PUBLIC HEALTH ENGLAND GUIDANCE ON INFECTION CONTROL states that “The transmission of COVID-19 is thought to occur mainly through respiratory droplets generated by coughing and sneezing, and through contact with contaminated surfaces. The predominant modes of transmission are assumed to be droplet and contact.
Droplets will generally fall out of the airstream within a short distance (depending on airflow speed and direction), hence the guidance to remain 2m apart. However, these may evaporate, reducing in size and mass, and travel further in air streams, contaminating surfaces and increasing the risk of airborne transmission in the way of an aerosol which is made up of very small micron particulates.
While airborne transmission is not thought to be a primary route of transmission, there is an emerging and growing body of evidence that the SARS-CoV-2 can also be spread through the air, particularly in poorly ventilated indoor spaces, and that ventilation provision in buildings should be reviewed in the light of this.
Proving modes of transmission during an outbreak is difficult. However, multiple recent studies are showing evidence of indirect contact (which may be linked to airborne spread) and have also linked airflow patterns to infection cases.
This has been particularly the case in high occupancy areas, in spaces with little outdoor air, and when people generate a lot of aerosols (e.g. shouting and singing, vigorous exercise). Given the growing body of evidence suggesting airborne transmission may be a route of infection and knowledge of aerosol generation and transport, it is prudent to ensure ventilation within your buildings is operating appropriately to offer some protection to the occupants.
The following measures, using outside air wherever possible, should help to reduce the risks from airborne transmission, but bear in mind the outside air, depending where you are situated, may need to be assessed for its own air quality and dealt with accordingly, say by using better filtration.
Prevention As We Know it
The primary mechanisms for preventing the transmission of coronavirus remain regular, that is thorough hand washing using soap and hot water for at least 20 seconds, the sanitization of your hands using a suitable alcohol based sanitizer. Couple this with strict adherence to social distancing requirements and staying at home as required.
Surfaces which may be contaminated also need regular cleaning following the protocols set out by Public Health England this will involve using good hygiene practices and using a combination of detergents and disinfectants.
The Dilution of internal air within a building should reduce any risk of potential airborne viral transmission by reducing exposure time to any airborne viral aerosols, and also reduce the chance for these aerosols to settle on surfaces. Evidence shows that virus can survive on some surfaces for at least 72 HOURS and hence any action to limit surface contamination is beneficial.
It is recommended that any ventilation or air conditioning system that normally runs with a recirculation mode should now be set up to run on full outside air where this is possible, but your assessments should take into account things like the outside ambient conditions and the filtration used within your Air Handling Units.
In buildings with mechanical ventilation systems extended operation times are recommended. In demand control systems CO2 set points should be set to 400ppm to increase the delivery of outside air. Ventilation should be kept on for longer, with lower ventilation rates when people are absent. It is not recommended to switch ventilation systems off in any buildings, even those temporarily vacated, but to operate them continuously at reduced speeds.
Recirculation of air between spaces, rooms or zones occupied by different people should be avoided. However, in the case of any systems serving a single space, partial recirculation of air within that space, such as through a local fan coil unit, is less of a concern. The reason is that the primary objective is to maximize the air exchange rate with outside air and to minimize the risk of any pockets of stagnant air.
If a local recirculation unit enhances air disturbance and hence helps reduce the risk of stagnant air then this should be considered when developing a strategy. Note that although these are relatively uncommon today, ceiling fans within a space can provide this function.
On colder days consideration must be given to human behavioural responses. A ventilation system on full outside air which is not adequately heated may create discomfort draughts. This may lead to users seeking to turn the system off, or with naturally ventilated spaces users may close vents or windows. These actions will reduce the air exchange rate and dilution of any contaminants (and not just any viral contamination) and overcome the primary objective of the ventilation strategy. It is important that where users can intervene in the control of the ventilation that they are made aware of the benefit of these for reducing the circulation of infectious material.
