Ventilation is important to reduce the spread of COVID-19 in indoor spaces. Increasing outdoor air ventilation rates and avoiding air recirculation can help remove virus-containing particles from occupied areas. For mechanically ventilated buildings, recommendations include extending ventilation system operation times, lowering CO2 setpoints, and keeping systems running at lower rates overnight and on weekends. For naturally ventilated buildings, opening windows more often is suggested. Room air cleaners with HEPA filters can also help locally if placed near occupants.

1. Part of the BRE Trust
COVID-19: some return to work ventilation
guidance
Stuart Upton, BRE
IAQM Indoor Air Conference
22 June 2020
© BRE

2. Background
– Many additional precautions are being put in place as people start to
return to workplaces.
– Personal hygiene, enhanced cleaning regimes and physical
distancing are obviously highly important.
– Ventilation of occupied spaces also has a key role to play.
– What else can be done going forward?
– Information given here is primarily based on:
– REHVA guidance (Federation of European Heating, Ventilation and Air
Conditioning Associations): REHVA COVID-19 guidance document, April 3,
2020. How to operate and use building services in order to prevent the spread of the
coronavirus disease (COVID-19) virus (SARS-CoV-2) in workplaces
– Plus a little bit of experience!
© BRE, 2011

3. Why ventilation is important: a bit of aerosol science……
Potential airborne transmission…
• Breathed/coughed/sneezed droplets are released, possibly containing virus, by
building occupants. Particle size distribution of coughed droplets is typically ~0.5 to
15µm
• ‘Large’ droplets are said in current guidance to ‘settle out close to their source’.
• Ok, so taking a 10µm droplet as an example: Particle settling velocity (vf) in still air
(e.g. indoors) is 3 mm s-1. So, if released at 2m height, it will be deposited to the
floor in ~11 minutes (so quite a long time)
• But…..
• A 10µm pure water droplet will typically evaporate in ~0.2s (and exhaled droplets
are largely water) leaving any residue present airborne – which could be a virus.
• The size of a coronavirus particle is approx. 80-160 nanometres.
• A particle of this size will remain suspended in the air for a much longer time – in
fact almost indefinitely! (100nm particle vf = ~0.0003 mm s-1)
• Therefore, increasing ventilation rates to remove as many exhaled particles as
possible from occupied spaces is a good idea!

4. What about external pollutant ingress with increased ventilation?
Reducing the ingress of outdoor air pollutants is, of course, very
important – but you have to ‘balance the potential problems’!
• Marylebone Road, London - monitoring station with high air pollution:
Number of days with ‘moderate’ (or greater) levels of air pollution:
• PM2.5 particles: 2019 (7); 2018 (2); 2017 (7); 2016 (10)
• Nitrogen dioxide: 2019 (0); 2018 (20); 2017 (18); 2016 (31)
• So, there are a lot of days per year when air pollutant levels (even in
Central London) are lower than the ‘moderate’ level.
• Under ‘normal’ circumstances there is a benefit in reducing
ventilation rates when outdoor pollution levels are higher – and doing
this dynamically is always best if possible.
• However, while the pandemic lasts, there seems to be a clear benefit
in simply keeping building ventilation rates higher than normal.

5. REHVA: Practical recommendations for building services operation
General principle: Increase the air supply and exhaust ventilation in buildings!
In buildings with mechanical ventilation systems:
• Extended ventilation system operation times are recommended
• Demand-controlled ventilation systems: change CO2 setpoint to a lower value
(possibly a default 400 ppm setting?).
• Keep the ventilation on 24/7, with lowered ventilation rates when people are
absent (overnight and weekends).
• In vacated buildings, maintain the ventilation to operate continuously, but at a
reduced rate (i.e. basic normal ‘non-virus’ building maintenance).
• In springtime/summer with small heating and cooling needs, there will be limited
energy penalties, while helping to remove virus particles from buildings.
• The general advice is to supply as much outside air as is reasonably possible.
• If the number of employees attending is reduced, do not concentrate the
remaining employees in smaller areas but maintain physical distancing.
• Exhaust ventilation systems of toilet areas should always be kept on 24/7.

