Personal protective equipment during the COVID‐19 pandemic: a reply

I thank Professor Murphy for his interest [1] in my article [2]. There is a great danger that an anaesthetist steps well 'outside their lane' when discussing respiratory particle physics and fluid dynamics. These are complex sciences in their own right, in which I have no training. That said, I think we largely agree that respiratory secretions vary in size over several magnitudes and this will affect their behaviour when expelled from the respiratory tract. For clarity, I neither stated that particles above 5 µm were large nor referred at all to whether these are visible or not and I am unclear why these are referred to in Professor Murphy's letter. Particles of around 5 µm diameter are especially important because this is the (perhaps historical) cut-off used by most sources for defining behaviour as a droplet (>5 µm) or an aerosol (<5 µm) and because particles of these size are of the appropriate size to reach the alveoli rather than depositing higher up in the respiratory tract [2]. However, the behaviour of particles is highly complex, not dependent only on size and much debated: as I stated in my article "the dichotomy into> 5 and < 5 µm particles leading to droplet or airborne spread, respectively, is likely to be simplistic, with aerosols being maintained over a wider range of particle size". I infer that Professor Murphy's concerns are that larger particles than described contribute to aerosols and therefore that airborne transmission is a significant risk, and that the 2 m 'droplet zone' is insufficient. Respiratory aerosols are generated both by respiratory expulsive events and by aerosol generating (medical) procedures. Linking that quantitatively to infection risk is considerably more complex [2, 3]. Despite numerous challenges, all the major organisations including the World Health Organization [4] the US Centers for Disease Control and Prevention [5], its European counterpart, and the UK [6], Canadian, and Australian governments, all ascribe to a view that the predominant route of transmission of SARS-CoV-2 is via droplet and contact (fomite) transmission, with airborne spread being a minor and undefined contributor. The danger of excessive focus on risk of airborne transmission is that it leads to perverse emphasis, for instance on factors such as masks for inborne protection of the wearer. It is important to focus on the greater risk of droplet and fomite transmission including facemasks to protect transmission to those around the wearer, scrupulous cleaning of clinical areas and use of droplet precautions when within the 'droplet zone' [2, 6]. All the above organisations describe, directly or indirectly, a zone of increased risk that varies between 1 and 2 m. The UK's Public Health England in its initial (referenced) document published in March 2020 referred to a safe distance to avoid droplet contamination as 1 m. In April 2020, after consultation with the Royal Colleges and a guidance update, this was changed to 2 m. Much as with the size of particles, the 'droplet zone' cut-off is likely to be somewhat artificial. Logically, the further the distance from an expulsive respiratory event the lower the risk, and for patients who have particularly strong coughs, or sneeze excessively, the 2 m zone may be inadequate. It maybe that recommendations will change further as new evidence emerges. In the meantime, the 2 m droplet zone is likely a practical recommendation and an improvement on the previous 1 m recommendation.