Long-term impacts of prenatal and infant exposure to fine particulate matter on wheezing and asthma

Background: This systematic review aimed to summarize epidemiologic evidence regarding long-term effects of prenatal and infant particulate matter with an aerodynamic diameter less than 2.5 [micro]m (PM2.5) exposure on wheezing and asthma. Methods: Epidemiologic data investigating the associations between ambient PM2.5 exposures during prenatal or the first 2 years of life and wheezing or asthma throughout life were extracted from five databases. All included studies were assessed according to the Critical Appraisal Skills Programme checklists. We performed meta-analyses if >=2 studies estimated the effects of continuous PM2.5. Results: Nine of 18 eligible studies were suitable for meta-analyses. For prenatal PM2.5 exposure and asthma by 10 years of age (n = 4), the overall risk estimate per 10-unit increase (95% confidence interval) was 1.12 (1.00, 1.26). Although meta-analysis of prenatal exposure and wheezing by 4 years of age (n = 5) was not possible due to inconsistent exposure and outcome assessments, four studies found strong positive associations with wheeze by 2 years of age. The overall risk of developing asthma (n = 5) and wheezing (n = 3) by 8 years of age for infant PM2.5 exposure was 1.14 (0.96, 1.35) and 1.49 (0.99, 2.26), respectively. One large high-quality study reporting risk differences not suitable for meta-analysis demonstrated significant associations between prenatal or infant PM2.5 exposure and childhood asthma. High heterogeneity was present among studies of prenatal exposures and asthma, whereas studies of other associations showed low heterogeneity. There was insufficient evidence about susceptible subgroups. Conclusions: The limited and inconsistent evidence is suggestive of an association between early life PM2.5 exposure and wheezing/asthma. Large standardized studies are needed to explore the associations and identify vulnerable populations.