Quantitative Risk Assessment and Tiered Classification of Indoor Airborne Infection Based on the REHVA Model: Application to Multiple Real-World Scenarios

Abstract

The COVID-19 pandemic highlighted the need for a scientific framework that enables quantitative assessment and control of airborne infection risks in indoor environments. This study identifies limitations in the traditional Wells-Riley model-specifically its assumptions of perfect mixing and steady-state conditions-and addresses these shortcomings by adopting the REHVA (Federation of European Heating, Ventilation and Air Conditioning Associations) infection risk assessment model. We propose a five-tier risk classification system (Monitor, Caution, Alert, High Risk, Critical) based on two key metrics: the probability of infection (Pn) and the event reproduction number (R_event). Unlike the classical model, our approach integrates airborne virus removal mechanisms-such as natural decay, gravitational settling, and filtration-with occupant dynamics to reflect realistic contagion scenarios. Simulations were conducted across 10 representative indoor settings-such as classrooms, hospital waiting rooms, public transit, and restaurants-considering ventilation rates and activity-specific viral emission patterns. The results quantify how environmental variables (ventilation, occupancy, time) impact each setting's infection risk level. Our findings indicate that static mitigation measures such as mask-wearing or physical distancing are insufficient without dynamic, model-based risk evaluation. We emphasize the importance of incorporating real-time crowd density, occupancy duration, and movement trajectories into risk scoring. To support this, we propose integrating computer vision (CCTV-based crowd detection) and entry/exit counting sensors within a live airborne risk assessment framework. This integrated system would enable proactive, science-driven epidemic control strategies, supporting real-time adaptive interventions in indoor spaces. The proposed platform could serve as a practical tool for early warning and management during future airborne disease outbreaks.