The findings are reported in the second issue of the December 2005 American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.
As part of the California-based Child Health Study, Frank D. Gilliland, M.D., Ph.D., of the Keck School of Medicine at the University of Southern California in Los Angeles, and eight associates analyzed 1,351 fourth grade students attending elementary school in southern California.
The researchers performed DNA analysis of cells from each child's cheek (buccal cells) and examined all participants' school absence reports. They found that genetic susceptibility caused by a variant genotype called "tumor necrosis factor 308A" influences the risk of respiratory-related school absences due to second-hand smoke.
Genotyping showed that 24 percent (324) of the fourth graders possessed one or more copies of the problem-causing variant.
According to baseline questionnaires completed by the children's parents, 20 percent had been exposed to second-hand smoke at home. Of those, 6 percent lived with one or more smokers.
Such exposure rates resulted in a 51 percent greater risk of lower respiratory illness compared with those who were not exposed. The researchers qualify this finding by noting that approximately 15 percent of the children involved in the study had physician-diagnosed asthma, which was associated with a 50 percent increase in risk for illness-related school absences.
The study also found that in children who possessed at least one copy of the tumor necrosis factor variant, exposure to two or more household smokers was associated with a four-fold risk of school absence due to lower respiratory illness, when compared with children who had the same variant, but who were not exposed to second-hand smoke.
The researchers added that second-hand smoke exposure increases the possibility of absence by raising the risk for and the severity of respiratory infection, as well as by increasing asthma-related airflow obstruction, inflammation and other symptoms.
"Adverse respiratory outcomes caused by second-hand smoke exposure include increased occurrence and severity of respiratory symptoms, respiratory infections, physician visits, emergency room visits, hospital admissions and transient changes in lung function," said Dr. Gilliland.
According to the Third National Health and Nutrition Examination Survey, the problem of second-hand smoke exposure in children is a serious one: in the United States, 43 percent of kids between the ages of 4 and 11 years are exposed to second-hand smoke in the home.
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