| Summary: | This study aimed to improve indoor air quality (IAQ) in an existing college building in London by addressing two key pollutants: PM<sub>2.5</sub> particles (from indoor and outdoor sources) and SARS-CoV-2 as a biological contaminant. Various mitigation strategies were assessed, including hybrid ventilation that combined CIBSE-recommended rates with partial window and door opening. The effectiveness of HEPA-based air purifiers (APs) and upper-room ultraviolet germicidal irradiation (UVGI) systems with different intensities was also evaluated for reducing viral transmission and the basic reproduction number (R<sub>0</sub>). To manage PM<sub>2.5</sub> in the kitchen, HEPA and in-duct MERV13 filters were integrated into the ventilation system. Results showed that hybrid ventilation outperformed mechanical systems by achieving greater reductions in infection probability (P<sub>I</sub>) and maintained higher performance as the number of infectors increased, showing only a 2.5–16% drop, compared to 35% with mechanical ventilation. An R<sub>0</sub> analysis indicated that UVGI is more suitable in high-risk settings, while APs combined with hybrid ventilation are effective in lower-risk scenarios. The findings also emphasize that combining Supply–Exhaust ventilation with APs or MERV13 filters is crucial for maintaining safe IAQ in kitchens, aligning with the WHO’s short- and long-term exposure limits.
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