Air purifiers did not protect against respiratory infections
In Russia, as in many other countries, the season of respiratory infections has begun. Many people use air purifiers as a preventive measure. Especially their popularity increased after the coronavirus pandemic. However, scientists have not found evidence of the effectiveness of such devices.
Installing technologies to remove or deactivate respiratory pathogens from indoor air is one of the most common strategies for non-pharmaceutical control of infectious diseases. There are two main types of air purification devices: filters and decontaminators. Filters remove particles from the air that may contain viruses. Air disinfectors use ultraviolet light or ozone to inactivate pathogens.
During the Covid-19 pandemic, such devices were advertised as a practical mitigation measure in settings where social distance was difficult to maintain. Governments around the world considered installing purifiers in many institutions, especially schools. These aspirations were hampered by high costs and uncertainty about which devices might actually be effective. Cluster randomized controlled trials were then initiated to provide possible evidence of effectiveness in schools and nursing homes, but results are not yet available.
Recently, researchers from Norwich Medical School (UK) examined 32 clinical and cohort studies on this topic conducted between 1970 and 2022. The results are published in the journal Preventive Medicine.
The authors looked for data on infections and disease outcomes in people who spent at least 20 hours a week in rooms with air purifiers. But they could not find evidence that these technologies could clear the air of respiratory or gastrointestinal pathogens or reduce their ability to penetrate human cells.
"Although the number of environmental and surface samples decreases after air treatment, especially bactericidal exposure and high-efficiency particulate filtration, strong evidence of efficacy for the risk of infection in real-world settings has yet to emerge," the researchers reported.
When looking at laboratory-confirmed cases of influenza or norovirus infection, there was a trend toward fewer infections. However, these data seemed biased to the authors.
Scientists have not been able to study the effectiveness of purifiers on coronavirus: there are no available studies on this topic yet.
Nevertheless, in July this year, German scientists presented the results of a study in which they examined the effect of high-efficiency particulate air (HEPA) filters on coronavirus in kindergartens. The researchers compared the incidence of the disease in schools where the new filters worked and those that didn't. It turned out there was no significant difference between the two. Infection rates were even found to be slightly higher among children in schools with filters.
Another method of non-pharmaceutical control of respiratory diseases is considered to be the maintenance of high absolute (not relative) air humidity. However, work on it has so far been done only for guinea pigs, and there are no studies on humans. Nevertheless, a number of studies suggest that, at least for influenza, maintaining at least 40% humidity in a warm room may indeed reduce the likelihood of infection.