Tobacco smoke exposure

Antenatal smoke exposure is an important risk factor for increased respiratory symptoms and lung function abnormalities in infants and children. In children less than 2 years of age, the risk for lower respiratory illness has been found to be increased by 72% if the mother smoked. Although the observed increase in risk is lower in older children, parental smoking may nevertheless account for approximately 20% of all asthma in childhood. Maternal environmental tobacco smoke (ETS) exposure during the third trimester of pregnancy is associated with asthma and allergy-related symptoms in pre-school children. Certain infants may have a genetic susceptibility to the adverse effects of environmental smoke exposure, both maternal during pregnancy and in infancy. Antenatal smoking exposure has been demonstrated in some, but not all, studies to have an adverse effect on lung function in infancy. In older children, antenatal smoke exposure has been associated with a reduction in airway function. The effects of passive smoking exposure vary with genetic factors, sex, race and exposure to other pollutants. Exposure to ETS and subsequent active smoking both aggravate symptoms and have a negative effect on lung function. Passive smoke exposure in the first 3 months after birth also increases the risk of hospital admission for infectious illness. The association is strongest in the first 6 months after birth, but in vulnerable groups, such as prematurely born infants, the association has been shown to hold through to 8 years of age. Bronchiolitis also occurs more frequently in infants of mothers who smoke. Exposure in later childhood to ETS is associated with increased respiratory symptoms, although the effect appears to diminish with increasing age of the child.

Infants less than 6 weeks old
Pre-term birth
Chronic illness
                          Bronchopulmonary dysplasia
                          Other chronic lung disease
                          Congenital heart disease
                          Neurological disease
                          Immunodeficiency
Maternal factors  
                          Asthma
                          Smoking during pregnancy
Environmental factors  
                          Poverty
                          Over-crowding
                          Passive smoking
Table 1 – Risk factors for severe respiratory syncytial virus infection. Modified from Greenough and Broughton, 2005.

Antenatal smoking exposure may have a more deleterious effect than passive smoke exposure after delivery. Analysis of the British Births Survey data bank revealed that the incidences of admissions to hospital for a LRTI in the first 5 years after birth and of episodes of bronchitis were 2.3% and 14.1%, respectively, in infants of nonsmokers, 3.1% and 18.2% in infants whose mothers smoked only after birth, but 5.9% and 18.9% in infants whose mothers smoked only during pregnancy.

See the entire Early-life events Chapter