Age and sex modify the association between secondhand smoke exposure and self-reported asthma in school-aged children (6–11 years): a cross-sectional study from NHANES 2005–2020

Abstract Background Secondhand smoke (SHS) is a known risk factor for childhood asthma, but whether susceptibility varies among school-aged children remains unclear. We aimed to investigate whether age and sex modify the association between SHS exposure and asthma in a nationally representative sample of U.S. children. Methods We analyzed data from 5,662 children aged 6–11 years from the National Health and Nutrition Examination Survey (NHANES) 2005–2020. Asthma status was determined by self-reported physician diagnosis. SHS exposure was defined based on serum cotinine levels (≥ 0.05 ng/mL). Multivariable logistic regression was used to assess the association, adjusting for demographic, socioeconomic, and environmental factors, including age, sex, BMI, race, poverty-income ratio(PIR), household smoking, maternal smoking during pregnancy, and family history of asthma. Multiplicative and additive interactions were examined for age and sex. Results After adjustment for demographic, socioeconomic, and environmental factors (Model 2), SHS exposure was significantly associated with increased odds of self-reported asthma (OR = 1.28; 95% CI: 1.1–1.5). This association was significantly modified by both age (P for interaction = 0.03) and sex ( P for interaction = 0.02). The effect was most pronounced in younger children (< 9 years; (OR = 1.59, 95% CI: 1.23–2.05) and in females (OR = 1.54, 95% CI:1.18–2.00). In contrast, the association was attenuated and non-significant in older children (≥ 9 years; OR = 1.02, 95% CI: 0.80–1.31) and in males ((OR = 1.09, 95% CI: 0.86–1.28, P = 0.46). Conclusions In this cross-sectional study of school-aged children, SHS exposure was associated with higher odds of self-reported asthma. This association was modified by age and sex on the multiplicative scale, with stronger relative associations observed in younger children (< 9 years) and females. These findings underscore the need for targeted tobacco control interventions and suggest that future research should consider demographic factors as effect modifiers rather than mere confounders.