Ambient air pollution exposure and adult asthma incidence: a systematic review and meta-analysis.

Background: Ambient (outdoor) air pollutant exposures have emerged as a plausible risk factor for incident childhood asthma. However, the effect of ambient air pollutant exposures on risk of incident adult asthma is unclear. We aimed to investigate associations between specific ambient air pollutants and the risk of incident adult asthma.

Methods: In this systematic review and meta-analysis, we searched MEDLINE, Embase, Cochrane Central Register of Controlled Trials, and Web of Science from inception to Nov 27, 2023. We included observational studies with the outcome of new-onset asthma during adulthood (onset at ≥18 years), and metric of exposure of ambient air pollutants (particulate matter [PM], nitrogen dioxide [NO], ozone [O], and sulphur dioxide [SO]). Study data were extracted independently by two reviewers and study quality was assessed using the Newcastle-Ottawa scale. When four or more eligible studies were available for a given pollutant, we applied meta-analysis using inverse variance weighting in a random effects model to estimate pooled relative risk (RR), and used meta-regression to explore sources of heterogeneity. The protocol was registered with PROSPERO, CRD42023420139.

Findings: Our search identified 1891 references. After excluding 651 (34%) duplicates and ineligible studies, we included 25 studies in the systematic review. After excluding studies with overlapping populations or reporting effect estimates that could not be pooled, we performed meta-analysis for PM (nine studies), NO (nine studies), and O (four studies). Pooled random effects RRs for incident adult asthma per 5 μg/m increase in PM were 1·07 (95% CI 1·01 to 1·13) and per 10 μg/m in NO were 1·11 (1·03 to 1·20). We found no significant association between increasing O concentration and incident adult asthma (per 60-μg/m increase in O, pooled RR 1·04 [0·79 to 1·36]). We found substantial heterogeneity across studies (I=88% for all analyses). In exploratory meta-regression, average exposure level was a significant source of heterogeneity for the pooled NO estimate (95% CI -0·0077 to -0·0025 per μg/m).

Interpretation: Exposure to increased ambient PM or NO might present an additional risk factor for incident adult asthma, although high heterogeneity among included studies warrants caution in interpretation. Evidence was inconsistent for O and insufficient for SO. To increase confidence and population representation in pooled estimates, further primary investigations are necessary, ideally with aligned methodology and reporting.

Funding: None.