Air Guidance Relating to COVID-19
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.
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.
- 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".
- 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. It is shown that rotary heat exchangers, which are properly constructed, installed and maintained,
have almost zero transfer of particle-bound pollutants (including air-borne bacteria, viruses and
fungi), but the transfer is limited to gaseous pollutants such as tobacco smoke and other smells xxvii .
Thus, there is no evidence that virus-bearing particles starting from 0.1 micron would be an object
of carry over leakage. Because the leakage rate does not depend on the rotation speed of rotor, it is
not needed to switch rotors off. Normal operation of rotors makes it easier to keep ventilation rates
higher. It is known that the carry-over leakage is highest at low airflow, thus higher ventilation rates
If leaks are suspected in the heat recovery sections, pressure adjustment or bypassing (some systems
may be equipped with bypass) can be an option in order to avoid a situation where higher pressure
on extract side will cause air leakages to supply side. Pressure differences can be corrected by
dampers or by other reasonable arrangements. In conclusion, we recommend to inspect the heat
recovery equipment including the pressure difference measurement. To be on the safe side, the
maintenance personnel should follow standard safety procedures of dusty work, including wearing
gloves and respiratory protection.
Virus particle transmission via heat recovery devices is not an issue when a HVAC system is equipped
with a twin coil unit or another heat recovery device that guarantees 100% air separation between
return and supply side