Parental smoking and lower respiratory illness in the first three years of life.

The relationships between parental smoking and the rates of lower respiratory illness during the first three years of life were examined for a birth cohort of 1265 New Zealand children. Lower respiratory illness varied significantly with maternal smoking for the first year; there was equivocal evidence of a relationship between maternal smoking and lower respiratory illness in the second year; and by third year the relationship had clearly disappeared. Paternal smoking had no significant effect on rates of lower respiratory illness at any time. Application of logistic regression showed that for the first year rates of lower respiratory illness were approximately linearly related to maternal smoking: increases of five cigarettes a day resulted in an increase of 2.5 to 3.5 incidents of lower respiratory illness per 100 children at risk. Statistical control for maternal age, education, family size, and family living standards showed that the relationship between maternal smoking and rates of lower respiratory illness was not significantly influenced by these factors.