COVID-19 Air Guidance for Ventilation Systems in London

Air Guidance Relating to COVID-19

Published: 24/05/2020
Updated: 24/06/2020

This article may be updated to reflect newly released governmental guidance or scientific data in relation to Coronavirus (COVID-19/SARS-CoV-2). In addition, the available evidence specifically regarding COVID-19 is limited therefore some data is based on the previously studied coronavirus outbreaks; SARS (SARS-CoV-1) and MERS (MERS-CoV). NIJHUIS DEBA excludes any liability for any direct, indirect, incidental damages or any other damages that would result from, or be connected with the use of the information presented in this document. The scope of this article is limited to commercial and public buildings (e.g. offices, schools, shopping areas, sports premises, etc) where only occasional occupancy of infected persons is expected.

How to Prevent Airborne Spread of Coronavirus (COVID-19) in Workplaces

Vital for every epidemic is the identification of transmission routes of the infectious agent. In relation to COVID-19 the understanding is that the following two transmission routes are especially present: via large air droplets (emitted when sneezing or coughing or talking occurs) and via surface contact (hand-hand, hand-surface etc.).

Identified Exposure Mechanisms

Close Contact Transmission - Large Particles (> 10 Microns)
Droplets are created by coughing and sneezing and fall on nearby surfaces and objects such as desks and tables. Infection can be caused by a person touching contaminated surfaces or objects; and then touching their eyes, nose, or mouth. If people are standing within 1-2 meters of an infected person, they can become infected directly by breathing in droplets.

Airborne Transmission - Small Particles (< 5 Microns)
Small particles may stay airborne for hours and can travel long distances. These are also generated by coughing, sneezing, and talking. Small particles are formed from droplets that evaporate and desiccate. The size of a coronavirus particle is between 80-160 square nanometres. SARS-CoV-2 remains active up to 3 hours indoors whilst airborne and up to 3 days on room surfaces in common conditions. Small virus particles remain airborne and can be carried long distances by air movements within the room or in the extraction ducts of ventilation systems.

Air transmission has been demonstrated to cause infections of SARS-CoV-1 in the past and therefore it is reasonable to assume COVID-19 also carries such risk. There is no documented evidence to rule
out the transmission of the airborne-particles. SARS-CoV-2 has been identified from swabs taken from exhaust vents in rooms occupied by infected patients. This evidence suggests that keeping 1-2 m distance from infected persons might not be sufficient and therefore increasing the ventilation rate is beneficial to extract more particles.

COVID-19 Transmission Via Air

Poor Indoor Ventilation Risks
A recent study that investigated transmission events demonstrated that closed environments with poor ventilation significantly contributed to a higher number of secondary infections.

Reducing Risk Factors to Staff, Customers, Residents & the Public

- Increase Exhaust Ventilation Output
In buildings with mechanical ventilation systems it is recommended to extend the operation times. Change the settings of system timers to begin ventilation at least 2 hours prior to the building usage time and switch to a lower speed 2 hours after the building use has ended.

When using demand-controlled ventilation systems change Co2 setpoint to 400ppm (or lower) to ensure the nominal speed is maintained. Keep the ventilation system running 24/7, with a lower ventilation rate when people are absent. In buildings that have been vacated it is not recommended to switch the ventilation off, but instead to operate the system continuously at a reduced speed. There should not be a significant financial outlay to maintain these levels of ventilation.

- Increase Air Supply
The general advice is to supply as much outside air as is reasonably possible. The key element is the
volume of fresh air that is supplied per person within the area you are responsible for. If the number of employees has been reduced, do not allow the remaining employees to gather or work in smaller spaces but keep to or increase social distancing (minimum physical distance 2-3 m between persons). This will enable ventilation systems to have a better effect.

