More siblings, less hay fever: more evidence.

The prevalence of allergic rhinitis is increasing. Many authors have noted the prevalence of hay fever is less in children from large families and the 'hygiene hypothesis' has been suggested as an explanation. We looked at the association of sibling number and other variables on the prevalence of hay fever and perennial rhinitis. From 26100 households selected at random, 34362 questionnaires were returned from individuals over 14 years of age; 19.3% had symptoms of hay fever and a further 6.0% had symptoms of perennial rhinitis. A logistic regression analysis showed that for individuals with two or more siblings, the odds of suffering from hay fever are less than the odds for people with no siblings and that these odds decrease as the number of siblings increases. This effect was not found in subjects with perennial rhinitis. This study adds weight to the 'hygiene hypothesis'.