Exposure to ambient air pollutants and the risks of metabolic syndrome and elevated white blood cell count Among Adults: A rapid review

• Major components of PM 2.5 show consistent positive effects on the risk of MetS. • Greater heterogeneity was observed in the associations between gaseous oxidants and WBC count. • The level of WBC count may mediate the effects of air pollution exposure on metabolic syndrome risk. • Identifies critical research gaps to guide future research work. Ambient air pollution is increasingly implicated in metabolic and inflammatory dysregulation, yet the evidence base specific to metabolic syndrome and white blood cell count remains fragmented, prompting a need to clarify whether exposure to major pollutants and particulate matter components materially contributes to these outcomes in adults. In this rapid review, conducted according to PRISMA-RR guidance, we synthesized twenty-six observational and clinical studies that examined associations between exposure to PM 2.5 , PM 10 , CO, NO 2 , O 3 , SO 2 and five PM 2.5 components including sulfate, nitrate, ammonium, black carbon and organic matter, and found that most investigations reported positive relationships with the prevalence of metabolic syndrome and elevated white blood cell count, with the most consistent signals observed for particulate matter across diverse geographic settings. Several studies suggested that exposure to particulate matter may activate systemic inflammatory processes, reflected in higher white blood cell count, which is biologically consistent with inflammation-related pathways linking pollution exposure to metabolic dysfunction. However, the strength and clarity of this pathway were limited by differences in exposure modelling, variation in metabolic syndrome criteria, and inconsistent adjustment for behavioral, clinical, and socioeconomic factors. Substantial evidence gaps persisted. Taken together, these findings point to a need for longitudinal, mechanistic and methodologically rigorous studies that apply refined exposure assessment, adopt standardized metabolic syndrome definitions, evaluate mediation and susceptibility profiles, and integrate atmospheric metrics with metabolic and inflammatory phenotyping in order to generate policy relevant evidence that generate policy-relevant evidence that supports multi-pollutant air quality management and the integration of environmental risk into clinical screening and prevention strategies for metabolic health in adults.