Cross-boundary Intrusion and Exposure Risk: Migration and Attribution of Fecal-source Bioaerosols in Urban and Rural Buildings with Different Ventilation Modes

摘要

Urban buildings are widely regarded as "healthy fortresses" against external pollution, yet their defensive capabilities against long-range transported biological contaminants remain unclear. This study aims to investigate the cross-boundary intrusion mechanisms of fecal-source bioaerosols (BA) and compare the resulting indoor exposure risks under different ventilation modes. Through parallel, year-long field monitoring in an urban-rural binary scenario (a rural residential house 3 km from a wastewater treatment plant vs. an urban office building 15 km away), this study found that: during external pollution events, the indoor total bacterial concentration in the urban office with a mechanical fresh air system (average 1.5×10³ CFU/m³) was systematically higher than that in the naturally ventilated rural house (average 8.9×10² CFU/m³). Analysis based on metagenomic sequencing and an original "Source-Pathway-Shell-Indoor" (S-P-S-I) attribution model attributed this phenomenon to the "pollutant enrichment effect" generated by the continuously operating mechanical ventilation system when facing external pollution. The findings challenge the linear assumption that "more modern buildings are safer," revealing ventilation mode as the dominant factor determining indoor exposure risk to long-range biological pollution. This work provides crucial scientific evidence for reassessing modern building ventilation strategies and for constructing intelligent defense systems based on dynamic risk assessment.