Acute lower respiratory infections include pneumonia (infection of the lung or alveoli), as well as infections affecting the airways such as acute bronchitis and bronchiolitis, influenza and whooping cough. They are a leading cause of illness and death in children and adults across the world. The importance of lower respiratory infections may be underestimated.
Evidence that passive smoking increases risk of lower respiratory infection
- Exposure of infants to passive smoke in the first 2 years of life increases their risk of developing lower respiratory infections
- Infants exposed to passive smoke from both parents are 1.82 times more likely to develop a lower respiratory infection compared to those who are not exposed
What evidence was used?
The evidence presented here is derived from a systematic review published by Jones (2011) that assessed the effect of exposure of infants to passive smoking on the incidence of lower respiratory infections (LRI) occurring in the first 2 years of life. An updated search for additional original studies was performed starting in 2011 up to February 2013. From these searches, a total of 34 studies were identified.
The outcomes assessed in the 34 included studies varied. Five studies assessed acute respiratory infection, eight studies assessed bronchitis and/or bronchiolitis, and in 21 studies the type of LRI was not specified.
Measurement of infant exposure to passive smoke was determined through self-report in 29 studies and by measuring levels of biochemical indicators of smoking in the body in five studies.
Eighteen of the included studies considered the effects of any household member smoking, six of both parents smoking, nine of paternal smoking, 16 of maternal smoking, and 11 of pre-natal maternal smoking (some studies covered more than one exposure).
Seventeen of the included studies were conducted in Europe.
The methodological quality of the 34 studies, as judged by the Newcastle-Ottawa scale score, gave an overall median score of 6 (range 4─8). Before the quality was evaluated, scores of <7 were designated as indicative of low quality and scores of ≥7 were designated acceptable as high quality. Therefore, 16 of the 34 studies were judged to be of high quality.
Publication bias was assessed visually using a funnel plot for the association between exposure to household passive smoke and the risk of LRI. There was no evidence of publication bias identified from this funnel plot.
Detailed findings and data
Effects of any household member smoking
Eighteen studies investigated the effect of any household member smoking. The evidence showed that infants exposed to smoking by any household member were 1.43 times more likely to develop LRI compared to those not exposure to any smoking in the home (pooled relative risk ratio 1.43, 95% confidence interval 1.28 to 1.59). Click here to see a forest plot of the findings ─ Figure A
To explore the impact of geographical location, the analysis was grouped based on where the studies were conducted. Studies conducted in European countries showed similar risks of infants developing LRI from passive smoke exposure as compared to studies conducted elsewhere. Click here to see a forest plot of the findings ─ Figure B
To explore whether methodological quality had any possible impact in the 18 studies that investigated the effect of passive smoking of household members, the analysis was grouped into higher versus lower quality studies. Similar increases in the risk of infants developing LRI were seen in the high, 1.49 times, and low, 1.35 times, quality studies. Click here to see a forest plot of the findings ─ Figure C
Effect of smoking by both parents
Infants exposed to both parents smoking were 1.82 times more likely to develop LRI compared to those not exposed. Click here to see a forest plot of the findings ─ Figure D
Effect of paternal smoking
Those infants exposed to paternal smoking were 1.15 times more likely to develop LRI when compared to those not exposed. Click here to see a forest plot of the findings ─ Figure E
Effect of maternal smoking after birth
Infants exposed to maternal smoking were 1.62 times more likely to develop LRI than those not exposed. Click here to see a forest plot of the findings ─ Figure F
Effect of prenatal maternal smoking
Infants exposed to prenatal maternal smoking were 1.19 times more likely to develop LRI compared to those who had not been exposed to prenatal smoking. Click here to see a forest plot of the findings ─ Figure G
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