An Aerosol Generating Procedure (AGP) describes an activity that can result to the release of small airborne particles (aerosols) or droplets. Under certain conditions, the release might contain potentially transmissible quantities of virial material; the current focus of this document.
The mechanisms of aerosol release are associated with:
Low temperature (usually under 60°C) evaporation processes where virial material could survive and be suspended in air
Operations generally involving high shear stresses or/and rates on materials that could potentially be suspended in air
Aerodynamic interactions between materials/streams of materials that could generate or/and intensify or/and spread new or pre-existing aerosols/pre aerosolised material
AGPs with virus transmission potential are diverse and could include, apart from medically associated operations, common activities such as playing wind instruments1, singing2, sneezing, coughing, physically intensive activities that involve vigorous exhalation etc.
Public Health England (PHE) lists the following medical procedures for COVID-19 associated with increased risk of respiratory transmission3,4:
tracheal intubation and extubation
manual ventilation
tracheotomy or tracheostomy procedures (insertion or removal)
bronchoscopy
dental procedures (using high speed devices, for example ultrasonic scalers/high speed drills)
upper ENT airway procedures that involve respiratory suctioning
upper gastro-intestinal endoscopy where open suction of the upper respiratory tract occurs
high speed cutting in surgery/post-mortem procedures if respiratory tract/paranasal sinuses involved
A more recent review by PHE5 (January 2021) aimed to provide advice on high risk AGPs from outputs presented by the independent high risk AGP panel. The outputs focused on medical procedures which are not currently classified as high risk by WHO. These are: nasogastric tube insertion, cardiopulmonary exercise and lung function tests, spirometry, swallowing assessment, nas(o)endoscopy and suction in the context of airway clearance (not associated with intubation or mechanical ventilation). The review concluded that there is insufficient evidence to demonstrate the level of high-risk otherwise associated with these procedures (contrary to what WHO has suggested). The evidence presented in the document was not coherent from study to study, nor was it robust, which is something PHE has recognised in their review. Nevertheless, PHE stated that the findings lacked the indicators required to class the examined procedures as high risk and proposed not to include those examined procedures in the extant UK AGP list. Finally, PHE advised that future coordinated, standardised clinical studies including specialists from various dental and medical fields are required to fill the AGP gaps of knowledge observed. They have suggested that this need could be addressed via coordinated funding calls both nationally and internationally.
A broader and more detailed list of procedures is created via an extensive literature review which adds the following procedures to the PHE list or generalises some specific processes already listed6.
Breaking closed ventilation systems (intentionally or unintentionally)
Amount and variability of overall viable SARS-CoV-2 virons released from each AGP and associated transmissivity thread.
Types/measurement methods to adequately quantify/measure released material.
For dental operations and their subsequent use of high rotating speed pneumatic powered equipment (e.g. microturbine handset drills), a suggestion of using lower speed, high torque, electrically powered micromotor handsets seems to be reducing the amount of expelled material7,8.
Mitigation routes (including dynamic prediction of fallow times) per process.
There is a lack of high quality studies which have examined the risk of transmission of microbes responsible for acute respiratory infections to healthcare workers caring for patients undergoing AGP8.
Lack of precision in the definition for aerosol generating procedures9.
Unknown:
Amount of actual virial material in released emissions per droplet/particle especially when AGP involves secondary fluids (e.g. water for instrument cooling etc.)
Threshold of virial content volume in droplet to transmit the virus
Aerosol and droplet emission generation mechanisms at the microscale to allow specific mitigation processes as a result of altered/modified operation of action or/and design of instrumentation used.
Characteristics of expelled particles/droplets for each procedure including velocity and trajectory from source.
References
He, R., Gao, L., Trifonov, M. & Hong, J. Aerosol generation from different wind instruments. J. Aerosol Sci.151, 105669 (2021).
Bahl, P. et al. Droplets and Aerosols Generated by Singing and the Risk of Coronavirus Disease 2019 for Choirs. Clin. Infect. Dis. 2019–2021 (2020). doi:10.1093/cid/ciaa1241
Public Health England. COVID-19: Guidance for the remobilisation of services within health and care settings. Infection prevention and control recommendations. Infection prevention and control recommendations (2020).
Assessing the evidence base for medical procedures which may create a higher risk of respiratory infection transmission from patient to healthcare worker. (2020).
Public Health England. Independent High Risk AGP Panel Summary of recommendations arising from evidence reviews to date. (2021).
Jackson, T. et al. Classification of aerosol-generating procedures: a rapid systematic review. BMJ Open Respir. Res.7, e000730 (2020).
Implications of COVID-19 for the safe management of general dental practice A practical guide. 06-16 (2020).
Mitigation of Aerosol Generating Procedures in Dentistry. (2020).
Tran, K., Cimon, K., Severn, M., Pessoa-Silva, C. L. & Conly, J. Aerosol generating procedures and risk of transmission of acute respiratory infections to healthcare workers: A systematic review. PLoS One7, (2012).
