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Study Links Early Exposure to Herbicides and Wood and Oil Smoke to Childhood Asthma Risk: Presented at ATS

By Peggy Peck

SEATTLE, WA -- May 26, 2003 -- Early-life exposure to cockroaches, herbicides, and wood and oil smoke were cited as major risk factors for childhood asthma based on the latest results from the Children's Health Study.

Since 1993, this case-control study has been following 3,000 Los Angeles children for respiratory health. The findings were presented here on May 21^st at the American Thoracic Society International Conference.

Frank D. Gilliland, MD, PhD, professor of preventive medicine at the Keck School of Medicine, University of Southern California, Los Angeles, United States, and colleagues identified 338 children who had physician-diagnosed asthma before age 5 and 570 controls who were asthma free. Participants were matched for age, exposure to maternal smoking during gestation, and community of residence.

Several findings in the current analysis were surprising, Dr. Gilliland said in an interview. "We expected that breastfeeding would be protective, but it was not. We expected that day care would be protective, but exposure to day care before 4 months of age actually increased risk by a factor of 2.34," he said. The most surprising finding, he noted, was "that maternal age greater than 35 was associated with a 70% increase in risk for asthma."

Dr. Gilliland said that, while early day care was associated with increased risk, "having siblings was associated with a reduced risk." Exposure to farm crops before the age of 1 year increased risk nearly two times, while exposure to wood or oil smoke or exhaust anytime from birth to age 5 increased risk more than 50%. Exposure to herbicides increased risk by 4.5 times, and exposure to pesticides increased risk by 2.4 times.

"We saw no effect for pets," said Dr. Gilliland, contradicting several other studies, which reported that children who are raised in homes with pets are less likely to develop asthma.

Dr. Gilliland said one disturbing finding was that "about 90% of women who smoke continue to smoke during pregnancy, even though smoking during pregnancy is associated with a twofold increase in risk of asthma.

Homer A. Boushey, Jr., MD, professor of medicine, chief of the division of allergy/immunology and director of the Asthma Clinical Research Center at the University of California, San Francisco, United States, questioned several of the associations reported by Dr. Gilliland's team.

"Cockroaches, wood or oil smoke, older siblings -- that makes sense, and these findings are in concordance with associations reported in other studies," said Dr. Boushey. He pointed out that several studies, however, found that pets and farm animals are protective: "In one European study there was an association with the level of endotoxins -- we are really talking about manure -- found in [farm-animal] bedding. Higher levels were protective [N Engl J Med 2002 Sep 19;347(12):869-77]."

Dr. Boushey, who was not involved in the study, is president of the American Thoracic Society.


[Study title: Early Life Risk Factors for Asthma: Findings from the Children's Health Study. Abstract D011]

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