Towards an understanding of expiratory droplets and SARS ward ventilation

Abstract

Large droplet transmission and/or airborne transmission are believed to be the major modes of transmission for many respiratory diseases such as tuberculosis, influenza and SARS. The important role of droplets in disease transmission requires reliable data on the number and size of respiratory droplets as well as their movement and evaporation in air after being expelled. As a result of SARS, the Hong Kong SAR Government promptly constructed more than 500 new state-of-the-art isolation rooms with more than 1300 beds in 14 hospitals by the end of 2003. We measured the numbers and sizes of droplets exhaled by healthy Chinese adults with various expiratory activities and those from the use of a nebuliser and took nasopharyngeal aspirates in both laboratory and hospital environment. We determined how far droplets could travel in a room environment as well as the penetration of breathing flows. We measured the ventilation effectiveness in nearly 50 selected isolation rooms in nine major hospitals. The average size of droplets due to coughing and other respiratory activities can be larger than 50-100 microns; however, such droplets tend to evaporate quickly. Nebulisers generate very fine droplets (less than 1 micron) and large amounts of these fine droplets can escape through the holes in facemasks. 'Large droplets' (i.e. those larger than 50-100 microns) cannot be removed effectively by current ventilation systems and are deposited on nearby surfaces (floor, bed, patient's body, etc). For optimum protection, the distance between beds in an isolation ward should be larger than the distance travelled by the large droplets (~1.5 m). The bed distance is determined by the mechanisms of droplet transmission, not airborne transmission. All of the wards investigated satisfied the fundamental infection control requirement of no air leakage to the corridor. However, at the time of measurement, 28% of the tested newly renovated isolation rooms in existing hospital wards did not achieve the recommended 12 air changes per hour (ACH) ventilation rate, and 60% had reverse airflow direction for the toilets/bathrooms, suggesting room for improvement. A new type of isolation room with better performance in removing both large droplets and fine droplet nuclei is also recommended from this study.