Exposure and infection risk from exhaled contaminants at varying occupancy levels: a comparison of ventilation rate recommendations for non-residential spaces

We compared two IAQ standards—EN 16798-1:2019 and ASHRAE 62.1—and four infection control documents: ASHRAE Standard 241, and guidelines from REHVA, WHO, and the Lancet COVID-19 Commission. The objective was to estimate exposure and infection risk for exhaled contaminants across various occupancy levels in classrooms, restaurants, and offices using simplified steady-state mass balance models. Ventilation design methods in these documents rely on five basic principles: occupancy, floor area, air change rates, and formulas considering occupant numbers and room volume. We assessed exposure risk through metrics like ΔCO₂ (indoor CO₂ above outdoor levels), infection risk probability (P), event reproduction number (R), and quanta independent metrics relative risk reduction (r), and maximum absolute difference ( ΔP_abs.max). The latter two metrics address variability in virus emission. The infection risk was calculated using SARS-CoV-2 data. We also assessed the methods based on specific fan energy consumption. Our analysis revealed that across the design methods, exposure-related metrics did not respond uniformly to changing occupancy. Depending on which metric was assessed and which method was used, they could increase, decrease, or remain (approximately) constant as occupancy density changed. As no agreed method links maximum occupancy density to prescribed ventilation rates, we propose either defining maximum occupancy based on existing ventilation rates or, conversely, calculating minimum ventilation rates from a chosen maximum occupancy density. To improve protection, we also proposed two alternative calculation approaches that maintain a constant reproduction number (R) and ( ΔP_abs.max) regardless of occupancy density.