Parental smoking and childhood asthma: longitudinal and case-control studies

Abstract

Background-The relation of parental smoking to wheezing and asthma occurring after the first year of life was assessed by a systematic quantitative review of case-control and longitudinal studies, complementing earlier reviews of cross sectional surveys and wheezing in early childhood. Methods-Fifty one relevant publications were identified after consideration of 1593 abstracts selected by electronic search of the Embase and Medline databases using keywords relevant to passive smoking in children. The search was completed in April 1997 and identified six studies of asthma incidence, seven of prognosis, 22 case-control studies, and 10 case series addressing disease severity. Results-Maternal smoking was associated with an increased incidence of wheezing illness up to age 6 (pooled odds ratio 1.31, 95% CI 1.22 to 1.41), but less strongly thereafter (1.13, 95% CI 1.04 to 1.22). The long term prognosis of early wheezing illness was better if the mother smoked. The pooled odds ratio for asthma prevalence from 14 case-control studies was 1.37 (95% CI 1.15 to 1.64) if either parent smoked. Four studies suggest that parental smoking is more strongly associated with wheezing among non-atopic children. Indicators of disease severity including symptom scores, attack frequency, medication use, hospital attendance, and life threatening bronchospasm were in general positively related to household smoke exposure. Conclusions-The excess incidence of wheezing in smoking households appears to be largely non-atopic "wheezy bronchitis" with a relatively benign prognosis, but among children with established asthma, parental smoking is associated with more severe disease. This apparent paradox may be reconciled if environmental tobacco smoke is considered a co-factor provoking wheezing attacks, rather than a cause of the underlying asthmatic tendency.

Relationships of active smoking to asthma and asthma severity in the EGEA study

“…In a longitudinal study, conducted in the general population of Copenhagen, in which the 15-yr FEV1 slope was at least twice as steep in asthmatics as in nonasthmatics in both males and females, smoking further significantly accelerated the relative decline in lung function in both asthmatic and nonasthmatic adults [21]. Results in adults from the EGEA are consistent with findings in children, for whom there is good evidence that parental smoking influences asthma severity in children [2]. Significant associations of parental smoking with asthma severity in children have been observed considering various severity dimensions, such as the frequency of wheezing episodes, the persistence of symptoms between attacks, life-threatening events or composite severity scores [2].…”

“…Results in adults from the EGEA are consistent with findings in children, for whom there is good evidence that parental smoking influences asthma severity in children [2]. Significant associations of parental smoking with asthma severity in children have been observed considering various severity dimensions, such as the frequency of wheezing episodes, the persistence of symptoms between attacks, life-threatening events or composite severity scores [2].…”

“…The definitions of asthma used varied between studies, which could partly explain differences in results. For now, there is limited evidence that smoking is a risk factor for asthma, but this remains puzzling as tobacco consumption is an established risk factor for intermediate phenotypes of asthma, such as elevated immunoglobulin E levels [23] and bronchial hyperresponsiveness [24] and maternal smoking relates to childhood asthma [2].…”

“…Associations of environmental risk factors, such as smoking or air pollution [2], with asthmatic symptoms, lung function, composite severity scores or even fatal or near-fatal attacks could reflect an acute effect of these factors, inducing exacerbations of the disease rather than an increase in the underlying severity. Few studies have differentiated between acute and chronic effects [26].…”

“…It is recognized that maternal smoking is a risk factor for asthma in children [2] and that smoking increases the incidence of asthma in subjects exposed to some occupational agents [3]. However, there is conflicting evidence as to whether active smoking in general may be a risk factor for asthma, although there are more studies showing no association [4] than studies supporting that hypothesis [5].…”