Cardiorespiratory benefits of reducing indoor particulate matter depend on immune improvements: Insights into individual heterogeneity from a randomized, double-blind crossover trial

Indoor air pollution and health have emerged as a significant environmental and public health concern globally. However, individual heterogeneity in immune response to indoor particulate matter (PM) reduction and its role in cardiorespiratory benefits remain unclear. This randomized, double-blind crossover trial was performed among 80 healthy college students in Beijing, China, including two 4-day indoor air purification sessions (real and sham), separated by a 16-day washout period. Real-time indoor PM levels were monitored. Nasal mucosal lining fluid was collected one day before cardiorespiratory assessments and fasting blood collection to detect immune molecules. Linear mixed-effect models were used to estimate intervention effects. We found reducing indoor PM significantly improved nasal and systemic immunity, but not overall cardiorespiratory function. Further individual-level analysis revealed heterogeneous changes in immune and cardiorespiratory indicators. Specifically, respiratory benefits occurred in individuals with greater nasal immunity improvements, while cardiovascular benefits primarily depended on improvements in systemic immunity. Both respiratory and cardiovascular benefits were observed in individuals with improvements in both nasal and systemic immunity. Notably, even when indoor PM concentrations were below WHO air quality guidelines, further reducing PM exposure still yielded immune improvements. These findings support precision interventions in indoor PM pollution tailored to individual immune sensitivity.