Secondhand Smoke Linked to Reduced Lung Function in Children With Cystic Fibrosis

Secondhand smoke exposure (SHSe) is associated with a decrease in pulmonary function in children with cystic fibrosis (CF), according to a study in the Journal of Cystic Fibrosis.

Secondhand smoke exposure is harmful for children with CF. Researchers therefore conducted a systematic review and meta-analysis aimed at identifying and quantifying the effects of secondhand smoke exposure on forced expiratory volume in 1 second percent predicted (FEV₁pp) in children with CF.

The researchers conducted a literature search from 1990 to the present in the PUBMED, SCOPUS, and MedLine databases for peer-reviewed English language studies that reported results including mean or standard deviation (SD) FEV₁pp measurements for children with CF, with and without SHSe. The review yielded 6 studies for meta-analysis, published from 1992 to 2022, with FEV₁pp as an outcome measure.

A Bayesian meta-analysis was carried out with use of a Bayesian random-effects model, and a random-effects model was used to pool effect sizes among the studies.

In the primary meta-analysis, the estimated mean difference between the SHSe and non-SHSe cohorts in pooled effect was -5.11pp (95% CI, -7.20 to -3.47), indicating that children with SHSe had average lung function, as measured by FEV₁, that was 5.11% predicted lower compared with children who did not report SHSe. Estimated between-study heterogeneity was 1.32% predicted (95% CI, 0.05, 4.26), indication significant heterogeneity.

In other analyses, the pooled standardized mean difference (effect size) was -0.25 (95% CI, -0.39 to -0.11), which was statistically significant. The finding suggests that lung function was reduced in the children with SHSe vs without that exposure by an average of 0.25 SD, which was considered to be a small effect size.

A conventional (frequentist) meta-analysis confirmed results from the Bayesian methods in the primary analysis. The bias-corrected pooled mean difference was -5.42% predicted (95% CI, -8.54 to -2.30), which was statistically significant (P =.007). The restricted maximum likelihood method determined a between-study heterogeneity of 2.06, which was greater compared with the Bayesian estimate. The P value for the test of heterogeneity was 0.022, which shows statistically significant heterogeneity between the groups. The heterogeneity statistic (I² = 61.9%; 95% CI, 7.3-84.4%) suggested moderate to substantial heterogeneity and that about 62% of the data variation was accounted for by between-study heterogeneity.

Among several limitations, it is possible that additional unidentified studies were missed, and there is potential for missing data on SHSe in the US Cystic Fibrosis Foundation Patient Registry that could be inferred as under-reporting. In addition, the studies had differing lengths of follow-up, which may potentially bias the estimates, as well as age-related differences. The inadvertent overlap among patients across studies is another potential limitation.

“This meta-analysis identified clinically relevant drops in lung function in young people with CF, which were associated with SHSe. Exposure to secondhand smoke contributed to an additional average loss of 5% predicted of lung function, compared to individuals who were unexposed,” the researchers concluded. These quantifiable impacts of SHSe in CF may be a consideration for future research into environmental health, they added.