Online Data Supplement to Effects of Maternal Smoking during Pregnancy and Environmental Tobacco Smoke on Asthma and Wheezing in Children

Questionnaires were collected from parents of 5762 students in the 12 study communities (Table E1). The majority of students were 10 yr of age or less, white, and from households with health insurance and high educational attainment. Twenty-three percent of participants had no siblings at the time of interview, 19.5% a family history of asthma, 47.7% a family history of atopy, and 35.3% a personal history of atopy. Premature birth occurred for 10.8% of children and 9.6% required neonatal special care. A severe chest illness before age 2 occurred in 10.6% of participants.

In utero exposure to maternal smoking occurred for 18.8% of children and 39.5% had any lifetime exposure to ETS (Table E2). Both ETS and in utero exposure varied by age and cohort, reflecting decreases in the prevalence of parental smoking. ETS exposure was highest among African American children, children with a parent who had less than a college education, children from low-income families, families with one child, and for those whose questionnaire was not completed by a parent. In utero exposure was lower among Asian and Hispanic children than among non-Hispanic white children and among children whose parent had a college degree than those whose parent had a lower educational attainment. The number of siblings was inversely associated with in utero exposure.

The lifetime prevalence of wheezing was 33.7% and was higher in males than females (Table E2). A physician diagnosis of asthma was reported for 14.6% of children and males had a higher prevalence than females. Most cases were diagnosed by age 5 and the majority of children who were ever diagnosed with asthma continued to require medication or had persistent symptoms since entry into 1st grade.

Of the potential determinants of physician-diagnosed asthma shown in Table E1, tobacco smoke exposure, race/ethnicity, hay fever, family histories of asthma or allergy, gestational age, pets, and use of a humidifier were univariately associated with asthma (Table E3a and Table E3b). The odds of physician-diagnosed asthma were higher, especially among boys, for African Americans compared with non-Hispanic whites, and lower for children with a parent who had less than a high school education. However, household income was not strongly associated with physician-diagnosed asthma. Children who were born prematurely were more likely to be diagnosed with asthma than children born at full-term.

We found that in utero exposure to maternal smoking without subsequent postnatal ETS exposure was associated with physician-diagnosed asthma and wheezing. In the univariate analyses, exposure to maternal smoking in utero was associated with physician-diagnosed asthma (Table E4). The odds of physician-diagnosed asthma for in utero exposure alone was 1.8-fold higher (95% CI 1.1 - 2.9) than for no involuntary tobacco smoke exposure and did not vary substantially between males and females.

The effect of involuntary tobacco smoke exposure appeared to vary by age at asthma diagnosis (Table E4). Among children who were diagnosed by age 5 years, in utero exposure to maternal smoking alone had an adjusted OR of 2.1 (95% CI 1.2 - 3.7) compared with no tobacco smoke exposure (Table E5). In contrast, among children diagnosed after age 5 years, in utero exposure to maternal smoking had an adjusted OR of 1.2 (95% CI 0.5 - 2.8). Although estimates were imprecise, the effect of ETS exposure appeared to be larger in the older age at diagnosis group, especially among girls (OR 1.4, 95% CI, 0.9 - 2.1).

In contrast to the findings for physician-diagnosed asthma, both in utero exposure to maternal smoking alone and ETS exposure alone were independently associated with lifetime history of wheezing (Table E5). In general, joint exposure appeared to increase the individual effects of ETS exposure, but not in utero exposure, on wheezing; however, the point estimates were imprecise. The relationships were not accounted for by adjustment for the potential confounders presented in Table E1. Indoor exposure variables including pets, pests, air conditioning, gas stove, water damage, and humidifier use did not confound the relationship between ETS and asthma or wheezing. The effects of ETS and in utero exposure on wheezing did not vary substantially between males and females.