Early life second-hand smoke exposure and serious infectious morbidity during the first 8 years: evidence from Hong Kong's "Children of 1997" birth cohort.

BACKGROUND: Second-hand smoke (SHS) exposure is a modifiable cause of ill health. Despite the smoking ban in public places introduced in Hong Kong in 2007, infants and children continue to be exposed within the home. AIMS: To determine the critical windows of SHS exposure and the duration of its impact on serious infectious morbidity in the first 8 years of life.
METHODS: The Hong Kong "Children of 1997" birth cohort is a prospective, population-based study of 8327 children comprising 88% of all births in April and May 1997, of whom 7402 (89%) were followed up until their eighth birthday in 2005. We used multivariable Cox regression to assess the relation between postnatal SHS exposure and risk of first admission to public hospitals (together accounting for >95% total bed-days overall) for respiratory, other and all infections from birth to 8 years of age, for all individuals and for vulnerable subgroups.
RESULTS: Overall, household SHS exposure within 3 metres in early life was associated with a higher risk of admission for infectious illness up until 8 years of age (hazard ratio 1.14, 95% CI 1.00 to 1.31), after adjustment for sex, birthweight, gestational age, feeding method, maternal age, highest parental education and proxies of preferred service sector. The association was strongest in the first 6 months of life (HR 1.45, 95% CI 1.15 to 1.83). In vulnerable subgroups such as premature babies, the association held through to 8 years of age (HR 2.00, 95% CI 1.08 to 3.72). Infants exposed to SHS in the first 3 months of life were most vulnerable to infectious causes of hospitalisation.
CONCLUSION: Household SHS exposure in early infancy increases severe infectious morbidity requiring hospital admission. Reducing SHS exposure in infants and particularly in more vulnerable infants will lower the bed-days burden due to infectious causes.