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Background: The COVID-19 pandemic has raised concerns on whether colonoscopies (CS) carry a transmission risk. The aim was to determine whether colonoscopies are an aerosol-generating procedures.

Methods: This was a prospective observational trial including all patients undergoing CS in Prince of Wales Hospital from 1st June to 31st July 2020. Three particle counters were placed 10cm from the patients' anus, near endoscopists' and nurses' mouth. The particle counter recorded the number of particles of size 0.3μm, 0.5μm, 0.7μm, 1μm, 5μm, and 10μm. Patient demographics, seniority of endoscopists, use of CO2 and water immersion technique and air particle count (particles/cubic feet, dCF) were recorded. Multilevel modelling was used to test all the hypotheses with a post-hoc analysis.

Results: 117 patients were recruited. During CS, the level of 5μm and 10μm were significantly higher than the baseline period (p=.002). Procedures performed by trainees had a higher level of aerosols when compared to specialists (0.3μm: p <.001; 0.5μm and 0.7μm: p<0.001). The use of CO2 and water immersion techniques had significantly lower aerosol generated when compared to air (CO2: 0.3μm, 0.5μm and 0.7μm: p<0.001; water immersion: 0.3μm: p=0.048; 0.7μm: p=0.03). There were no significant increases in any particle sizes during the procedure at the endoscopists' and nurses' mouth. However, 8/117 (6.83%) particle count tracings showed a simultaneous surge of all particle sizes at the patient's anus, endoscopists' and nurses' level during rectal extubation.

Conclusion: CS generates droplet nuclei especially during rectal extubation. The use of CO2 and water immersion techniques may mitigate these risks.