Comparison of droplet spread in standard and laminar flow operating theatres: SPRAY study group

Background: Reducing of COVID-19 transmission relies on controlling droplet and aerosol spread. Fluorescein staining reveals microscopic droplets. We used this technique to compare the droplet spread in a standard theatre (ST) and a laminar air flow theatre (LAF).

Methods: We used a ‘cough-generator’ fixed to a theatre trolley at 45-degrees. Fluorescein stained ‘secretions’ were projected onto a series of calibrated targets. These were photographed under UV light and a ‘source detection’ software measured droplet splatter size and distance.

Results: The smallest droplet detected was ≅ 120 μm and the largest ≅ 24,000 μm. We detected an average of 25,862 spots in the ST, compared with 11,430 in the LAF (56% reduction). The LAF mainly affected the smaller droplets (<1000 microns). The surface area covered with droplets was: 6% at 50 cm, 1% at 2 m and 0.5% at 3 m in the ST; and 3%, 0.5% and 0.2% in the LAF respectively.

Conclusion: Accurate mapping droplet spread in clinical environments is possible using fluorescein staining and image analysis. The laminar flow affected the smaller droplets but had limited effect on larger droplets in our AGP cough model. Our results indicate that LAF require similar post-surgery cleaning to those of ST and staff should consider full PPE for medium and high-risk patients.