HVAC Ventilation Services London

- Toilet Ventilation
Exhaust ventilation systems in toilets should always be kept on (24/7). Ensure that under-pressure is created to avoid the risk of faecal-oral transmission. The use of open windows in toilets with passive-stack or mechanical exhaust ventilation may cause contaminated airflow from the toilet to other rooms, which could be an indication that the ventilation is traveling in a reverse direction.

Open toilet windows, therefore, should be avoided unless there is inadequate exhaust ventilation in operation. It is important to also keep other windows open in nearby parts of the building in order to achieve cross-flows to reduce risk.

- Utilise Openable Windows
In buildings that lack mechanical ventilation it is recommended to make proper use of openable windows to increase ventilation (even in cold weather). Consider briefing staff on your window airing procedure and display reminders in rooms for people to see upon entering. A suggestion is open windows for circa 15 minutes upon entering a room (especially important if the room was occupied by others shortly beforehand). In buildings that have mechanical ventilation, window airing can be used to further increase ventilation provided some assessment has been made regarding the direction of flow in relation to potential "particle hotspots".

Open Windows Covid-19 Coronavirus London
Pay attention to ventilation systems and the flow of air through the building. Utilise windows where appropriate.

- Heat Recovery Systems
In certain situations virus particles in extracted air can re-enter the building, increasing the risk. Heat recovery devices may carry over particles from the exhausted air to the supply air via leaks. Regenerative air-to-air heat exchangers may be prone to considerable leaks if poorly designed or maintained. For properly operating rotary heat exchangers, fitted with purging sectors, and correctly configured, leakage rates are low.

For any installation, the leakage should be below 5%, and should be compensated with an increase of outdoor air ventilation according to EN 16798-3:2017. Do not assume any rotary heat exchanger is "just fine". As a person of responsibility, consider having your heat recovery and ventilation system inspected. NIJHUIS DEBA provides thorough system checking and will advise if there are any issues that require attention. Properly installed and maintained rotary heat exchangers have only minimal transfer of particle-bound pollutants. Maintaining the normal operation of rotors is beneficial in keeping the ventilation rates high. Higher ventilation rates are recommended.

If you suspect leaks in your heat recovery sections, be sure to act promptly to address the issue. NIJHUIS DEBA can provide risk assessments, testing, system adjustments and remedial works to restore optimal system performance.

- Recirculation System Risk Factors
Virus particles can also re-enter a building where air handling units feature recirculation systems. It is recommended to avoid central recirculation during coronavirus outbreaks. Close the recirculation dampers. Standard recirculation filters are not enough to prevent the spread of virus particles unless they are HEPA rated (most are not). Fan coil and induction systems operate local (room level) circulation. These units are recommended to be switched off to prevent resuspension of virus particles in the room. If fan coils cannot be deactivated, it is recommended that the fans are run continuously.

HSE Guidance Update - Added 24/06/2020

Air conditioning

  • The risk of air conditioning spreading coronavirus (COVID-19) in the workplace is extremely low.
  • You can continue using most types of air conditioning system as normal. But, if you use a centralised ventilations system that removes and circulates air to different rooms it is recommended that you turn off recirculation and use a fresh air supply.
  • You do not need to adjust air conditioning systems that mix some of the extracted air with fresh air and return it to the room as this increases the fresh air ventilation rate. Also, you do not need to adjust systems in individual rooms or portable units as these operate on 100% recirculation.

If you’re unsure, ask the advice of your heating ventilation and air conditioning (HVAC) engineer or adviser.

General ventilation

  • Employers must, by law, ensure an adequate supply of fresh air in the workplace and this has not changed.
  • Good ventilation can help reduce the risk of spreading coronavirus, so focus on improving general ventilation, preferably through fresh air or mechanical systems.
  • Where possible, consider ways to increase the supply of fresh air, for example, by opening windows and doors (unless fire doors).
  • Also consider if you can improve the circulation of outside air and prevent pockets of stagnant air in occupied spaces. You can do this by using ceiling fans, desk fans or opening windows, for example.
  • The risk of transmission through the use of ceiling and desk fans is extremely low.


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