The COVID-19 pandemic prompted widespread concerns about indoor air quality and its potential impact on virus transmission. While claims were made regarding the efficacy of air treatment devices in reducing the spread of respiratory infections, real-world evidence supporting these assertions has been lacking.
In a review conducted by researchers, including the author, a comprehensive examination of the evidence pre-dating COVID-19 revealed that air treatment technologies, including filters and air disinfectors, did not significantly reduce the frequency or severity of respiratory illnesses. Two primary types of air treatment devices were considered: filters, which remove particles containing infectious viruses, and air disinfectors, which use ultraviolet radiation or ozone to inactivate viruses in the air.
The systematic review encompassed 32 observational and experimental studies conducted between 1970 and 2022. While there appeared to be a trend towards fewer laboratory-confirmed influenza or norovirus infections, the findings were tempered by evidence of strong publication bias, wherein studies with positive results were more likely to be published than those with negative results.Â
Publication bias can skew the perceived impact of interventions, making them seem more effective than they actually are. Consequently, the review concluded that there is no robust evidence supporting the notion that air treatment technologies effectively reduce the risk of respiratory-transmitted illnesses. It is essential to note that none of the studies directly focused on COVID-19, as they were conducted before the emergence of the virus.Â
A more recent German study, published in July, specifically investigated the impact of high-efficiency particulate air (HEPA) filters on COVID-19 in kindergartens. The study compared illness rates in schools with newly installed filters to those without. Surprisingly, there was no significant difference between the two groups, and infection rates were slightly higher in schools with the filters.Â
The review also highlighted the absence of consideration for ventilation in the analyzed air treatment studies. While ventilation, such as keeping windows open, is another aspect of indoor air quality, the available evidence on its impact was limited and of questionable quality. Researchers acknowledged the need for further investigation to establish the efficacy of ventilation in reducing the risk of respiratory infections.Â
The majority of studies (n = 28) did not provide information regarding the costs associated with the technology under investigation. Ultraviolet Light (GUVL) as a “relatively low-cost intervention.” In a more detailed breakdown, specified that installing GUVL in an office building with 1000 staff would entail approximately $52,000 in initial costs and about $14,000 in annual running expenses (covering electricity and replacement bulbs). This translated to an investment cost of $52 and annual running costs of $14 per employee.Â
Regarding High-Efficiency Particulate Air (HEPA) filtration, Salam et al. (2010), focusing on devices used in private homes, mentioned that two portable HEPA filtration units were priced at approximately $900 each, with annual running costs around $500. In a different context, Butz et al. (2011), who studied devices employed in hospital rooms, estimated likely costs to range from $200 to $400 per installed unit.
Sustainability and maintenance issues related to device operation were often addressed through citations of other documents by the authors, noting that HEPA device filters typically required replacement once a year. A document reported a change in HEPA filters during the intervention period after 12 weeks.Â
Despite the lack of concrete evidence supporting the effectiveness of air treatment technologies, questions arise as to why these methods may not be the panacea that some initially claimed. Several factors contribute to this skepticism. First, the transmission risk of respiratory viruses is heavily dependent on proximity to an infected person.
Close contact poses a higher risk, and early pandemic research demonstrated a significant drop in infection risk with increased distance from an infectious individual. The efficacy of air treatment in altering such close person-to-person transmission remains doubtful.Â
Second, even if air treatment were effective within a specific indoor space, individuals regularly move between different environments, where air treatment measures may not be in place. Consequently, protection within one setting does not extend to other environments, limiting the overall effectiveness of air treatment technologies.Â
Finally, the epidemic dynamics of infections with short-duration immunity, such as COVID-19, introduce complexities. Infections like COVID, characterized by the potential for multiple reinfections as immunity wanes, deviate from standard epidemic models.
The SEIRS (susceptible, exposed, infected, recovered, susceptible) model better captures the dynamics of infections with short-lived immunity. In this model, interventions like air filtration or mask-wearing become less effective as most infections become reinfections. The driving factor for infection rates shifts to the rate at which people lose their immunity.Â
The available real-world evidence challenges the efficacy of air treatment technologies in reducing the risk of respiratory infections, including COVID-19. While there is a slightly more favorable view toward increased ventilation, the evidence is far from conclusive. As the world continues to grapple with respiratory viruses, a nuanced understanding of the limitations and complexities of various interventions is essential for informed decision-making.Â
Journal Reference Â
Julii Brainard et al, Effectiveness of filtering or decontaminating air to reduce or prevent respiratory infections: A systematic review, Preventive Medicine (2023). DOI: 10.1016/j.ypmed.2023.107774Â
