How much atopy is attributable to common childhood environmental exposures? A population-based birth cohort study followed to adulthood.

Background: The rising prevalence of atopic diseases implies a strong influence of environmental determinants. Epidemiological studies have identified several early life exposures that appear to influence the risk of developing atopic sensitization, but the combined influence of these exposures is unknown. We sought to estimate the proportion of atopy that could be attributed to common childhood exposures associated with atopic sensitization in adolescence and young adulthood. Methods: Atopic sensitization was measured by skin-prick tests for common aeroallergens in a population-based New Zealand birth cohort at ages 13 and 32 years. The independent effects of previously identified risk and protective factors for atopic sensitization were assessed using multiple logistic regression. Population attributable fractions were calculated for atopic sensitization in childhood and adulthood. Results: Tobacco smoke exposure, dog and cat ownership, nail-biting and thumb-sucking, attending pre-school day care, and household crowding were associated with a lower risk of atopic sensitization whereas breastfeeding was associated with a higher risk. Population attributable fractions for combined effects of these environmental factors suggest that they may account for 58% of atopic sensitization at age 13 and 49% at age 32 years. Conclusions: A substantial proportion of atopic sensitization appears to be attributable to common childhood environmental and lifestyle factors, and the influence of these exposures persists into adulthood. The absolute risks attributable to these exposures will be different in other cohorts and we cannot assume that these associations are necessarily causal. Nevertheless, the findings suggest that identifiable childhood environmental factors contribute substantially to atopic sensitization.

Thumb-Sucking, Nail-Biting, and Atopic Sensitization, Asthma, and Hay Fever.

BACKGROUND: The hygiene hypothesis suggests that early-life exposure to microbial organisms reduces the risk of developing allergies. Thumb-sucking and nail-biting are common childhood habits that may increase microbial exposures. We tested the hypothesis that children who suck their thumbs or bite their nails have a lower risk of developing atopy, asthma, and hay fever in a population-based birth cohort followed to adulthood. METHODS: Parents reported children's thumb-sucking and nail-biting habits when their children were ages 5, 7, 9, and 11 years. Atopic sensitization was defined as a positive skin-prick test (≥2-mm weal) to ≥1 common allergen at 13 and 32 years. Associations between thumb-sucking and nail-biting in childhood, and atopic sensitization, asthma, and hay fever at these ages were assessed by using logistic regression with adjustments for sex and other potential confounding factors: parental atopy, breastfeeding, pet ownership, household crowding, socioeconomic status, and parental smoking. RESULTS: Thirty-one percent of children were frequent thumb-suckers or nail-biters at ≥1 of the ages. These children had a lower risk of atopic sensitization at age 13 years (odds ratio 0.67, 95% confidence interval 0.48-0.92, P = .013) and age 32 years (odds ratio 0.61, 95% confidence interval 0.46-0.81, P = .001). These associations persisted when adjusted for multiple confounding factors. Children who had both habits had a lower risk of atopic sensitization than those who had only 1. No associations were found for nail-biting, thumb-sucking, and asthma or hay fever at either age. CONCLUSIONS: Children who suck their thumbs or bite their nails are less likely to have atopic sensitization in childhood and adulthood.