The association of family size with atopy and atopic disease.

Studies in children have shown that family size is negatively associated with atopy and atopic disease. To describe the association of family size with atopy and atopic disease in young adults. A randomly selected sample of 1159 men and women aged 20-44 years provided information on respiratory symptoms, hay fever and eczema. Blood samples were taken for assessment of total IgE and specific IgE to house dust mite, grass, cat, Cladosporium and birch. The association of family size and birth order with respiratory symptoms, atopy and total IgE was assessed by multiple logistic and linear regression. There was a negative association between family size and the reporting of 'wheeze with breathlessness' (adjusted odds ratio for an increase of one sibling 0.85; 95% confidence interval 0.75-0.98), 'wheeze without a cold' (adjusted odds ratio for an increase of one sibling 0.85; 95% confidence interval 0.75-0.98) and 'asthma attacks' in the last 12 months (adjusted odds ratio for an increase of one sibling 0.77; 95% confidence interval 0.61-0.97), current 'hayfever and nasal allergies' (adjusted odds ratio for an increase of one sibling 0.84; 95% confidence interval 0.75-0.94) and sensitization to grass (adjusted odds ratio for an increase of one sibling 0.87; 95% confidence interval 0.76-0.99). Birth order was negatively associated with 'hayfever and nasal allergies' only. A decreased risk of sensitization to grass in those from large families did not fully explain the negative association between family size and hayfever. No statistically significant (P > 0.05) association of family size or birth order with the reporting of other respiratory symptoms, eczema, sensitization to the other allergens or total IgE was observed. There is a negative association between family size and some symptoms suggestive of asthma, 'hayfever and nasal allergies' and sensitization to grass in young adults. There is no consistent, significant association between family size and eczema, total IgE or sensitization to other allergens.