title: Thumb-Sucking, Nail-Biting, and Atopic Sensitization, Asthma, and Hay Fever.
abstract: 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.
  
  
  
  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.
  
  
  
  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.
  
  
  
  Children who suck their thumbs or bite their nails are less likely to have atopic sensitization in childhood and adulthood.
authors.affiliation: Department of Preventive and Social Medicine, Dunedin School of Medicine, University of Otago, Dunedin, New Zealand; and.
authors.forename: Stephanie J
authors.initials: SJ
authors.lastname: Lynch
authors.affiliation: Firestone Institute for Respiratory Health, Michael de Groote School of Medicine, McMaster University and St Joseph's Healthcare, Hamilton, ON, Canada.
authors.forename: Malcolm R
authors.initials: MR
authors.lastname: Sears
authors.affiliation: Department of Preventive and Social Medicine, Dunedin School of Medicine, University of Otago, Dunedin, New Zealand; and bob.hancox@otago.ac.nz.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Hancox
mesh_terms: D000293:Adolescent; D000328:Adult; D001249:Asthma / Q000453:epidemiology*; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D005260:Female; D005386:Fingersucking; D005500:Follow-Up Studies; D006801:Humans; D006969:Hypersensitivity, Immediate / Q000453:epidemiology*; D008297:Male; D009259:Nail Biting; D006255:Rhinitis, Allergic, Seasonal / Q000453:epidemiology*; D018570:Risk Assessment