This guidance will apply in most workplaces – it will help you and your workers:
assess the risk from aerosol transmission in enclosed areas
identify poorly ventilated areas
decide on the steps you can take to improve ventilation
Why ventilation is important
A good level of ventilation reduces how much virus remains in the air after a person with the virus has been in the same enclosed area. It should be noted that ventilation will reduce the aerosol risk but has minimal impact on:
Identifying poorly ventilated areas
You should identify areas of your workplace that are usually occupied and are poorly ventilated, prioritising these areas for improvement to reduce the risk of aerosol transmission. There are some simple ways to identify poorly ventilated areas:
Why monitor the levels of CO2
Carbon Dioxide levels in fresh air (atmospheric) are 400ppm (parts per million), as we breathe we generate excess carbon dioxide which increases the overall concentration of this gas in the indoor environment. A poorly ventilated workplace with very little fresh air can easily see levels in excess of 1200ppm as the working day progresses.
The chart below demonstrates the changes in the carbon dioxide levels in a typical office over a number of days. As staff enter the office at 8.30am each day the levels can be seen to increase to a peak in the late afternoon as the ventilation systems struggle to supply enough fresh air. When staff leave the office at 5pm the levels can be seen to steadily reduce back to atmospheric levels.
If pre-symptomatic or asymptotic staff were present within the office the viral load of aerosols within the air would be represented in the same way.
Assessment of fresh air in the workplace
There are a number of factors to consider when deciding on the ventilation needed in your workplace:
How do you provide fresh air (ventilation) to your workplace?
Adequate ventilation reduces how much virus is in the air and therefore reduces the risk from aerosol transmission for workers in that area. Your workplace may have different means of providing ventilation for different areas. It may be helpful when doing your assessment to make a list of areas in your workplace and how they are ventilated.
Remember to include changing rooms and areas used for breaks, such as canteens. If you are not able to easily tell how an area is ventilated, it may be because it is poorly ventilated.
How many people use or occupy the area?
The more people who use or occupy an area the greater the risk that an infected person is there, increasing possible exposure to aerosol transmission. Reducing the number of people who use or occupy an area reduces this risk.
How much time do people spend in the area?
The longer people use or occupy an area, the greater the risk. Consider how many people use or occupy an area for a sustained period, and how many come and go throughout the day. Can you reduce this in any way?
How large is the area?
The larger the area, the lower the risk. This is because larger areas:
have more air to help dilute the virus
tend to be designed with higher ventilation rates
mean it takes longer for aerosols to build up
What tasks or activities take place in the area?
Activities that make you breathe deeper, for example physical exertion or shouting, will increase generation of aerosols and increase the risk of transmission.
These activities increase transmission risk even in areas with adequate ventilation.
Do you use desk or ceiling fans?
Desk or ceiling fans should not be used in poorly ventilated areas.
How to improve natural ventilation
You can improve natural ventilation by fully or partially opening windows, air vents and doors. Buildings are designed to provide an adequate amount of ventilation and, where this is through windows and air vents, you should be able to open them. If they cannot be opened whilst occupied, the ventilation in that area will be affected.
Purging of unoccupied rooms (in-between shifts or on breaks) by opening all windows and doors fully can improve ventilation levels.
How to improve mechanical ventilation
Mechanical ventilation brings fresh air from outside into a building. You should speak to the people who manage the day-to-day operations of your workplace ventilation systems to:
understand how they operate
make sure they’re supplying fresh air into an area and how much
make sure they’re maintained in line with manufacturers’ instructions
You should base ventilation rates on the maximum ‘normal’ occupancy of an area.
These mechanical systems will provide adequate ventilation if they are set to maximise fresh air and minimise recirculation.
If your system draws in fresh air, it can continue to operate. You need to know how much fresh air it draws in and if this provides adequate ventilation. You may need to increase the rate or supplement with natural ventilation (opening doors, windows and air vents) where possible.
You can also consider extending the operating times of mechanical ventilation systems to before and after people use work areas.
It is preferable not to recirculate air from one space to another. Recirculation units for heating and cooling that do not draw in a supply of fresh air can remain in operation provided there is a supply of outdoor air, for example windows and doors left open.
Recirculation units (including air conditioning) can mask poor ventilation as they only make an area feel more comfortable.
Air cleaning and filtration units
You can use local air cleaning and filtration units to reduce airborne transmission of aerosols where it is not possible to maintain adequate ventilation. These units are not a substitute for ventilation. You should prioritise any areas identified as poorly ventilated for improvement in other ways before you think about using an air cleaning device.
If you decide to use an air cleaning unit, the most suitable types to use are:
Any unit should be appropriate for the size of the area they’re used in to ensure they work in the way they are intended to.
Carbon dioxide (CO2) detectors are not suitable for use in areas that rely on air cleaning units to provide ventilation. This is because filtration units remove contaminants (such as coronavirus) from the air but do not remove CO2.