Pulmonary function changes on exposure to air pollutants: Inferences from systematic review and meta-analysis of observational studies involving traffic regulators

Introduction: Air pollution poses a considerable threat to health across the world by increasing the risk for multiple systemic ailments, including that of respiratory ailments. We systematically reviewed the literature and quantified the pulmonary dysfunction through evidence synthesis of comparison between individuals with high exposure to air pollutants, i.e., traffic regulators and relatively unexposed comparators. Methods: Observational studies that reported pulmonary function comparisons between traffic related air pollutants exposed group (traffic regulators) and healthy matched controls were systematically searched in PubMed, Scopus, and EMBASE databases since inception to January 2022. The PRISMA guidelines were adhered during the entire process of study execution. The mean difference in forced expiratory volume during the initial first second (FEV1), forced vital capacity (FVC), the ratio of FEV1 and FVC, peak expiratory flow rate (PEFR), and forced expiratory flow between the two extreme quartiles (FEF25%-75%) were pooled using random-effects model. Heterogeneity was assessed using Cochran-Q test and I-2 statistic. Risk of bias of studies and the quality aqnd strength of evidences were as assessed using the risk of bias in non-randomized studies of exposures and navigation guide. Relevant sub-group, sensitivity and meta-regression analyses were performed where data permitted. Results: Twenty studies including 1952 traffic regulators and 1414 control individuals were included in the quantitative synthesis. The pooled mean difference of FEV1 (from 19 studies), FVC (16 studies), FEV1/FVC (13 studies), PEFR (13 studies) and FEF25% -75% (6 studies) were-0.49L (-0.71 to -0.28; I-2 = 96.49),-0.33L (-0.59 to -0.07; I-2 = 96.65), 2.86% and 95% CI of-6.16 to 0.43),-0.89 L/s (-1.65 to-0.12; I-2 = 98.12) and-0.6L (-0.96 to-0.23; I-2 = 94.06) respectively. Majority of the included studies were rated as moderate to severe risk of bias. The strength of evidence ranged from "limited " to "inadequate ". Assuming a cumulative exposure of 5 years as reported in majority of the studies, the estimated reduction in FEV1 & FVC were approximately 100 ml/year and 70 ml/year respectively in the exposed population. Discussion: We observed the pulmonary function derangements associated with exposure to air pollution. However, considering high heterogeneity and low quality of primary evidence, there is need for high-quality studies with longitudinal design demonstrating pulmonary function changes over time to understand the association. Nevertheless, present observations should caution the policymakers towards drafting regulations for reducing air pollution and the need for preventing steps for reducing the exposure.