Care should be taken with any ventilation grills that can be blocked, e.g. floor grilles for displacement ventilation, and occupants educated on the purpose and benefits of these.
The potential benefit to public health at this time outweighs the reduction in energy efficiency caused by not re-circulating air. Airborne contaminants may be minimized by proper and effective filtration and regular maintenance. Viral material that settles in ductwork will become unviable over time. In the event that some viral material entered ventilation and air-conditioning systems prior to buildings being vacated due to the current restrictions, it is extremely unlikely that that material will pose any risk when those buildings are re-occupied.
Where cleaning or planned maintenance of ventilation systems is required, such as in catering premises, it should be undertaken in line with agreed industry guidance, including that relating to site operations under social distancing requirements. Appropriate PPE should be worn and all materials, including old filters, should be carefully bagged and disposed of safely. Given the requirement for many business premises to close for the immediate future, there is unlikely to be a requirement to undertake work on their ventilation or air conditioning systems at this time.
In poorly ventilated spaces with a high occupancy and where it is difficult to increase ventilation rates it may be appropriate to consider using air cleaning and disinfection devices. The most appropriate devices are likely to be local HEPA filtration units or those that use germicidal UV (GUV) radiation. GUV devices use radiation in the UV-C spectrum and have been shown to inactivate coronaviruses, although there is not yet specific evidence of the efficacy of UV-C irradiation for SARS-CoV-2.
There are currently uncertainties about a variety of factors affecting UV performance including dosage, wavelength and exposure time. In addition, consideration will need to address the specific room and system configuration, air flow, distribution and humidity.
Any potential equipment will need to be properly tested, validated and quality assured and demonstrated to provide the specific irradiation properties it is designed to and nothing else. It is essential that appropriate safety interlocks are installed to ensure UV cleaning equipment is not operated when people may be exposed to the radiation.
They can be applied as an upper-room system or a standalone consumer unit, but it is important that these are sized correctly for the room as many do not have the flow rate to be effective in larger spaces. In-duct UV-C is not recommended to control disease transmission unless it is to decontaminate air that is re-circulated.
Devices that emit ozone or other potentially hazardous by-products should not be used in occupied spaces. Further guidance on air cleaning and disinfection is given in CIBSE Guide A.
As mentioned before in several articles the definition of the word droplet and Aerosol, droplets are usually larger particles that fall to ground quite quickly, and aerosols are the finer particulate that can travel further and remain airborne.
The internal air supply and the extraction within buildings plays a vital roll. As when the Air Handling Unit is re-circulating a percentage of its’ air, the smaller aerosols/particulate, could be re-circulating back into other areas/rooms, corridors, lobbies etc.
Filtration plays a vital part of the Air Handling Unit, because ill fitting filters or the wrong filter types, can cause its own problems. This can add to a high contamination of many types of particulates within the area.
The need to look after your AHUs and its’ associated ductwork is paramount. This also includes things like duct work cleaning, drive belt replacements, bearings, coil cleaning and condensate wastes, and more importantly the fresh air intakes.
If total fresh intake is going to be used, then certain factors have to be taken into account. The biggest factor is ‘If I go to total fresh air Intake, can my filters cope with the dirt build up on my filters’ (depending on my Location). Also remember the whole system has been balanced to achieve certain airflows/patterns (where specified) within each area, and also give the correct air change rates according to CIBSE guidelines.
The Air Change rates within buildings play a vital role, especially where you have classrooms, libraries, Gyms, shopping Mals etc. and areas of high occupancy/footfall.
Remember the outside air is not filtered, and depending where you are located this has a major impact on your Air Handling Systems and the internal air within the building. Because the only thing between you and the outside is the Filtration that is installed within the AHUs and if these are not fitted correctly or are the wrong grade, then the air being delivered into your buildings will not be clean.
REF: CIBSE & REHVA.
Keith Wallace, Envirofil Ltd