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

I wish to make a few observations with respect to Professor Cook's recent review [1]. First, whereas there is no consensus definition of what ‘relatively large’ means with respect to respiratory droplets, the spectrum of aerosol emissions from the airway has been observed to be in the range of 0.1–1000 μm [2]. A 5‐μm particle is therefore much closer to the lower end than the upper end. Second, a droplet is not visible to the unaided eye of most people when smaller than 50 μm. Stoke's law predicts the terminal settling velocity of a 5 μm droplet under the influence of gravity in perfectly still air to be of the order of 0.1 cm.h⁻¹. At 50 μm, it is closer to 6 cm.h⁻¹ [Stariolo, unpublished observations, https://arxiv.org/abs/2004.05699]. So, while those droplets are subject to gravitational forces, the inference of Professor Cook's review that they fall out under gravity within 1 m of expulsion is simply not consistent with well‐established aerosol physics [3].

These facts have important implications for the conclusions of Professor Cook's review with respect to the suitability of personal protective equipment for different task scenarios.