Abstract

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 8th birthday in 2005. We used multivariable Cox regression to assess the relation between post-natal 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 8.0 years of age, for all individuals and for vulnerable sub-groups.

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 (HR 1.14, 95% CI 1.00 to 1.31), after adjustment for sex, birth-weight, 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 sub-groups such as premature babies, the association held through to 8 years old (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.