More autonomy over applied COVID-19 Secure controls has seen workplaces adapting their requirements to best suit, although the emphasis on effective ventilation and the provision of a suitable fresh air supply will remain key control measures in the immediate future.

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 continues with the different factors and influences that we need to consider.

What are the main transmission routes for COVID-19?

There are several ways that the SARS-CoV-2 virus can spread. An infected person coughing, sneezing, or talking produces small liquid particles containing the virus. These liquid droplets vary in size from larger respiratory droplets to smaller aerosols. The larger droplets (>5-10 μm in diameter) contaminate the immediate atmosphere (typically not further than 1-2 metres). Whereas the smaller aerosols (<5μm in diameter), remain suspended in the air for longer periods of time, and can travel further distances.

Illness is thought to be primarily caused because of short-range contact with an already infected person (typically less than 1 metre) producing these larger droplets and aerosols which are then directly inhaled by another. It is also thought to be spread over longer ranges (typically over 1 metre) in poorly ventilated or crowded indoor spaces in which people spend longer periods of time. People can also contract the virus when they touch surfaces contaminated by infected droplets, secretions or other body fluids (fomites) and then touch their mouth, nose or eyes.

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 you can prepare your strategy for successfully managing air quality.

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 (there is a link at the end of this guide).

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 it. 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 when 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, considering 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.

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; 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.

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.

Consideration should be given to seasonal variations in temperature, 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 an effective solution in combination with purge ventilation.

Indoor air quality monitoring

Having implemented measures to increase ventilation rates in your buildings, you should consider trying to identify any areas within your building which may remain poorly ventilated. Indoor air quality monitoring can help to identify any areas which remain poorly ventilated. These areas can then be prioritised for improvement to reduce the risk of viral transmission via contaminated aerosols. If improvements can’t be made in a poorly ventilated space, you should consider other control measures that can be put in place, for example, restricting the number of people that enter the space, or, restricting the time spent in the space, or stopping the use of the space entirely.

Carbon dioxide (CO2) is exhaled by people when they breath, and as such, can be used to determine whether a space is adequately ventilated. Outdoor air levels are typically around 400 parts per million (ppm). For indoor air, CO2 levels at or below 800 ppm are likely to mean that a space is well ventilated. Levels which exceed 1500ppm are likely to indicate that a space is poorly ventilated, and further action is required to improve such spaces.

More recent advice on the introduction of CO2 monitoring, including from the Health and Safety Executive (HSE) has seen an increase in use of these in workplaces. It must be remembered thought that these are intended to identify and help in manage “poorly ventilated areas” rather than indicate the overall quality of air. They should not be confused with, or therefore act as a substitute for independent, accredited and professionally performed air quality assessments. Properly managed air quality has benefits beyond COVID-19 control and should from part of your proactive and ongoing workplace management. Having monitored air quality in 1,000’s of building environments, both prior to and during the pandemic, well maintained forced air ventilation systems in our experience, continue to provide a very good quality of air to occupied spaces.

It is also worth noting that there are certain types of spaces in which CO2 monitoring may not provide reliable information regarding ventilation. Such areas would include small spaces such as toilets, changing rooms or lifts; and areas occupied by people for short periods of time, or varying periods of time.

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 most virus particles will not colonise and grow in your ductwork distribution systems – very few things do anyway – and if entering, they will either “blow through” or die trying.

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, considering the pressure differentials between supply and exhaust side air streams. You will need to make sure 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 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.

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 practices, especially hand washing.


  1. COVID-19: Advice for employers and businesses (GOV.UK)
  2. Coronavirus (COVID-19): latest information and advice (HSE)
  3. Ventilation (HSE)
  4. Working safely during coronavirus (Covid-19) (GOV.UK)
  5. Coronavirus disease (COVID-19) pandemic (World Health Organisation)
  6. Air pollution and COVID-19 (WHO)
  7. Emerging from Lockdown (CIBSE)
  8. CORONAVIRUS, SARS-COV-2, COVID-19 AND HVAC SYSTEMS (CIBSE),-sars-cov-2,-covid-19-and-hvac-systems
  9. EMG: Role of ventilation in controlling SARS-CoV-2 transmission, 30 September 2020 (SAGE)

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