Aerosol clearance times to better communicate safety after aerosol‐generating procedures

Abstract

As infection prevention and control, specifically transmission-based precautions and personal protective equipment, have become important topics in recent months, one issue has become a particular bugbear. This is the time that should be allowed after an aerosol-generating procedure before the ‘all clear’ is declared and staff can return, all other things being equal, to droplet precautions. Poor understanding of this and poor communication creates a safety issue and certainly significantly impedes efficiency, particularly in operating theatres at the start and end of operations. The mathematics is fairly simple, although the evidence source is less clear. The Public Health England guidance until April 2020 referred to 63% of circulating aerosol/viral load being cleared by each air exchange [1]. The guidance referenced Coia et al. [2] as the source of the evidence, but this in turn does not contain the original data, referring to a personal communication from P Hoffman in 2012. The US Centers for Disease Control and Prevention website includes a table of times taken to clear the room of 99% or 99.9% of aerosols [3] and cites a 1994 document relating to infection control around tuberculosis as the source [4]. This in turn refers to a 1973 document [5] in which air exchange is calculated using basic principles of physics. We have not been able to find evidence that confirms the degree of air replenishment that actually takes place with each air exchange, but the 63% rule is certainly widely applied [1, 3]. If each air exchange actually does replace 63% of the air, and thereby the same proportion of potentially virus-bearing aerosols, the remaining quantity is 37% after one exchange, and after n air exchanges 0.37n remains. So, after two exchanges 14% remains, after five exchanges 0.7% and after 10 exchanges 0.004%. One of us (TC) recently referred to the time that should be left before the room is ‘clear’ as the ‘viral clearance period’ or VCP [6]. However, the aerosol may or may not contain virus and, after discussing this, we propose the term ‘aerosol clearance time’ (ACT): the time taken in minutes for an air exchange in a room, being equal to 60 divided by the number of air changes per hour measured (ideally) or estimated for that room. Appending a number to the term indicates how many such time periods have elapsed. Different sources recommend as little as two ACTs (ACT2) [1] or up to 10–15 (ACT10–ACT15) [7]. Hospitals cannot interpret recommendations without knowing the air exchange rates in their hospital locations, although typical exchange rates may be 2.h−1 on a ward, 6.h−1 on an intensive care unit (ICU) and 20.h−1 in an operating theatre. Using the ACTn term allows each hospital, using its own policy in terms of which, ACT2, ACT5, ACT10 or ACT15, is the acceptable safe time period, in order to calculate these according to the known air exchange rates of clinical areas and to communicate this clearly. Example. This room has six air exchanges per hour ACT2 = 20 min ACT5 = 50 min Hospital policy uses ACT5: do not enter without wearing airborne precautions until 50 min after any aerosol-generating procedure. Therefore, in a typical ICU, ACT2 = 20 min and ACT5 = 50 min, while in an operating theatre, ACT2 = 6 min and ACT5 = 18 min. Using this nomenclature, each location can be labelled in a language that is clear and may improve communication and thereby safety and efficiency.

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