In assessing how you may best manage your buildings and their air systems and services; we have provided the following guidance to help reduce the potential spread of COVID-19 around your buildings.

This document establishes first principles of the transmission routes for the virus and goes on to talk about the different factors and influences that we need to consider.

What are the transmission routes for COVID-19?

There are two dominant ways that the SARS-CoV-2 virus – as well as many others - can spread. An infected person coughing, sneezing or talking etc. and producing fine droplets (>5-10 μm in diameter) containing the virus that contaminate the immediate atmosphere (typically not further than 1-2 metres). Illness is then caused as a result of this “close contact” with an already infected person, where:

  1. Someone else then respires these contaminated droplets directly: or
  2. Someone else contacts a surface contaminated by infected droplets, secretions or other body fluids (fomites) and then touch their mouth, nose or eyes.

Evidence has also grown for the airborne transmission of the SARS-CoV-2 virus. This is when very fine droplet nuclei (<5μm in diameter), again generated through an infected person coughing, sneezing or talking, remain airborne and can be carried long distances by natural airflows in rooms, or potentially extract systems. Although this route of transmission is difficult to confirm, our knowledge of other viruses points towards this a likely transmission route. There is also growing evidence of higher infection rates in poorly ventilated indoor spaces; highlighting the need for adequate ventilation in order to reduce the potential of airborne transmission of the SARS-CoV-2 virus.

Updated in June 2020 Public Health England (PHE) report 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.”

Another possible route could be via faecal-oral transmission and SARS-CoV-2 viruses have been detected in stool samples. Flushing toilets with open lids can create a plume of droplets and an associated droplet residue. So potentially this could be a localised means of transmitting contaminated droplets into the local environment and adjacent surfaces. A number of studies are reporting this as a potential route of transmission although, as of the 29th March 2020 the World Health Organisation (WHO) stated “There have been no reports of faecal−oral transmission of the COVID-19 virus to date.” This advice does not appear to have changed in the intervening period.
WHO also report that “there is no evidence to date of viruses that cause respiratory illnesses being transmitted via food or food packaging. Coronaviruses cannot multiply in food; they need an animal or human host to multiply. It is highly unlikely that people can contract COVID-19 from food or food packaging.”

So to conclude “COVID-19 is a respiratory illness and the primary transmission route is through person-to-person contact and direct contact with respiratory droplets generated when an infected person coughs or sneezes.”

Coronavirus and forced air ventilation/air conditioning systems

On the 22nd May 2020 the Health and Safety Executive (HSE) published a bulletin stating, “The risk of air conditioning spreading coronavirus is extremely low”.

Multiple updates of advice in relation to COVID-19 has also come from organisations such as WHO, HSE and The Chartered Institution of Building Services Engineers (CIBSE).

Currently the HSE state:

“The law requires employers to ensure an adequate supply of fresh air in the workplace and this has not changed during the pandemic.

Good ventilation, together with social distancing, keeping your workplace clean and frequent handwashing, can help reduce the risk of spreading coronavirus.

Good ventilation reduces the concentration of the virus in the air and therefore reduces the risks from airborne transmission. This happens when people breathe in small particles (aerosols) in the air after someone with the virus has occupied an enclosed area.

However, ventilation will have little or no impact on droplet or contact transmission routes.

You should consider ventilation alongside the relevant control measures required to reduce the risk of transmission as part of making your workplace COVID-secure.”

Know what systems you have in place

Air conditioning in general terms can be a catch all phrase often used to cover any type of forced air ventilation. Different systems however have different characteristics and knowing what you have in place and where and what it serves will dictate how your strategy for successfully managing air quality can be prepared.

While much of the guidance considers system types in isolation (e.g. natural, extract only, localised individual units, mixed mode, mixing boxes, mechanical ventilation/air conditioning) in reality many workplaces have a mixture of ventilation types. So, your controls need to reflect this too as well as how you are proposing to occupy and use the space.

In addition to talking to your HVAC engineers and independent specialists, CIBSE have produced some good guidance on different type of systems and controls in their “Emerging from Lockdown” and “Coronavirus, SARS-CoV-2, COVID-19 and HVAC Systems“ publications (link at the end of this guide).

Filter upgrades and duct cleaning?

