“Unlike outdoor air pollutants, far less is known about the sources, transformation and fate of indoor air pollutants.

The provision of clean air indoors has become an increasingly important issue over the last 2 years, driven by concerns over Covid-19 transmission indoors. However, thinking about indoor air quality in developed countries like the UK, has been a focus for many scientists since the 1980s. Unlike outdoor air pollutants, far less is known about the sources, transformation and fate of indoor air pollutants.

Maybe this is the reason why air cleaning devices (ACDs) have become so popular since the pandemic began. Air cleaning technology comes in all shapes, sizes and prices. On the face of it, air cleaning could be an attractive solution to poor indoor air quality. After all, many of them just need to be plugged in and left running. However, the market for ACDs is currently unregulated. We only have the manufacturers’ assurances that they are effective, and they are often limited to the removal of one pollutant (or virus) at a time. Worse still, many ACDs can produce air pollutants as they operate.

At the end of 2020, the UK Government asked the SAGE Environmental Modelling Group to investigate the potential for different methods of air cleaning to reduce the potential for transmission of Covid-19 indoors. The report suggested that ACDs were only likely to have limited benefit in spaces that were already well ventilated, unless there were specific risks identified.

If ventilation was poor and ACDs were needed, those based on fibrous filtration (e.g. HEPA filter devices) or germicidal UV could be beneficial. However, those based on ionisers, plasma, chemical oxidation, photocatalytic oxidation and electrostatic precipitation had limited evidence to demonstrate they were effective against Covid-19. Further, their use could lead to the production of harmful secondary air pollutants, including ozone and nitrogen oxides

The report also noted that for all ACDs, the safety and efficacy should be evidenced by relevant test data before they are used. This last point is important. What do we mean by relevant test data? We need the ACDs to be tested in realistic conditions. Controlled laboratory conditions are not the same as the widely varying temperature and humidities experienced in a typical household, or the movement of air that might be experienced as 30 children move around a classroom.

There are other practicalities that need to be considered for pollutant removal more generally. Is the ACD big enough for the space that needs cleaning? If it is unable to deliver sufficient clean air, a false sense of security may be assumed. Conversely, if the ACD is used in a smaller space than intended and it produces a secondary pollutant such as ozone, resulting concentrations could increase to unsafe levels. Some of the units are noisy and can end up being run at sub-optimal speeds to reduce the background noise. Think about a classroom where a teacher needs to talk over an ACD, so that all of the pupils can hear the lesson clearly.

So what is needed? Regulation is the answer. ACDs need to be tested in realistic environments (maybe in special laboratories) to make sure that they are effectively removing pollutants, and not making any new ones. It needs to be clear that they are safe during operation, to those maintaining and using them and that this continues over time.

Such a regulatory system should be welcomed by ACD manufacturers who are already thinking about these issues, but it would also weed out those ACDs that are ineffective, or even dangerous.

And what about the real elephant in (or outside) the room? If the air quality outside was good, we could open windows to improve indoor air quality and ACDs would no longer be needed in most buildings. Although improving outdoor air quality will not remove all indoor air pollutants (e.g. those generated indoors through cooking and cleaning), it does mean that ventilation of those indoor spaces becomes much simpler.

Further Resources

As well as the SAGE report mentioned above, more information can be found with the following resources:


And a report aimed more at schools:


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