6. REHVA: Practical recommendations for building services operation
In buildings with natural ventilation systems:
• Window opening is usually the only way to boost air exchange (ventilation)
rates – so use more window opening (‘airing’ or purge ventilation).
• It is recommended to actively use openable windows much more than normal
- even when this may cause some thermal discomfort. (So tell staff to ‘bring in
a jumper’ – which should certainly be ok over the summer period!).
• Try to stay away from (or not use) any poorly-ventilated spaces (i.e. spaces
not near to, or without, opening windows).
• Any localised humidification and air-conditioning (heating/cooling) systems
have no practical effects as far as potential virus levels are concerned.

7. REHVA: Practical recommendations for building services operation
Safe use of mechanical ventilation heat recovery systems
• Under certain conditions virus particles in extracted air can re-enter a
building.
• Some heat recovery devices may carry-over viruses attached to particles
from the exhaust air side to the supply air side via leaks.
• Regenerative air-to-air heat exchangers (i.e. rotors, also called enthalpy
wheels) may have air leakage paths if poorly designed and/or maintained.
Possible virus recirculation should not be an issue with HVAC systems equipped
with twin-coil units or heat recovery devices that give 100% air separation
between return and supply.
If in doubt – check. There is currently still plenty of time before the heating
season starts!

8. REHVA: Practical recommendations for building services operation
Mechanical ventilation with air recirculation: Basic principle - don’t use
this feature at the moment!
Virus particles can re-enter a building when centralised air handling units are
equipped with recirculation systems/sectors.
Close any air recirculation dampers (via the Building Management System or
manually).
This may lead to problems with cooling or heating capacity – but avoiding
contamination is currently more important.
Ensure that such actions taken do not lead to any HVAC equipment overheating
- giving an increased risk of fire.
Some air handling units and recirculation sections have return air filters. Unless
these are HEPA filters (generally unlikely in HVAC systems) do not recirculate air
– and even then it is worth checking that there are no air leakage paths around
the filtration units!

9. REHVA: Practical recommendations for building services operation
• Duct cleaning has no practical effect on virus levels – it is much more
important to increase the fresh air supply and avoid any air recirculation.
• More frequent changing of outdoor air filters at ventilation system inlets is not
necessary.
• Free-standing room air cleaners can be useful:
• Those that remove particles from the air can provide a similar effect to
increased ventilation.
• To be effective against the virus, such cleaners must have at least HEPA
filter efficiency. Devices that use electrostatic filtration principles (not
room ionisers!) also work quite well.
• There can be a limited area of beneficial effect from these devices
(depending on room size, ventilation rate, airflow patterns) – so locate
devices near to occupants if possible.

10. BRE: tests carried out on some room air filtration systems
BRE has recently carried out testing of various room air cleaners in controlled
environmental chambers with introduced pollutants (gases, particles and
vapours).
Those with HEPA filters can be very effective at removing airborne particles
down to ~20 nanometres.
BRE also has HVAC testing and consultancy capabilities available if required.

11. Summary of suggested ventilation measures during COVID-19
• If possible, increase the ventilation rate of occupied spaces with outdoor air.
• Extend periods of mechanical ventilation system operation.
• Possibly change (lower) any demand-controlled ventilation set-point for CO2.
• Do not switch ventilation off overnight/weekend. Keep systems running at
lower rate.
• Open windows in naturally-ventilated buildings
• Keep toilet ventilation operating 24/7.
• Switch air handling units with recirculation to 100% outdoor air if possible (i.e.
don’t recirculate air).
• Inspect any heat recovery equipment to be sure of no air leakages paths.
• No need to:
• Change heating, cooling and possible humidification set-points.
• Plan additional duct cleaning for this period.
• Replace central outdoor air inlet filters (follow normal maintenance
schedule).
• Room air cleaners (with HEPA filtration) can be very useful locally –
especially in naturally-ventilated buildings as the heating season approaches
and windows will increasingly need to be closed.

12. Thank you for your attention!
BRE Contact for further information:
Stuart Upton
BRE
E-mail: stuart.upton@bregroup.com
Telephone: +44 (0) 1923 664479
Website: www.bre.co.uk