At approximately 0.1 of a micron SARS-CoV-2, virus particles are much smaller than bacteria and many other particles in our atmosphere. Even F8 filters will only “catch” possibly up to 80% or 90% of particles of 1 micron in size, so they will not provide a direct barrier. Add to this the likelihood that SARS-CoV-2 concentrations in outdoor air will only be very small anyway, the risk is extremely low. Maintaining your systems and their filters and checking performance is the best course of action.

There is the potential for those individuals who are changing filters within air handling units to be exposed to microbiological matter; particularly with regards to changing extract air filters. Although the risk is minimal, you should assume that filters are contaminated with viable viruses. These filters in turn should be changed in line with standard safety procedures: the air handling unit should be switched off; appropriate PPE should be worn; and filters should be disposed of in a sealed bag.

As most of the likely sources of transmission will be the people and surfaces in the office, duct work cleaning will have little effect on this. Equally the majority of virus particles will not colonise and grow in your ductwork distribution systems – very few things do anyway – and if entering, it will either “blow through” or die trying.

Maintain or increase air supply and extract ventilation

By keeping air moving through your building, the supply and extract systems act as a natural diluter for any contaminants within. The supply air adding fresh, uncontaminated air and the extract removing the stale air.

Keeping mechanical ventilation systems operating and, if possible, increasing the run times and ‘fresh’ air content (reducing the recirculation rates) are good options. Consider operating your ventilation systems to obtain the maximum air change rates you can relative to the space and occupancy levels.

You could run your air system 24/7 for the greatest effect; you could choose to lower these rates (but not switch off) when there are fewer occupants in the building or it is being used intermittently. A secondary option is to extend the current operation times of your ventilation system, so that it starts two hours earlier than normal, and finishes two hours later. At nights and weekends, do not switch ventilation off, but run systems at lower speed. With any of these options you must also weigh up the benefit versus the additional costs, maintenance and environmental impact of your choice.

These options aim to provide the maximum amount of fresh air to your occupants whilst removing any potential virus particles that have been transmitted within your building.
Review your system operation to check it is properly balanced, taking into account any changes in occupancy and/or space utilisation, so you are achieving uniform air distribution throughout these areas.

Where you have in room or room to room recirculation (e.g. comfort cooling units), these typically have no fresh air make up so are in essence just recirculating the air within the room (albeit tempered). Provided the areas have a good fresh air ventilation supply (either natural or mechanical), the action of these units can help to de-stratify the air, reducing the chances for pockets of stagnant air, and can help contribute to the overall dilution of airborne virus particles.

See also “What are you recirculating”, “Heat recovery systems” and “Toilet systems” below.

Naturally ventilated buildings

In buildings without a mechanical ventilation system, consider opening windows more frequently, and for longer periods, to achieve a similar diluting effect. Obviously be aware of the possible effect this may have on the thermal balance of the internal environment, or the potential for other pollutants to be drawn into the building.

During the winter months, it is important to strike the right balance between appropriate ventilation and thermal comfort. Continuous ventilation via open windows is the most effective way of diluting the air in a naturally ventilated building. However, this will have the biggest impact on your occupants with regards to reduced indoor temperatures. Intermittent airing/ purge ventilation is another option; this is when a building is purged of its stale air for a period by opening windows and doors intermittently.

For spaces which may be occupied sequentially by different groups, you may also wish to have fallow periods in between uses to help dilute the air of any contaminants. This can be effective in combination with purge ventilation.

See also “Toilet systems” below.

What are you recirculating?

Recirculated air has the potential to reintroduce airborne contaminants into the indoor environment, so be careful how you manage this in your building.

Consider avoiding, where you can, recirculating air through your building from the main air handling system. However, be aware of the consequential effect reducing the amount of recirculating air may have on, for example, temperature control. In the winter months, some recirculation may be acceptable if it means there is a greater overall ventilation rate without causing thermal discomfort to your occupants.

Secondary systems (e.g. fan coil units) can remain switched on provided the areas have a good fresh air ventilation supply (either natural or mechanical), the action of these units will help to de-stratify the air, reducing the chances for pockets of stagnant air, and can help contribute to the overall dilution of airborne virus particles. If however, there is very little fresh outside air ventilation (either natural or mechanical), then the advice is to switch off the unit.

