They are more effective in rooms where the ventilation rate is lower than desired and/or is difficult to provide ventilation consistently. This can include rooms that rely on opening windows for ventilation.

There are a range of different options, including portable plug-in devices or ones which are installed on a wall or ceiling. They all work the same way – a fan draws air through the filter which removes particles. If the filter is a high grade, it can remove over 99% of all particles in the air that passes through it.

The rate at which the air is cleaned depends on how much air passes through the device. This article describes how to choose what size device is required. (Covid: How to best use air-cleaning units in schools | Tes)

In larger rooms it is often better to have several smaller devices rather than one very large device. Devices are most useful in rooms where people spend longer periods of time together.

HEPA-filter based devices can also reduce exposure to other particles in air such as pollen, dust, smoke, and soot. There are several considerations with using HEPA-filter units including noise, the physical location and robustness of the device, the cost to purchase, install and run, and the maintenance of the device.

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