The paper Aerosol and Droplet Generation from Singing, Wind Instruments (SWI) and Performance Activities by Public Health England (PHE) and the Environmental and Modelling Group of the Environment Agency (EMG) was considered at meeting #51 of the Scientific Advisory Group for Emergencies (SAGE) on 13 August 2020, and published on 4 September. The SWI working group consensus reviewed the international evidence base and commissioned two research trials (PERFORM and SOBADRA) to investigate droplet and aerosol production in performance events. Using the knowledge obtained through these scientific enquiries, it considered the real-world implications for the management of these activities to minimise the risks associated with them.
Our post Aerosols and droplets in church – breathing, speaking, shouting and singing (24 August) includes a summary of, and link to, the pre-peer review preprint of PERFORM project and other related material, and has now been updated to include a summary of the SOBADRA study, extracts of which are reproduced below [emphasis in original]. This earlier post notes that the culture secretary, Oliver Dowden, stated “We have worked closely with medical experts throughout this crisis to develop our understanding of Covid-19, and we haves now updated our guidance in light of these findings so people can get back to performing together safely.”
Meeting #51 of the SAGE group took place prior to this statement, and it may be deduced that its assessment of the PERFORM project and of the SOBADRA study (details of which are in Appendix 2) were taken into consideration in the production of the updated guidance referred to by Oliver Dowden. A further update would therefore seem dependent upon the consideration of additional information and analysis, as identified in paragraphs [6.5] and [6.6], below. Nevertheless, the report is important since it includes an independent assessment of the PERFORM project; this is supplemented by the additional work of the SOBADRA study at PHE Porton into the sampling for oral bacteria in aerosols and droplets from respiratory activities.
The report notes that the analysis is based on measurement of inert droplets and aerosols and oral bacteria. Research to relate these data to viral aerosol generation is challenging, but important to fully characterise risks [6.5]. Aerosol generation is identified as likely posing an important risk. Research to fully understand the risks this poses in different performance venues, including details of the mitigation provided by ventilation and the influence of spatial variation in airflow patterns created by occupants and the building systems, is a high priority. This should include experimental and computational simulation models coupled to airborne transmission risk models [6.6].
Summary of key findings from PERFORM and SOBADRA
2.1. Droplets refer to respiratory particles that rapidly deposit onto surfaces. These are measured through detecting deposition of particles onto water sensitive paper or through microorganisms that settle on petri dishes. Social distancing is a primary mitigation measure for minimising the risk of droplet transmission.
2.2. Aerosols refer to respiratory particles, typically less than 10 µm in diameter, that can remain airborne for long periods of time. These are measured using aerosol particle counters or bioaerosol samplers. Ventilation and social distancing are important mitigation measures for minimising the risk of aerosol transmission. Aerosol risk is also higher closer to the source, particularly in an indoor environment.
2.3. It should be noted that these studies do not measure virus. The studies use aerosol particles and/or oral bacteria as proxy markers for the potential for virus to be dispersed in droplets and aerosols. The transmission risk is assumed to be related to the amount (mass) of droplets and aerosol that is generated, but this cannot be confirmed currently.
Risks posed by droplets
3.1. The total mass of droplets generated from singing is a similar order of magnitude to speaking at a comparable volume for the same time duration. (high confidence level).
3.2. Droplet deposition onto surfaces from singing and speech does not generally extend beyond 2m from the subject (high confidence level).
3.3. No >40µm droplet production was identified from 12 wind and brass instruments studied (high confidence level).
3.4. Oral bacteria can be detected in droplets and aerosols generated during respiratory activities, including singing. This shows that droplets can carry microorganisms.
3.5. Significantly fewer oral bacteria are dispersed during singing and speech than during more ‘dramatic’ respiratory activities (e.g. coughing) (medium confidence level).
3.6. The variance in droplet generation and dispersion of oral bacteria between individuals is high (high confidence level).
4. Risks posed by aerosols
4.1. Breathing, speaking and singing all produce aerosol (high confidence level).
4.2. Regular conversational speaking produces approximately the same amount of aerosol (in terms of total mass across all particle sizes, referred to simply as “aerosol mass” below) as breathing (high confidence level).
