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

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.

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.

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.

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. 

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. ​​​​​

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.