The link between aerosol dispersion, infection risk and air filtration

Portable HEPA filters are widely used to reduce airborne disease transmission in indoor environments. However, their performance is commonly evaluated using aerosol decay rates rather than infection risk. This study applies a radius-resolved infection risk framework that combines measured aerosol concentrations with CFD simulations to quantify infection risk and maximum occupancy time in classrooms with one or three portable HEPA filters, using a stale-air classroom as the baseline. An Aerosol Removal Model (ARM) is also developed to evaluate filtration performance. For the specific HEPA design and intake–discharge arrangements tested, the single-HEPA arrangement performed worse than the stale-air case under unfavourable airflow conditions because filter-induced circulation promoted infectious-aerosol dispersion. In contrast, the three-HEPA configuration substantially reduced exposure, although its effectiveness remained sensitive to filter placement and airflow alignment. The ARM predicted a removal time of approximately 50 min for the three-HEPA configuration, compared with about 5 hours for the single-HEPA configuration. The three-HEPA system approximately doubled the maximum occupancy relative to the stale-air case, whereas activating the tested single-HEPA arrangement after occupancy resulted in all occupants exceeding the infection threshold within approximately 2 min. These findings support operating HEPA filtration before classroom occupancy. The results further reveal a non-linear relationship between clean air delivery rate (CADR), airflow-induced mixing, and infection risk, demonstrating that filtration effectiveness depends not only on CADR but also on the airflow patterns induced by filter placement and orientation. The proposed framework provides a practical tool for optimising the design and operation of portable HEPA filtration systems.