ReviewPortable air purification: Review of impacts on indoor air quality and health
Graphical abstract
Introduction
Individuals in developed countries spend up to 90% of the day in indoor environments such as homes, offices and schools. Indoor air pollutant concentrations, including particulate matter (PM), carbon monoxide (CO), ozone (O3) and volatile organic compounds (VOCs) can at times be several times higher than those in the outdoor air, indicating a significant potential for detrimental health impacts (ONS, 2017; US EPA, 2018b; NICE, 2020).
Technology is rapidly evolving in an attempt to mitigate existing indoor air pollution and air purifiers for the indoor environment are one of the latest technologies attempting to reduce concentrations of harmful pollutants in the indoor air of both indoor and outdoor origin (Kelly and Fussell, 2019). There are a wide range of air purifiers which use various technologies; however, some of these have been shown to have potentially adverse health impacts such as the generation of harmful intermediates such as O3 (see Box 1) (ASHRAE, 2015; US EPA, 2018c).
PM can originate from outdoor sources, such as fuel combustion from vehicles (along with non-exhaust emissions such as physical road wear particles from vehicles) and industry, uncontrolled emissions from fires and flue emissions from fireplace and wood burning stoves, as well as natural sources including wind-blown soil and dust and can infiltrate indoors through windows, vents and leakage through cracks and gaps in the building envelope, or via mechanical ventilation systems, where filtration is not present (Chen and Zhao, 2011; PHE, 2018; US EPA, 2018b). Potential indoor sources of PM include cooking, smoking, emissions from wood stoves and fireplaces, heating, cleaning and other occupant activities (Hill et al., 2001; McGrath et al., 2017; Defra, 2019; O'Leary et al., 2019; NICE, 2020; RCPCH, 2020).
A good understanding of all the sources, both indoors and outdoors, contributing to overall PM exposure is required to determine the potential implications for health (Cincinelli and Tania, 2017). Long-term exposure to PM less than 2.5 μm in diameter (PM2.5) is associated with chronic conditions such as cardiovascular and respiratory diseases and cerebrovascular complications, leading to reduced life expectancy (COMEAP, 2009). Short-term exposure to PM2.5 can also cause a variety of health impacts including exacerbation of asthma and increases in respiratory and cardiovascular hospital admissions and mortality (COMEAP, 2009; PHE, 2018). Symptoms can be exacerbated in those already diagnosed with heart and lung conditions as well as having a significant impact on more vulnerable populations, such as older people, children (whose lungs are developing) and pregnant women. PM is also listed as a cause of lung cancer (IARC, 2013; WHO, 2013; Hamra et al., 2014).
Controlling the emissions of pollutants at source is the optimal strategy to reduce overall air pollution and protect health; however, where this is not practical then the pollutant concentrations should be targeted, with prevention and abatement techniques employed to aid in exposure reduction from the remaining air pollution.
Kitchen extractor fans or cooker hoods are the most commonplace intervention in homes for reducing emissions from combustion and cooking processes by being situated near the source, reducing the amount of pollution dispersed into the indoor air, although not always very effectively (O'Leary et al., 2019). With the exception of some mechanical systems, for example, mechanically ventilated heat recovery (MVHR) systems, extractors work intermittently and do not deal with any continuous emission sources. In the case of schools or offices, buildings can either be naturally or mechanically ventilated with or without PM filtration (DfE, 2018).
There are a wide range of air purifying technologies currently available aimed at reducing the pollutants in the room where located. These devices are promoted to deal with various pollutants for which, in the absence of source control, cannot necessarily be removed by extraction processes and occupant ventilation practices, which are presented in Box 1.
Over the last couple of decades, previous reviews on air purification in various indoor environments focussed on the comparison between different types of technologies currently available, the targeted pollutants, the potential advantages and disadvantages and applicability to various environments. There is a lack of research around interventions appropriate for the general population and their efficacy.
