Measurement of ventilation and airborne infection risk in large naturally ventilated hospital wards

Airborne pathogens pose a significant threat to human health and this is especially the case in hospital environments which house patients with weakened immune systems. Good ventilation design can reduce risk, however quantifying ventilation performance and its influence on infection risk is difficult, particularly for large naturally ventilated environments with multiple openings. This study applies a pulse-injection gas tracer method to assess potential infection risk and local ventilation rates in a naturally-ventilated environment. Experiments conducted in a 200 m3 cross-ventilated Nightingale ward show that local external wind speeds in the range 1-4 m/s lead to indoor ventilation rates of between 3.4 and 6.5 air changes per hour (ACH). Natural ventilation is shown to be effective in open wards with an even distribution of potential airborne infection risk throughout patient locations. Comparison with a partitioned ward highlighted the potential for protecting neighbouring patients with physical partitions between beds, however, higher tracer concentrations are present in both the vicinity and downstream of the source. Closing the windows to represent winter conditions dramatically increases infection risk, with relative exposure to the tracer increased fourfold compared to the scenarios with the windows open. Extract fans are shown to alleviate this problem suggesting that a hybrid approach utilising the respective strengths of natural and mechanical ventilation may offer the best year-round solution in this and similar settings.