Assessing the effects of transient weather conditions on airborne transmission risk in naturally ventilated hospitals

Background Many UK hospitals rely heavily on natural ventilation as their main source of airflow in patient wards. This method of ventilation can have cost and energy benefits, but it may lead to unpredictable flow patterns between indoor spaces, potentially leading to the unexpected transport of infectious material to other connecting zones. However, the effects of weather conditions on airborne transmission are often overlooked. Methods We proposed a multi-zone CONTAM model of a naturally ventilated hospital respiratory ward, incorporating time-varying weather. Coupling this with an airborne infection model, we provided an assessment of the variable risk in inter-connected spaces, focusing particularly on occupancy, disease and ventilation scenarios based on a UK respiratory ward. Results Our results suggest that natural ventilation with varying weather conditions can cause irregularities in the ventilation rates and inter-zonal flow rates of connected zones and thus, lead to infrequent but high peaks in the concentration of airborne pathogen in particular rooms. This transient behaviour increases the risk of airborne infection, particularly through movement of pathogens between rooms, and highlights that under certain conditions large outbreaks may be more likely. We demonstrated how ventilation rates achieved by natural ventilation are likely to fall below the recommended guidance, and that the implementation of supplemental mechanical ventilation can both increase ventilation rates and reduce the variability in infection risks. Conclusion Our model emphasises the need for consideration of transient external conditions when assessing the transmission risk of airborne infection in indoor environments.