Aerosol generating procedures: are they of relevance for transmission of SARS-CoV-2?
Authors: Fergus Hamilton, David Arnold, Bryan R Bzdek, James Dodd, Jonathan Reid, Nick Maskell, Carrie White, James Murray, Johannes Keller, Jules Brown, Andrew Shrimpton, Anthony Pickering, Timothy Cook, Mark Gormley, David Arnold, George Nava, Jonathan Reid, Bryan R Bzdek, Sadiyah Sheikh, Florence Gregson, Fergus Hamilton, Nick Maskell, James Dodd, Ed Moran
Journal: The Lancet Respiratory Medicine (2021)

It is now generally accepted that SARS-CoV-2 can be spread by aerosols as well as larger droplets from the upper respiratory tract, although the relative importance of aerosol transmission remains incompletely answered.1Pöhlker ML Krüger OO Förster J-D et al.Respiratory aerosols and droplets in the transmission of infectious diseases.arXiv. 2021; (published online 1 March.)http://arxiv.org/abs/2103.01188Google Scholar Despite this, current UK infection control guidance for hospitals is centred on the premise that aerosols are only generated by specific medical interventions designated as aerosol generating procedures (AGPs).2WHOInfection prevention and control of epidemic- and pandemic-prone acute respiratory infections in health care. World Health Organization, Geneva2014Google Scholar This draws from epidemiological observations during the 2003 outbreak of severe acute respiratory syndrome, during which certain procedures appeared to be associated with an increased risk of staff infection (particularly tracheal intubation), and these procedures had a theoretical risk of viral aerosolisation.3Tran K Cimon K Severn M Pessoa-Silva CL Conly J Aerosol generating procedures and risk of transmission of acute respiratory infections to healthcare workers: a systematic review.PLoS One. 2012; 7e35797Crossref PubMed Scopus (1322) Google Scholar However, the evidence supporting aerosolisation during these procedures was, before the pandemic, remarkably slim, with aerosolisation being assumed on the basis of the precautionary principle and low quality mechanistic studies.4Jackson T Deibert D Wyatt G et al.Classification of aerosol-generating procedures: a rapid systematic review.BMJ Open Respir Res. 2020; 7e000730Crossref PubMed Scopus (118) Google Scholar This view of aerosol generation subsequently led to a dichotomisation—later codified in international guidance2WHOInfection prevention and control of epidemic- and pandemic-prone acute respiratory infections in health care. World Health Organization, Geneva2014Google Scholar—that categorised all medical activities into either AGPs, where potentially infectious aerosols are generated, versus everything else, where the risk of potentially infectious aerosol is presumed to be negligible. The logical extension of this dichotomy has resulted in health-care workers in many countries undertaking interventions classified as AGPs wearing higher levels of personal protective equipment (PPE), such as FFP3 or N95 masks, whereas those health-care workers providing other medical care have not been afforded the same protection, as infectious aerosol is not considered a risk outside of AGPs.5Public Health England6. COVID-19 infection prevention and control guidance: aerosol generating procedures.https://www.gov.uk/government/publications/wuhan-novel-coronavirus-infection-prevention-and-control/covid-19-infection-prevention-and-control-guidance-aerosol-generating-proceduresDate: 2020Date accessed: April 1, 2021Google Scholar Although this dichotomy was reasonable at the start of the pandemic, recent aerosol sampling studies from multiple groups investigating several currently defined AGPs have revealed more information on the potential risk of aerosols from these procedures. In fact, the aerosol emissions from tracheal intubation, high-flow nasal oxygen, and non-invasive ventilation are low, with similar sampled aerosol concentrations to tidal breathing and speaking.6Wilson NM Marks GB Eckhardt A et al.The effect of respiratory activity, non-invasive respiratory support and facemasks on aerosol generation and its relevance to COVID-19.Anaesthesia. 2021; (published online March 30.)https://doi.org/10.1111/anae.15475Crossref Scopus (80) Google Scholar, 7Hamilton F Gregson F Arnold D et al.Aerosol emission from the respiratory tract: an analysis of relative risks from oxygen delivery systems.bioRxiv. 2021; (published online Feb 1.)http://medrxiv.org/lookup/doi/10.1101/2021.01.29.21250552Google Scholar, 8Brown J Gregson FKA Shrimpton A et al.A quantitative evaluation of aerosol generation during tracheal intubation and extubation.Anaesthesia. 2021; 76: 174-181Crossref PubMed Scopus (134) Google Scholar, 9Alsved M Matamis A Bohlin R et al.Exhaled respiratory particles during singing and talking.Aerosol Sci Technol. 2020; 54: 1245-1248Crossref Scopus (145) Google Scholar Critically, these aerosol studies also confirm that coughing by both healthy volunteers, and patients (with or without COVID-19), generates orders of magnitude more aerosol than many AGPs.6Wilson NM Marks GB Eckhardt A et al.The effect of respiratory activity, non-invasive respiratory support and facemasks on aerosol generation and its relevance to COVID-19.Anaesthesia. 2021; (published online March 30.)https://doi.org/10.1111/anae.15475Crossref Scopus (80) Google Scholar, 7Hamilton F Gregson F Arnold D et al.Aerosol emission from the respiratory tract: an analysis of relative risks from oxygen delivery systems.bioRxiv. 