A report to Health Canada by Wallace (2008) highlighted the importance of the clean air delivery rate (CADR), which is calculated by multiplying the efficiency by the air flow rate through the filter, as this determines how much clean air is being supplied. The report concludes that air cleaners can reduce particulate levels in homes, but that attention should be paid to the CADR and the efficiency of the filter. Zhang et al. (2011) conducted a literature review on the use of air cleaning technologies to improve indoor air quality. The included studies demonstrated that mechanical filters can efficiently remove particles, suggesting a higher efficiency removal rate for larger particles, but limited and inconsistent evidence to support use for other pollutant removal. The review also reported that the filters themselves, if not properly used, can become a source of pollution and that the filtration efficiency was sometimes less than that reported by the manufacturers. Luengas et al. (2015) also reviewed various types of indoor air treatment stating that ‘mechanical filtration is a simple and extensively used technique for removing suspended particles from indoor air’. The review states that gases are not retained, and that filter performance is dependent on several factors including: filter type, material and air flow. The authors also conclude that device efficiency is reduced over time due to the filters becoming saturated, therefore highlighting the need to maintain and replace them.
Apart from the impact of air purifiers on indoor air quality, some epidemiological studies have been carried out to evaluate the possible health benefits of air filtration, including PAPs. These studies have generated mixed results. Reisman (2001) reviewed the epidemiological evidence published up to 2000 that focussed on patients with clinical allergic disease. He concluded that there was inadequate data available on the potential value of air purifiers for the prevention and treatment of allergic disease. Another systematic review and meta-analysis was conducted on the effect of air filtration systems on asthma among adults and children (McDonald et al., 2002). Domestic air filters were associated with fewer allergic symptoms and less sleep disturbance, but no benefit was found on nasal symptoms, medication use, or peak expiratory flow (PEF) values based on 10 randomised controlled trials published from 1976 to 2000. Fisk (2013) investigated the health benefits of particle filtration in homes and commercial buildings with the main conclusion that it can have modest effect on adverse allergy and asthma outcomes. A recent review by Kelly and Fussell (2019) included studies published up to April 2018. They evaluated the efficacy of different air cleaning technologies at reducing or removing indoor air pollutants and assessed any improvements in health. They considered a range of environments including homes, schools, offices, and transport. Their main conclusion was that they recommended further research to establish whether there are any links between air purification and improved health. They also predicted that there is likely to be increased demand for these technologies in the future.
The aim of the current work was to systematically review the current epidemiological evidence on the impact of PAPs using filtration technology on PM2.5 concentrations in the indoor environments (homes, schools, offices) and health. The focus was on PM2.5, due to its wide ranging and prevalence of sources, large scale contribution to overall pollution levels and significant known health implications.
Section snippets
Search strategy
The systematic review was conducted following the Preferred Reporting and Items for Systematic Review and Meta-Analysis (PRISMA) guidelines for reporting (Liberati et al., 2009).
Two literature searches were conducted in March 2020 using the following electronic databases: BNI, CINAHL, EMBASE and Medline. The first search was to identify the change in PM2.5 concentrations in home, school and office environments with the use of PAPs; the second search was to evaluate the effect of exposure to
Air purifiers and PM2.5 concentrations
Table 1 summarises the studies that considered the impact of air purifiers on PM2.5 concentrations. It was a mixture of developed and developing countries with 12 of the studies conducted in China, 12 in USA with the remaining eight spread across Canada, Denmark and Mongolia. The countries have substantially different climates, meteorology, average outdoor air pollution levels and sources of emissions. Reductions in PM2.5 concentrations across all the studies and environments ranged from
Conclusions- recommendations
In this systematic review, we evaluated the evidence base surrounding the utilisation of PAPs using filtration technology and its impact on PM2.5 concentrations in the indoor environment and on health. All the included studies conducted across all populations in homes and schools found that PAPs reduced PM2.5 concentrations by between 22.6 and 92.0% in homes and 49% in the schools. This variability can be attributed to a variety of factors including: study design, duration of the intervention,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was partially funded by EIT Digital, for the QUASIMODO Project (Quality of Indoor Air on Sites Matched with Outdoor Data-sets to Improve Wellbeing Outcomes).
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Contributed equally to the publication.