Discussion

In the confined space of an air ambulance helicopter, massive aerosol production, simulating the conditions of a patient with an airborne transmitted disease, led to direct exposure of the medical crew and sedimentation of particles on all surfaces in the cabin and, without a barrier, on aeronautic equipment in the cockpit, too. However, a simple curtain was sufficient to prevent contamination of the cockpit. In a scenario simulating intermittent, short production of droplets (mimicking a clinical situation of coughing or disconnection of a breathing circuit in a patient with a droplet transmitted disease), the medical crew was still exposed and sedimentation accumulated around the patient but not in the cockpit. This result indicates that the amount of aerosol determines the reach and degree of contamination. Thus, the simplified concept of airborne and droplet precautions is reasonable for clinical decisions to transport a patient by air ambulance, although more complex scenarios can be used for occupational health risk assessments in general. ^{Reference Jones and Brosseau2} Patients with an illness requiring airborne precautions should only be transported in air ambulance helicopters with a physical separation of cockpit and cabin. The curtain used for night vision goggle (NVG) flights in an H145 or H135 helicopter in our simulation seems to have been sufficient to prevent aerosols from entering the cabin. Procedural instructions for chemo-thermal washing and/or disinfection of curtains or separation walls should be developed.

Our study was limited by the model used for aerosol generation and by the number and duration of flights. Although we were able to simulate many real-life conditions on the ground and during flight, these experimental conditions clearly differ from those in commercial airplanes. ^{Reference Sze To3} In light of recent findings about virus shedding in exhaled breath, ^{Reference Leung, Chu and Shiu4} it seems unlikely that those aerosols would reach the cockpit just from intermittent coughing or exhalation. This risk could be further reduced if the patient wears a mask, which is recommended as part of droplet precautions. Personal protective equipment is required for the medical crew. Also, thorough cleaning of all surfaces is necessary in the cabin after transport because the contamination risk seems to be much higher in the confined helicopter cabin space compared to a normal hospital room where contamination is frequently found during routine care. ^{Reference Phan, Sweeney and Maita5}