The popularity of air-cleaning technology has increased in recent years (Siegel, 2016), even more so since the COVID-19 pandemic (SAGE, 2020). Numerous air-cleaning devices (ACDs) are available, with differences in technology adopted, shape, size, operating parameters and cost. Many of these ACDs focus on removing biological pollutants, although some also focus on removal of specific particulate and chemical pollutants. However, some ACD instruments can also form pollutants through their operating procedure, some of which can be harmful to health (e.g. ozone). At the moment, ACD instruments remain largely unregulated. Deciding which, if any, to use, and understanding the impact of doing so on indoor air quality (IAQ), can be challenging. 

ACDs typically adopt one or more techniques, including thermal or photocatalytic oxidation, adsorption, ozone-oxidation, plasma technology, filtration, UV germicidal irradiation, ion generation, and electrostatic precipitation (Zhang et al., 2011). However, none of these technologies remove all of the indoor air pollutants present and many generate undesirable secondary products (Zhang et al., 2011; US EPA, 2018). For instance, ozone-generators initiate chemistry indoors which leads to the production of a wide range of VOCs and aerosols, whilst ion generation could produce large quantities of ozone and nitrogen oxides (Zhang et al., 2011). The recent SAGE report (SAGE, 2020) noted that ‘ACDs have limited benefit in spaces that are already adequately ventilated, and are not necessary for adequately ventilated buildings unless there are identified specific risks’. It further suggested that if ACDs were required, those which used filtration or germicidal UV were likely to be beneficial if deployed correctly. 

Including activated carbon in a filter has been shown to reduce the ozone and organic pollutants in the air stream, and amount of carbon in the filters can be optimised depending on the ambient ozone concentrations in the airstream (Bekö et al., 2009). However, the activated carbon can become saturated, poisoned or fouled by particles (Siegel, 2016). The efficiency of filters varies over time, with the type (fibrous or electret), and the particle size (US EPA, 2018), and performance is likely to be different when challenged by real particles as opposed to test particles used to determine their efficiency in the laboratory (Kim et al., 2016; Schumacher et al., 2018). Poorly maintained filters can also be a source of pollution, either as they become loaded with pollutants which are then released, or when the filter load material (typically organic material associated with the particles) reacts with incoming air pollutants (e.g. ozone) to form secondary pollutants that can enter the space you are trying to clean (Bekö et al., 2009; Siegel, 2016).

Finally, ACDs can be deployed as a portable unit or as part of a centralised HVAC system. The mode of operation will affect how the impacts of using an ACD can be distributed around a building, and for both types, proper operation and  maintenance (e.g. regular filter replacement) is critical. What is now required is a standard testing procedure for ACDs, which considers (i) efficacy of removal of target pollutants and (ii) production of secondary pollutants through typical operating conditions and (iii) how these change over time and with maintenance.

There are three key aspects to consider. The measurement of the species the ACD is claiming to remove (often PM or biological contaminants). The second is the production of secondary pollutants, which are typically ozone, NOX and VOCs. Finally, we need to understand both of these with time, alongside issues such as operating costs, energy use and maintenance demands (such as needing to change filters).

The species above should be measured for a range of ACDs operating under typical conditions. We should also use models for additional insight, given it is not possible to routinely measure some species such as radicals which can be produced through ACD use and further impact the chemistry. Specifically:

Microbial pollutants: from our WG4 report

Particles: WG4: Full article: Particulate matter indoors: a strategy to sample and monitor size-selective fractions

Ozone : WG4 work described in Part 5.2.1, p13 in Sampling and analysis techniques for inorganic air pollutants in indoor air

VOCs: refer to Mapping organic constituents and WG4 Full article: An overview of methodologies for the determination of volatile organic compounds in indoor air

NO2: WG4 work described in Part 5.3.1, p546 in Sampling and analysis techniques for inorganic air pollutants in indoor air

ACD testing should ideally be carried out in realistic environments (not in carefully controlled laboratories), over a range of temperatures, humidities, and with different background concentrations of air pollutants.