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Detailed findings and data

Detailed findings and data

Wheeze

Overall

Children aged 2 years or younger who were exposed to prenatal maternal smoking had a 1.41 times increased risk of developing wheeze as compared to children who had never been exposed to passive smoke (pooled relative risk ratio 1.41, 95% confidence interval 1.19 to 1.67).

A similar size of effect was observed for the relationship between prenatal maternal smoking and the incidence of wheeze in children aged 3 to 4 years (1.28 times) and 5 to 18 years (1.52 times).

The strongest impact on the incidence of wheeze was seen in children exposed to postnatal maternal smoking, where children under 5 years exposed to postnatal maternal smoking were 1.65 times ─1.70 times more likely to develop wheeze when compared to children who had never been exposed to passive smoke (<2 years: pooled relative risk ratio 1.70, 95% confidence interval 1.23 to 2.35; 3 to 4 years: pooled relative risk ratio 1.65, 95% confidence interval 1.20 to 2.27). 

Children aged 5 to 18 years who were exposed to paternal smoking were 1.39 times more likely to develop wheeze when compared to those not exposed to passive smoke (pooled relative risk ratio 1.39, 95% confidence interval 1.04 to 1.85). No data were available to assess the effect of paternal smoking on the development of wheeze at younger ages. 

An effect of similar size was seen in those children of the same age exposed to household smoke (1.32 times). Click here to see a forest plot of the findings ─ Figure A.

Prenatal maternal smoking

Geography

To explore the impact of geographical location, the analysis was restricted to studies conducted in Europe.  The magnitudes of the risks of wheeze from prenatal maternal smoking in children aged 3─4 and 5─18 years were similar to the overall result; however, there was some evidence of a larger risk in children aged 2 years or younger (Europe only: pooled relative risk ratio 2.21; All studies: pooled relative risk ratio 1.41). Click here to see a forest plot of the findings ─ Figure B.

Quality

To explore the impact of methodological quality on the association between prenatal smoking and the development of wheeze, the analysis was restricted to studies which scored higher in methodological quality. The risk of wheeze from exposure to prenatal maternal smoking was similar in the higher quality studies compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure C.

Postnatal maternal smoking

Geography

To explore the impact of geographical location, the analysis was restricted to studies conducted in Europe. The risk of wheeze from exposure to postnatal maternal smoking was similar in studies conducted in Europe when compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure D.

Quality

To explore the impact of methodological quality on the association between postnatal smoking and the development of wheeze, the analysis was restricted to studies which scored higher on methodological quality. The risk of wheeze from exposure to postnatal maternal smoking was similar in the higher quality studies compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure E.

Paternal Smoking

Geography

To explore the impact of geographical location, the analysis was restricted to studies conducted in Europe. The risk of wheeze from exposure of paternal smoking was similar in studies conducted in Europe when compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure F.

Quality

To explore the impact of methodological quality on the association between paternal smoking and the development of wheeze, the analysis was restricted to studies which scored higher on methodological quality. The risk of wheeze from exposure to paternal smoking was similar in the higher quality studies compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure G.

Household Smoking

Geography

To explore the impact of geographical location, the analysis was restricted to studies conducted in Europe. The risk of wheeze from exposure to household smoking was similar in studies conducted in Europe when compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure H.

Quality

To explore the impact of methodological quality on the association between household smoking and the development of wheeze, the analysis was restricted to studies which scored higher on methodological quality. The risk of wheeze from exposure of household smoking in children aged <2 years and 5─18 years was similar in the higher quality studies when compared to the overall result including all studies. For children aged 3─4 years, a larger magnitude of risk of household smoking on the risk of wheeze was seen (Higher quality studies only: pooled relative risk ratio 1.20; All studies: pooled relative risk ratio 1.06). Click here to see a forest plot of the findings ─ Figure I.

Asthma

Overall

Children aged 2 years or younger who were exposed to prenatal maternal smoke were 1.85 times more likely to develop asthma when compared to those children who had never been exposed (pooled relative risk ratio 1.85, 95% confidence interval 1.35 to 2.54).

However, smaller magnitudes of risk were seen in children aged 3─4 years (pooled relative risk ratio 1.30) and those aged 5─18 years (pooled relative risk ratio 1.23).

Exposure to postnatal maternal smoke did not appear to significantly increase the risk of developing asthma in any age group.

Exposure to paternal smoking increased the risk of developing asthma. Children aged 3─4 years were 1.32 times more likely to develop asthma, but no but no significant effect was seen in older children (relative risk ratio 1.34). 

Children exposed to household smoking were 1.14 ─1.30 times more likely to develop asthma compared to children unexposed to household smoking.

Click here to see a forest plot of the findings ─ Figure J.

Prenatal maternal smoking

Geography

To explore the impact of geographical location, the analysis was restricted to studies conducted in Europe. The risk of asthma from exposure to prenatal maternal smoking was similar in studies conducted in Europe compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure K.

Quality

To explore the impact of methodological quality on the association between prenatal maternal smoking and the development of asthma, the analysis was restricted to studies which scored higher on methodological quality. The risk of asthma from exposure to prenatal maternal smoking was similar in the higher quality studies when compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure L.

Postnatal maternal smoking

Geography

To explore the impact of geographical location, the analysis was restricted to studies conducted in Europe. The risk of asthma from exposure to postnatal maternal smoking in children aged 3─4 years was similar in studies conducted in Europe when compared to the overall result including all studies. However, in studies conducted in Europe, children aged 5─18 years who were exposed to postnatal maternal smoking were 1.48 times more likely to develop asthma (All studies: pooled relative risk ratio 1.20). Click here to see a forest plot of the findings ─ Figure M.

Quality

To explore the impact of methodological quality on the association between postnatal maternal smoking and the development of asthma, the analysis was restricted to studies which scored higher in methodological quality. The risk of asthma from exposure to postnatal maternal smoking was similar in the higher quality studies when compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure N.

Paternal Smoking

Geography

None of the studies exploring the association between paternal smoking and the development of asthma were conducted in Europe; therefore, a sensitivity analysis based on geographical location could not be performed.

Quality

To explore the impact of methodological quality on the association between paternal smoking and the development of asthma, the analysis was restricted to studies which scored higher on methodological quality. Only one study conducted in children aged 5─18 years was deemed as higher quality; this study found the risk of asthma from exposure to paternal smoking was similar compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure O.

Household Smoking

Geography

To explore the impact of geographical location, the analysis was restricted to studies conducted in Europe. The risk of asthma from exposure to household smoking in children aged <2 and 3─4 years was similar in studies conducted in Europe compared to the overall result including all studies. However, in European studies conducted in children aged 5─18 years, exposure to household smoking was significantly associated with doubling the risk of developing asthma (pooled relative risk ratio 2.02). Click here to see a forest plot of the findings ─ Figure P.

Quality

To explore the impact of methodological quality on the association between household smoking and the development of asthma, the analysis was restricted to studies which scored higher on methodological quality. The risk of asthma from exposure to household smoking was similar compared to the overall result including all studies. Click here to see a forest plot of the findings ─ Figure Q.