Respiratory infections including Coronavirus (COVID-19): ventilation in the workplace

Guidance for employers on improving ventilation and the the supply of fresh air into the workplace.

Air cleaning technology

Air cleaning technologies (ACT) - also known as air filters or air purifiers - are one method to reduce the risk of the respiratory infections including COVID-19 being transmitted in indoor settings where it is not possible to maintain adequate ventilation.

However, neither ventilation nor air cleaning is a perfect solution; both can reduce the concentration of a respiratory virus in the air but will unlikely mitigate all transmission risks. These measures will not remove all virus particles and have limited effect on transmission when people are in close proximity to one another.

Air cleaners can be a useful supplement to ventilation, particularly where ventilation cannot be improved and/or where there are particular concerns over high levels of outside air pollution.

Air cleaning devices are never a replacement for ventilation and are never a reason not to ventilate a room, whether through natural or mechanical ventilation.

In many cases ACT should be considered as a short-term measure, either to mitigate the risk of viral transmission in a space while longer term upgrades to ventilation are made, or for use over short time periods to improve indoor air quality when external conditions mean that ventilation is very challenging (e.g. extreme weather or high levels of pollution).

If external conditions are such that air cleaners are required on a routine basis throughout the year, then it is likely better to invest in improved ventilation systems rather than use local ACT devices over the long term.

Types of air cleaners

Technologies based on particulate filters, usually HEPA filters, and/or germicidal ultraviolet (UVC) are likely to be most effective, particularly in the context of mitigating the transmission of COVID-19. Further information on the different kinds of air cleaning technologies can be found in COVID-19 CIBSE guidance.

Considerations for selecting and using ACT

The choice to use ACT in a room depends on a wide range of factors, and it will depend on the setting and activities that happen in it, its current ventilation, the occupancy and factors around usage and maintenance.

The following should be considered as part of the decision:

1. Assess ventilation


If ventilation is already effective, then the benefits of an air cleaner may be marginal.

If ventilation is very poor it is still necessary to ensure that there is some provision of ventilation implemented in a room regardless of a decision to use air cleaners.

CO2 monitors can be used as an indicator as to whether spaces are sufficiently well ventilated.

In a space suspected to be poorly ventilated, air cleaners may be a useful strategy if ventilation cannot be easily improved.

2. Select appropriate ACT equipment


In addition to selecting the type of technology (filter based, UVC etc), it is important to ensure that devices can effectively clean air in a room.

The performance of filter-based devices is usually reported as a Clean Air Delivery Rate (CADR) which represents the amount of air processed per hour by the unit for a particular particle size, and is usually given in m3/hr.

Enclosed UVC devices can also use similar metrics, however their performance will vary depending on particular microorganisms, so it is important that a manufacturer clearly indicates this.

A ventilation assessment is needed to evaluate the amount of clean air needed in a space.

Ideally the air cleaner plus the ventilation in the room should together provide the equivalent of 10 litres per second per person of clean air.

If information on the ventilation is not available, a good rule of thumb is to size air cleaners to provide around 6 litres per second per person.

Depending on the size of the space it is often better to have 2-4 smaller units with a lower CADR rather than a single large unit. This both reduces noise levels and provides a better distribution of air by spacing units out in a room.

The Chartered Institution of Building Services Engineers (CIBSE) provide more detailed COVID-19 guidance on selection of air cleaning devices including a flow chart to assess when air cleaners are useful and to assist with selection, and a calculator to estimate the effectiveness of units in different sized spaces.

3. Consider the noise generated by ACT units


All devices produce some noise and how noticeable this is will depend on the device, the environment and the occupants.

The actual noise experienced due to a device can be complicated to predict and will depend on the room size and its acoustics. COVID-19 guidance on appropriate noise levels for a range of different environments is given by CIBSE with further guidance for specific environments.

4. Consider practicalities of location and installation


The suitability of a particular ACT device will depend on the environment it is used in and the occupants.

Many devices are designed for the domestic market and may be suitable for smaller offices but are unlikely to be sufficiently robust for larger spaces.

Units vary in size and it is important to consider where they will be placed and how much space is available.

They will need to be plugged in, and it is important to consider whether devices are likely to be in a location where they could be interfered with or knocked over, block access to doors/windows/vents or be positioned in a way that they could create a tripping hazard from the device itself or the power cord.

Devices need to be positioned so that they can effectively distribute clean air in a room. Ideally, if there is a single unit it should be as central as possible, and if there are several units they should be distributed around the room.

Practicalities such as available sockets mean that devices are usually to the side of the room, however they should normally be placed away from walls and also away from curtains or furnishing which can block the flow.

In rooms with less space or where there are regularly people moving around, wall or ceiling mounted units may be more robust, however this is likely to require electrical work for installation. 

5. Carry out a cost benefit analysis


The cost of ACT devices varies significantly with units available from around £150 to over £1000 depending on the size of the device, the technology used and its features.

Running costs are typically low as ACT devices do not consume much power, however adding multiple units in a space or building may noticeably add to electricity consumption.

Filter replacement costs can vary substantially, and availability as well as cost needs to be factored in. The effectiveness of devices can degrade over time, and regular replacement of filters is needed to ensure devices operate as intended.

Investment in ACT devices should also be compared to the cost of measures to improve ventilation both in the short and long term.

If multiple ACT devices are being bought for an organisation, then it is worth considering whether it is more appropriate to have a support and maintenance contract rather than manage units in house.

All ACT devices will require ongoing maintenance and while this is relatively modest it needs to be factored into decision making.

Devices should be inspected on a regular basis for any signs of damage or electrical issues, and the outer surfaces should be kept visibly clean. The main maintenance requirement is replacement of filters and/or UVC lamps depending on the device; filters become clogged over time and therefore less efficient, while UVC lamp output degrades over time. ​​​​​

6. Ensure people know how to use the ACT devices


Although ACT devices are usually simple to use, it is important that those who are responsible for the spaces where they are located understand why the devices are there and understand what they need to do to operate the device properly; a device that is placed into “sleep” mode or tucked away in a corner because it is in the way will not be effective.

It is also important that people in the room are aware that the devices do not ventilate the room, and they still need to provide some ventilation, even when the devices are operated.



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