2021; (published online Feb 1.)http://medrxiv.org/lookup/doi/10.1101/2021.01.29.21250552Google Scholar, 8Brown J Gregson FKA Shrimpton A et al.A quantitative evaluation of aerosol generation during tracheal intubation and extubation.Anaesthesia. 2021; 76: 174-181Crossref PubMed Scopus (134) Google Scholar Despite this, the generation of a cough during a procedure is not considered sufficient for that procedure to be designated an AGP. Consequently, current UK infection control guidance advises the highest aerosol precaution PPE for procedures that are demonstrably not high risk (compared with coughing) and advises lower grade droplet precaution PPE when infectious aerosol risk is high (eg, caring for coughing patients with confirmed COVID-19 for a prolonged period of time in poorly ventilated health-care settings). Although we and others have sought to quantify aerosol generation, it should also be clear that a simple observation of increased aerosol emission does not confirm the potential for pathogen transmission; the much more exacting task of quantifying airborne infectious SARS-CoV-2 concentrations carried in aerosols has so far proven intractable. More research is ongoing to quantify other AGPs across a range of clinical settings. However, based on the research to date, a coughing patient with acute COVID-19 is likely to generate more infectious aerosol than many AGPs. This appears to be supported by the epidemiological evidence, which points to an increased risk of infection for ward medical staff (who care for acutely dyspnoeic, coughing patients with COVID-19) compared with intensive care staff—although it should be noted that interpretation of that data is confounded by patient mix, among other factors.10Cook TM Lennane S Occupational COVID-19 risk for anaesthesia and intensive care staff—low-risk specialties in a high-risk setting.Anaesthesia. 2021; 76: 295-330Crossref PubMed Scopus (21) Google Scholar We propose an end to the term aerosol generating procedure, as it is neither accurate (aerosol is not generated above a cough for many of these procedures), implies aerosol emission is only from specific procedures (rather than being generated during normal respiratory events), potentially misidentifies the source of infection risk, and applies a binary definition to a situation that is more complex. Instead, we propose that clinicians follow an evidence-based framework that accounts for the major drivers of risk, with a focus on physical exposure to patients with suspected or confirmed COVID-19 as the critical component (panel).PanelProposed factors to be included in risk matrix for respiratory transmission of SARS-CoV-2Patient risk (by far the largest risk factor)The probability of the patient having the infection, and time since acquisition. Risk based on symptoms, PCR positivity, and vaccination status. Note symptoms such as coughing and increased respiratory exertion are likely to be major factors in aerosol generation.Duration of exposureThe duration that the risk is in place. The length of time required to be in close proximity naturally increases risk of both aerosol and droplet transmission.Health-care practitioner risk from COVID-19Age, sex, body mass index, comorbidities, vaccination status.Proximity riskExposure to any care intervention requiring close patient contact increases risk. This includes personal care (such as mouthcare) and physical examination (especially relating to upper respiratory tract such as nasal or throat swab, nasendoscopy, or intubation)Environmental riskVentilation, humidity, temperature Patient risk (by far the largest risk factor) The probability of the patient having the infection, and time since acquisition. Risk based on symptoms, PCR positivity, and vaccination status. Note symptoms such as coughing and increased respiratory exertion are likely to be major factors in aerosol generation. Duration of exposure The duration that the risk is in place. The length of time required to be in close proximity naturally increases risk of both aerosol and droplet transmission. Health-care practitioner risk from COVID-19 Age, sex, body mass index, comorbidities, vaccination status. Proximity risk Exposure to any care intervention requiring close patient contact increases risk. This includes personal care (such as mouthcare) and physical examination (especially relating to upper respiratory tract such as nasal or throat swab, nasendoscopy, or intubation) Environmental risk Ventilation, humidity, temperature Subsequently, additional factors known to be relevant in viral transmission, such as ventilation, proximity, and duration of exposure to patients, should be included in assessing risk, while recognising the changing epidemiology of infection with setting. In summary, it is increasingly clear that transmission of SARS-CoV-2 via aerosol is possible and might represent a significant transmission route. However, emerging evidence indicates that many currently defined AGPs are unlikely to play any significant role in generation of infectious aerosol that poses a risk to staff. In view of this, the term AGP has neither face validity nor construct validity. Instead, we should focus on the risk in plain sight: close, physical exposure to people suspected, or known to have, COVID-19 for prolonged time or where ventilation remains poor. We declare no competing interests. Download .pdf (.15 MB) Help with pdf files Supplementary appendix