Background: Long-term exposure to air pollution is a risk factor for respiratory diseases, but most studies are from high income countries.

Aim: We aimed to investigate the evidence from epidemiological studies on long-term exposure to outdoor air pollution and respiratory diseases in low-and middle-income countries (LMICs) through a meta-analysis.

Methods: We searched for literature between 1990 and 2000 in Embase (Ovid), Medline (Ovid) and Web of Science (Core Collection). the air and gaseous pollutants studied included particulate matter (PM_2.5 and PM₁₀), nitrogen dioxide (NO₂) and black carbon (BC) and exposure was >1 year duration. We conducted a systematic review and meta-analysis with a random-effects model to calculate the pooled relative risk (RR) of the outcome per 10 µg/m³ increase in individual pollutant. The study protocol was registered in advance in PROSPERO- CRD42022311326.

Results: Of the 1246 studies identified, only six met our inclusion criteria, and these six reported on PM_2.5, PM₁₀, and NO₂ with asthma as main outcome. Three of these included studies were further included in meta-analysis because they had data on the same expsoure and outcomes (PM_2.5 and asthma). The results showed that 10 µg/m³ increase in PM_2.5 is associated with the risk of asthma development (pooled RR 1.21 95% CI 0.96, 1.46). There was evidence of considerable heterogeneity (I² =77.13%). Regression-based Egger test for small-study effects showed no significant publication bias among these three studies. Risk of bias was moderate.

Conclusion: Long-term exposure to PM_2.5 seems to increase the risk of asthma in LMICs, but studies are scarce and there is a large need for more research in LMICs in this field.