to ozone.

It is not clear whether the findings in the paper apply to all asthmatic children of premature birth or low birth weight around the country. The kids who participated in the study lived in neighborhoods where more than one-third of the residents had incomes below the poverty line. No attempt was made to compare results for kids from different socioeconomic groups or determine whether they had access to health care.

And the research sheds no light on what might be causing children to get asthma in the first place. About 5 million U.S. children suffer chronic inflammation of the airways, known as asthma, and, for reasons not yet understood, more and more are diagnosed every year.

Experts have favorite theories, but none so far seem able to fully explain the situation. Multiple factors may be to blame, including poverty, air pollution, insect fragments and sedentary habits in children. It remains an enigma science has yet to solve.

What is clear, however, is that asthma is becoming more prevalent worldwide. About 15 million Americans have been diagnosed with the illness--50% more than 20 years ago--and it results in 100 million missed days of school and work, and 6,000 deaths annually. It is now the most common, chronic childhood illness, said Dr. Timothy Bigby, spokesman for the American Lung Assn. of California and chief of the pulmonary and critical care unit at the Veterans Affairs hospital at UC San Diego.

“It’s a serious and an amazingly prevalent problem,” Bigby said. “The people who are affected are, tragically, young people that are otherwise quite healthy. We’re all kind of mystified and shocked.”