Chemical constituents of PM2.5 and their association with childhood asthma and lung function: a systematic review and meta-analysis

Over the past decades, a growing body of research has explored the associations between various chemical constituents of fine particulate matter (PM2.5) and childhood asthma. However, findings have been largely inconsistent. This study aims to synthesize evidence on the impact of short- and long-term exposure to PM2.5 chemical constituents on asthma risk and lung function changes in children and adolescents. The study protocol has been registered on PROSPERO (CRD 42024562521). We systematically searched for articles on the associations between the black carbon (BC), organic carbon (OC), ammonium (NH4+), nitrate (NO3-), sulfate (SO42-), and heavy metals in PM2.5 and the risk of asthma and lung function (forced vital capacity [FVC], forced expiratory volume in one second [FEV1], etc.) published up to January 2025, throughout databases of the Web of Science, MEDLINE, PubMed, and EMBASE. The overall effect size (OR: odds ratio; or SMD: standard mean difference) with a 95% confidence interval (CI) was calculated using a random-effect model. Heterogeneity among the research findings was assessed using the I-squared (I-2) statistic. A total of 24 original studies with a sample size of 6,978,265 were included. Twelve studies focused on long- and 12 on short-term associations. The inter-quartile range (IQR) values for BC, OC, SO42-, NO3-, and NH4+ in PM2.5 were 0.61, 1.68, 4.23, 3.48, and 1.38 mu g/m(3), respectively. For the long-term association, every IQR increase of OC, NH4+, and BC was associated with 37% (pooled OR = 1.370), 28.5% (pooled OR = 1.285), and 7.4% (pooled OR = 1.074) higher risk of childhood asthma; every IQR increase of BC and SO42- was associated with reduced FEV1 (SMD: 0.068 L and 0.639 L) and FVC (SMD: 0.144 L and 0.479 L). Every IQR increase of BC exposure was associated with reduced FEV1 (SMD: 0.061 L) and PEF (SMD: 0.130 L). Short-term exposure to SO42-, NH4+, NO3-, BC, Fe, and Zn was also associated with higher asthma admissions with no significant heterogeneity. Conclusion: Long-term and short-term exposures to BC, OC, and NH4+ are the main contributing chemical constituents in PM2.5 for childhood asthma. BC and SO42- are the main contributors for reduced lung function in children and adolescents.