Effects of long-term ambient fine particulate matter exposure on asthma: Evidence both for children and adults

Over the past few decades, adequate evidence has confirmed the adverse effects of short-term PM_2.5 exposure on asthma exacerbation, whereas the effects of long-term exposure on asthma morbidity and mortality, especially in adults, are still under debate. Benefiting from the recent explosion of relevant epidemiological studies, we comprehensively explored the impact of long-term ambient PM_2.5 exposure on both childhood and adult asthma in light of the emerging evidence by combining multiple state-of-the-art approaches.

First, we explored the association between long-term PM_2.5 exposure and risk of asthma by conducting a systematic review and meta-analysis. Through the systematic review, we identified a total of 3187 unique literatures, and found 32 on childhood asthma and 17 on adult asthma meeting the full eligibility criteria. According to the random-effects meta-analyses covering 10,519,588 children and 9,876,107 adults, we find that every 2 µg/m³ increment in PM_2.5 is associated with a 3.8% (95% CI: 1.6%–6.1%) and a 1.1% (95% CI: 0.1%–2.2%) increase in risk of childhood asthma and adult asthma, respectively.

We then explored the exposure-response effect of asthma at different exposure levels—i.e., estimated the exposure-response curves for asthma, by constructing exposure-response functions (ERFs) based on the data extracted from the systematic review. We find that risk of childhood asthma increases almost linearly with exposure concentrations, while the risk of adult asthma increases sub-linearly. We also find that the risk of childhood asthma is much higher than that of adult asthma at any given exposure level.

After confirming by the meta-analysis and exposure-response analysis that PM_2.5 exposure is statistically significantly associated with increased risk of asthma, we estimated the global burden of asthma attributable to long-term PM_2.5 exposure by applying the ERFs in an epidemiological model. We find that PM_2.5 exposure is responsible for 11.2 (95% CI: 7.4–14.1) million new cases of asthma and 58.3 (95% CI: 37.3–74.7) million prevalent cases in 2015, and children present majority of these cases.

Our study provides additional evidence on the effects of long-term PM_2.5 exposure on asthma by concluding a statistically significantly positive association between PM_2.5 exposure and the increased risk of asthma both in children and adults. Moreover, the substantial PM_2.5-attributable burden of asthma assessed in this study suggests a large impact of PM_2.5 on public health through asthma—i.e., the overall disease burden caused by PM_2.5 is much higher than previously thought. In light of these findings, we call for more attention to the effects of PM_2.5 exposure on asthma and for more stringent legislation to be designated sooner to improve air quality. In addition, the exposure-response curves established in our study—which incorporating evidence on high exposure levels that covering most of the worldwide exposure ranges—could be applied to assess the city to global scale asthma-related health benefits obtained from air pollutant reduction associated with policy scenarios.