Also check/inspect your heating and/or chilling batteries/coils and extract and return air grilles to make sure they are operating correctly, clean and free of any blockages and or restrictions.

Heat recovery systems

Like locally recirculated air, some heat recovery systems can allow extracted contaminants to be reintroduced to the building. For example, particles that become deposited on the extract side of a ‘thermal wheel’ could be blown back through on to the supply air side. These type of heat exchangers, and others which allow for the possibility of mixing extract and supply air flows, should be investigated for the potential of such ‘leaks’ occurring, taking into account the pressure differentials between supply and exhaust side air streams.

You will need to ensure that higher air pressure is present on the supply air side, to stop exhaust air from leaking into the supply air side. Higher ventilation rates are also recommended in order to minimise the potential for leaks. If there is no evidence of any leakage, then there is no need to switch such heat recovery systems off. If there is evidence of any bypass, then such systems should be temporarily switched off.

Toilet systems

Although still not yet established, there are a number of simple measures you could take to control possible faecal-oral transmission of SARS-CoV-2 – these will also prove more effective for other viruses (e.g. noroviruses) too. These could include:

  • Flushing toilets with the lid closed, thereby reducing droplet plumes being created. Therefore, minimising any faecal-oral transmission of SARS-CoV-2 virus particles.
  • Keeping your toilet extract systems maintained and operational. Consider bypassing any controls on your extract system (if you can without affecting other systems) and operating them continuously during occupation of the building.
  • Try to maintain negative air pressure in the toilets, this keeps smells as well as any generated contaminants out of adjacent areas.
  • Keep the plumbing systems well maintained and preserve water seals. For example, by stopping drains drying out.
  • Actively promote good hygiene practises, especially hand washing.

Social distancing of occupants

On the 11th May 2020 “Working safely during coronavirus (COVID-19)” was released through GOV.UK. This now offers guidance for 14 different types of workplace and remains regularly updated to keep people informed of the changing rules and restrictions. The advice covers:

  • Close contact services
  • Construction and other outdoor work
  • Factories, plants and warehouses
  • Heritage locations
  • Hotels and other guest accommodation
  • Labs and research facilities
  • Offices and contact centres
  • Other people’s homes - people working in, visiting or delivering to other people’s homes
  • Performing arts
  • Providers of grassroots sport and sport facilities
  • Restaurants offering takeaway or delivery
  • Shops and branches – effectively retail premises
  • Vehicles - People who work in or from vehicles, including couriers, mobile workers, lorry drivers, etc.
  • The visitor economy

As the number of occupants in buildings is likely to be reduced compared to the start of the pandemic, increased space and the ability to spread people throughout the building is more of a possibility. Be aware though that while this not only promotes social distancing as well as maximising the effect of the ventilation system in removing any airborne virus particles, it could have a negative effect on for example fire safety management.

Try and manage numbers of occupants and their locations accordingly to maintain social distancing requirements and indoor air quality, but not adversely affect fire and other health and safety, cleaning and security issues.

Cleaning and hygiene

Effective cleaning and hygiene (along with social distancing) is another key theme in keeping your workplace COVID-19 Secure. This is also covered in “Working safely during coronavirus (COVID-19)”. Reducing the potential for transmission of the virus through good hygiene practices could include the following:

  • Encourage staff to adopt good hygiene practises at all times and back this up with poster and other campaigns;
  • Amend cleaning regimes to target the most effective interventions. More regularly sanitising frequently used/touched surfaces such as handles, lift buttons, security pads, kitchen cupboards and utensils, vending machines, etc. will be time better spent than vacuuming.
  • Clean/sanitise desks regularly used and communal phones and equipment in “touchdown areas” or hot desks.
  • Keep consumables, soap, hand sanitisers, toilet rolls, etc. readily available and accessible.
  • Regularly remove waste and check areas such as washrooms and kitchens regularly.

Assurity Consulting will continue to take guidance from authoritative sources including Public Health England, WHO and the Government regarding COVID-19. As further information becomes available so we will update this guidance as required. Sources of information include:Assurity Consulting will continue to take guidance from authoritative sources including Public Health England, WHO and the Government regarding COVID-19. As further information becomes available so we will update this guidance as required.

This guide was first produced on the 3rd April 2020 (with updates on 22nd June 2020, and 8th January 2021)

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