TL/DR
Assuming 1m social distancing and appropriate hygiene measures are properly implemented, then any indoor premises with the ability to deliver clean unmixed air at a rate of 10 litres per occupant per second, starting from a minimum delivery of three air changes per hour and ramping up from there in accordance with CO2 levels, would present no more risk than a comparable outdoor space.
Background
Black Cat Worker Collective operate Krakatoa, a 200 capacity grassroots music venue in Aberdeen, Scotland. We followed the coronavirus saga with interest, and were so alarmed by reports of what was unfolding in Italy, that we voted to enter into lockdown on 12th March 2020. This was eleven days before the UK government first imposed a nationwide lockdown. Since then we’ve been monitoring the advice issued by SAGE (the UK’s Scientific Advisory Group for Emergencies). This release in April was particularly interesting, because it mentioned ventilation as one possible means of mitigation. A second report has since been released that expands on this topic.
We’ve spent the past six months investigating using fixed plate MVHR (Mechanical Ventilation Heat Recovery) systems as a solution to ventilating our building in order to mitigate the transmission of COVID-19. Key to this is fluid dynamics and how air moves around. This would be in addition to all the usual hygiene and distancing measures. As part of this we’ve developed a spreadsheet calculator (downloadable from this page) that can run various scenarios.
Please note that fixed plate MVHR systems differ from traditional rotary wheel AHUs (air handling units) in that the airstreams do not mix!
Based on this research we’re attempting to crowdfund a ventilation system for Krakatoa, if you find the information and tool on this page useful then please consider making a donation to our appeal via GoFundMe or PayPal, thank you!
Evidence
Many scientists are now of the opinion sufficient evidence exists that breathed out coronavirus can travel through the air, and remain infectious well beyond 2m social distancing.
This isn’t such an issue outdoors where the aerosol is quickly dispersed into the atmosphere and the virus irradiated (hence so few infections linked to BLM protest marches), but does present a significant hazard indoors, where it can build up and linger. On 7th July 2020 no less than 239 scientists signed a letter calling for international bodies such as the World Health Organization (WHO) to acknowledge the possibility of this type of airborne spread. On the 5th October 2020, the Centre for Disease Control acknowledged that airborne transmission can occur under special circumstances, and that adequate ventilation was a means of mitigating this.
“… it is especially important to address the issue now that people in many countries are returning to workplaces, restaurants and pubs, says signatory Julian Tang at the University of Leicester in the UK. Improving ventilation will reduce the risk, he says. “You can’t rely on people wearing masks.”
Calls to examine ventilation and air treatment solutions have recently further intensified.
If you’re interested to see how coronavirus spreads through the air then checkout this website.
Accuracy
The accuracy of this depends on various factors relating to the virus. Here’s some of what’s needed in order to build an accurate model:
- How much virus does a typical infected person exhale? From a recently published research paper this is estimated to be around 16,166 virons per minute. Assuming the worst case scenario, then at a typical respiration rate of 15 breaths per second works out at 1,078 virons being exhaled in every breath. Note however that spreaders might be breathing more rapidly than non-spreaders…
- How many virons are present in a quanta? From the research paper linked above this is estimated to be 970, and 1 plaque forming unit (PFU) is thought to be enough for an infectious dose (ID50).
We’d like to extend our thanks to Professor Jose Jiménez at the University of Colorado for making us aware of the above research.
Please get in touch if you have any information that would be useful to refining our model (such as links to scientific research), if you can see any flaws in it, or if you simply have any suggestions on how we could improve upon it.
Technology
We looked at both mechanical ventilation and UVC air sanitisation solutions in some detail. The main factors to consider are air changes per hour, and air change efficiency, where a “change” relates to the air being replaced or sanitised. For example if a room has a volume of 360m3, then a solution might flow 360m3 of air (through ventilation or sanitisers) every 6mins theoretically giving 10 changes per hour, but the effectiveness of this depends on air circulation.
In order to grasp the importance of this, imagine the exact same cubic meter of air in the aforementioned room is being cleaned every second. Even after 360m3 has flowed through, enough for a complete air change, contamination would actually have increased because the infected people in the room would have been adding more virus to the other 359m3 of air. Conversely imagine that a different cubic meter of air is being cleaned every second, then after one complete air charge the only contamination left will be what’s been breathed out by infected occupants in those 6mins. The higher the efficiency, the better the design.