4.3. Singing produces more aerosols (a statistically significant factor of between 1.5-3.4 in median number and mass concentration) than speaking at a similar loudness (medium confidence level).
4.4. The loudness of singing and speaking is a significant factor in determining the amount (total mass) of aerosol generated:
4.4.1. Singing and speaking at a low or medium loudness does not produce significantly more aerosol than breathing (medium confidence level),.
4.4.2. Very loud singing and speaking can generate around 20-30 times more aerosol (in terms of total mass) than breathing, quiet singing and speaking (high confidence level).
4.5. Some individuals produce a much greater mass of droplets and aerosol than other people, to the extent that breathing from a small number of people (2 out of 25 in the PERFORM study) generates as much material as singing at the loudest volume does by others. (high confidence level).
4.6. Further analysis of wind and brass instruments is awaited.
4.7. Clearing spittle/condensation from brass instruments does not produce aerosol particles. (high confidence level).
5. Real world implications of findings
5.1. The SARS CoV-2 virus can only be transmitted in a performance setting if there are infectious individuals present amongst the audience or performers. All relevant measures outlined in current Performing Arts Guidance that are required to make an event or performance setting COVID-Secure should continue to be applied.
5.2. In terms of droplet spread, social distancing is a prime mitigation. In terms of aerosol, social distancing and ventilation are important mitigation measures.
5.2.1. Since singing and speaking are broadly similar in terms of droplet production, extended social distancing (greater than 2m) to mitigate droplet transmission by singers does not seem necessary (high confidence). There is a remote chance of droplet transmission at a social distance of 2m.
5.2.2. The significance of extended social distancing (greater than 2m) to mitigate aerosol transmission by singers (singing generates an increase of 1.5-3.4 time more aerosol mass than speaking) is not known. It is likely that beyond 2m aerosol transmission risk will be determined by the room airflow pattern.
5.2.3. Since droplet spread from wind and brass instruments is negligible, extended social distancing (greater than 2m) to mitigate droplet transmission is not necessary (high confidence). Normal social distancing is still required as performers can produce aerosols and droplets when not playing the instrument. There is a remote chance of droplet transmission at a social distance of 2m.
5.3. Breathing, speaking and singing all produce aerosol; the contribution of potential aerosol contamination from audience and performers must be considered together. As singing at high volume can generate 20-30 more aerosol than breathing or quiet speaking/singing, ventilation provision should be enhanced to effectively dilute or remove these higher levels of aerosol. The enhanced ventilation will need to consider the duration of singing or loud speech during a particular performance or event. Ventilation provision for singers on a performer basis may therefore need to be up to 20 times that for audience members for the equivalent period during which they are actually singing.
5.4. Building on existing guidance, further recommendations should be based on the loudness, amplification and duration of the activity, the number of performers, the vulnerability of the performers and the audience, the size of the audience, the environment in which the activity occurs and the background prevalence of SARS CoV-2.
5.4.1. Small venues are likely to pose a significant challenge. Groups who use such venues should maximise ventilation and limit both numbers of people and the duration of activities. Where possible groups are advised to perform in larger spaces if these are available.
5.4.2. All singing and dramatic performances should consider limiting the number of performers and audience members, using larger halls/rooms, limiting the length of the activity, using microphones, and providing fresh air at a rate of at least 10l/s/person.
The rate of 10l/s/p reflects building regulations for offices rather than a result of this
study so confidence in this figure is lower; the higher the ventilation rate the better.
These measures will make transmission unlikely.
5.5. Socially-distanced, outdoor performances of singing or wind and brass instruments present a low risk of droplet or aerosol transmission. Transmission of SARS CoV-2 is highly unlikely if the space is fully open.
5.6. Analysis has been carried out predominantly for professional singers and musicians. There is no known reason why there should be a substantial difference in aerosol and droplet generation between amateur and professional musicians; however, the vulnerabilities between these groups may vary.
A risk model for simple airborne transmission was used to assess how the enhanced aerosol generation from singing/performance speech, the venue size and ventilation, and the occupancy density affect transmission risk. The two scenarios chosen were: a mid-sized theatre (250 seat); and a community venue such as a village hall.