‘Strong evidence’ for causal link between parental smoking, childhood asthma

By Andrew Czyzewski

Parental smoking is associated with symptoms of asthma in exposed children, report researchers who found the more parents smoked, the greater the risk to their children.

Lead author Edwin Mitchell (The University of Auckland, New Zealand) and colleagues say that "the presence of a dose-response effect is strong evidence for a causal relationship."

The effect of environmental tobacco smoke (ETS) exposure on respiratory health has been of interest for many years.

In 2006, the US Surgeon General concluded that: "the evidence is sufficient to infer a causal relationship between secondhand smoke exposure from parental smoking and the onset of wheeze illnesses in early childhood," and "that the evidence is suggestive but not sufficient to infer a causal relationship between secondhand smoke exposure from parental smoking and the onset of childhood asthma."

However, the data come predominantly from developed countries and the importance of the age of exposure is not well established.

In the current study, Mitchell et al examined the association between maternal and paternal smoking and symptoms of asthma, eczema, and rhinoconjunctivitis in 58 countries.

The parents or guardians of 220,407 children aged 6-7 years completed written questionnaires about their child's allergy symptoms and their own smoking in the child's first year of life.

In addition, 350,654 adolescents aged 13-14 years self-completed the questionnaires on their symptoms and whether their parents currently smoked.

Maternal and paternal smoking was associated with an increased risk for symptoms of asthma, eczema, and rhinoconjunctivitis in both age groups, although the magnitude of the odds ratio (OR) was higher for symptoms of asthma (OR=1.11-1.44) than the other outcomes (OR=1.04-1.20).

Indeed, for asthma symptoms there was a clear dose relationship (1-9 cigarettes/day, OR=1.27; 10-19 cigarettes/day, OR=1.35; and ≥20 cigarettes/day, OR=1.56).

Maternal smoking was associated with higher ORs than paternal smoking and maternal smoking in the child's first year of life was associated with a greater risk for symptoms for all outcome categories than current maternal smoking.

"This indicates that early exposure to ETS is especially important," Mitchell and co-authors comment.

The researchers also calculated population attributable risk (PAR) for the various exposures and prevalences of parental smoking.

"If maternal smoking is causally related to asthma then it might account for 5-7% of asthma cases in the world," they remark.

The research is published in Thorax.

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