Mechanical Ventilation
This boils down to the positioning of the intake and extract vents. The orthodox method is to position intakes closers to the edges of the ceiling, fitted with diffusers that direct the clean air off surrounding walls, whilst positioning the extracts more centrally in order to remove the contaminated air that’s being forced back into the middle of the room. In most cases any space would also benefit from being slightly over pressured (except for toilets).
Aircon with UVC
A ceiling mounted aircon unit is typically designed to suck air up the middle of and blow it out the sides, since that results in the most efficient airflow. As with ventilation these devices must be carefully positioned around a room to optimise the actual flow pattern. It is possible to retrofit UVC to some aircon units, but aircon doesn’t generally have sufficient flow rate to deliver the desired frequency of air changes.
Standalone UVC
There are devices that deliver high flow rates, and perhaps one of those per 20 occupants would deliver the total desired air change frequency. Unfortunately all the devices we identified were in the form of tubes that suck air in one end and blow it out the other. This would not provide adequate air change efficiency. Such units are intended to be installed on the basis of one unit per small enclosed area, rather than several positioned throughout a much larger space.
Issues
Concerns have been voiced that any solution that involves flowing air might actually assist the spread of the virus, but this paper appears to indicate that wouldn’t actually be the case. Firstly exhaled breath mostly dissipates in less than normal social distance. Secondly the chances of inhaling someone else’s breath are greatly reduced by breathing through the nose. Thirdly, such a mode of transmission only really presents a risk where people are face to face. Lastly correctly designed ventilation ducting will help dissipate the virus more quickly.
Download the Calculator
This is a complex workbook and uses features of MS Excel that might not display properly on some handheld device, suggest opening it on a computer. Be sure to read the instructions!
Changes for This Version
- “Safe Time” concept scrapped.
- Now estimates density of aerosol.
- Based on quanta.
Default Data
The default data shows a scenario with 1 spreader in a 466m3 space for 8.5 hours, against 1 MVHR unit delivering 5,500m3 of fresh air per hour combined, with the target entering the space one hour after the spreader. The system that is being installed at Krakatoa will actually be capable of flowing up to 6,900m3 per hour.
Assumptions
Current scientific thinking is that a person who is infected will typically exhale 970 quanta per hour whilst resting.
Limitations
This only deals with aerosol risk, not contact risk or larger expelled droplets being coughed in someone’s face etc. It’s also assuming that people are properly distanced, as it can only predict the aerosol risk at distances where the aerosol has mostly dispersed into the rest of the air.
A spreadsheet is not the best way to calculate anything involving air changes; this is better done recursively. The spreadsheet does therefore result in some minor discrepancies. It might be possible to improve on this with an engineering formula that can calculate the percentage of air changed over a set time period, but we’ve been unable to locate one.
Sample Outcomes
If you’re using a mobile device then please rotate it to landscape in order to view this section properly.
The following scenarios envisage venue with a volume of 466m3, and an infected party arriving one hour before a 3 hours long concert is due to start then staying for a total of 4 hours until the concert has ended. We look at the chances of infecting someone, who arrives just as the concert is starting, and who remains for 4 hours after it ends, for a total stay of 7 hours:
| T | -1hrs | infected party arrives one hour prior to concert starting |
| T | 0hrs | target arrives and concert commences |
| T | 3hrs | concert ends and infected party leaves after a 4 hour total stay |
| T | 7hrs | target leaves after a 7 hour total stay with a 3 hour overlap |
This scenario was run for infected party sizes of 1, 4, 12, and 25.
Spreaders = 1
with ventilation of 5,500m3 per sec @63.2% efficiency there is negligible chance of airborne infection
Spreaders = 4
with ventilation of 5,500m3 per sec @63.2% efficiency there is negligible chance of airborne infection
Spreaders = 12
with ventilation of 5,500m3 per sec @63.2% efficiency there is a low chance of airborne infection
Spreaders = 25
with ventilation of 5,500m3 per sec @63.2% efficiency there is a 50% chance of airborne infection after 3hrs
Conclusions
The calculator illustrates that a fixed plate MVPR system may afford a cost efficient solution for mitigating the risk from airborne infection, and that retrofitting such systems would be a useful measure in safely combatting the spread of this disease.
Assuming 1m social distancing and appropriate hygiene measures are properly implemented, then any indoor premises with the ability to deliver clean unmixed air at a rate of 10 litres per occupant per second, starting from a minimum delivery of three air changes per hour and ramping up from there in accordance with CO2 levels, would present no more risk than a comparable outdoor space.