Air purifiers: Most tests are insufficient to assess their effectiveness or safety

When it comes to protecting against infections, the actual effectiveness of portable air purifiers is rarely properly assessed. Furthermore, only a very small number of studies have assessed the potentially harmful effects that could result from the production of various toxic compounds by these devices. This is the conclusion of a detailed review of the scientific literature, conducted on nearly 700 studies, whose authors present the findings here.

Many respiratory viruses, such as the Sars-CoV-2 coronavirus responsible for the Covid-19 pandemic or the influenza virus, can spread through indoor air. Various so-called "infection control engineering" technologies aim to prevent the spread of viruses and other pathogens by cleaning indoor air, for example, using HEPA (High Efficiency Particulate Air) filters, ultraviolet light, or specific types of ventilation.

Together with our colleagues, who work at three academic institutions and two government scientific agencies, we identified and analyzed all studies aimed at evaluating the effectiveness of such technologies published between 1920 and 2023, representing 672 scientific articles.

To measure the performance of the tested devices in terms of air sanitation, three main approaches stand out:

• their ability to reduce infections in humans;
• their ability to protect laboratory animals, such as guinea pigs or mice;
• their ability to reduce the number of fine particles or suspended microorganisms.

Of all these studies, only about 8% of the studies used the first approach (testing effectiveness in humans), while more than 90% of the authors were content to evaluate sanitation devices in unoccupied places.

Thus, of the 44 studies that aimed to measure the effectiveness of photocatalytic oxidation (an air purification process that produces chemicals capable of killing microbes), only one examined the ability of this technology to actually prevent infections in humans. Other studies (35 in total) evaluated technologies based on the use of plasmas to destroy microbes, but none of them included human participants.

Finally, of the 43 studies on filters incorporating nanomaterials designed to both capture and destroy the microbes we identified, none have been tested in humans.

The Covid-19 pandemic, which has claimed millions of lives worldwide, strained healthcare systems, and led to the closure of countless schools and workplaces, has reminded us of how devastating the consequences of airborne infections can be. At the time, as early studies established that the Sars-CoV-2 coronavirus spread through the air , improving indoor air quality became a key focus in the fight against the pandemic .

The challenge remains: developing effective technologies to eliminate microorganisms from indoor air could not only have considerable public health benefits, but also help limit the economic damage that future pandemics could cause . And this would be possible without requiring any special effort from those protected, since such infection control engineering measures would operate in the background of their daily lives, without their awareness.

Manufacturers of portable air purifiers incorporating technologies that supposedly destroy microorganisms have understood this well. They do not hesitate to make ambitious claims about the ability of the products they market to purify the air and prevent infections, whether they are intended for the general public or for use in daycares, schools, clinics, or workplaces.

However, we found that most of these devices have not been properly tested for effectiveness. Without solid scientific results from human studies, it is therefore impossible to know if these marketing claims correspond to reality.

Our results suggest that consumers should exercise caution before investing in such air purification devices ( in France, in its expert report "Identification and analysis of different emerging indoor air purification techniques", published in 2017, the French National Agency for Food, Environmental and Occupational Health Safety noted that "the technologies implemented were often poorly described on the purification devices and that the claimed effectiveness was poorly justified", Editor's note).

The gap between commercial promises and actual evidence of effectiveness is hardly surprising, but the issue extends beyond marketing claims alone. Indeed, to destroy microbes, some of these air purification technologies generate chemicals such as ozone, formaldehyde ( classified as carcinogenic and mutagenic, editor's note) or hydroxyl radicals. All of these compounds can potentially harm health when inhaled .

Therefore, ensuring the safety of these devices should be a prerequisite for any large-scale deployment. However, of the 112 studies we analyzed that evaluated some of the currently commercialized microbicide technologies, only 14 investigated the presence of harmful by-products in the air treated by the devices tested.

The contrast is striking with the safety standards to which pharmaceutical research is subject.

More than 90% of the studies we reviewed evaluated air purification technologies by looking at the air itself – for example, by analyzing how well they were able to remove dust particles, microbes or certain gases.

The working hypothesis behind this approach is that breathing cleaner air should translate into a lower risk of infection. However, it is not known whether these atmospheric measures actually translate into an effective reduction in infections among people living in environments treated in this way, nor, if such a reduction exists, what its magnitude is.

Ultimately, identifying the safest and most effective air purification technologies will require further research to detect potential toxic by-products and to evaluate these technologies in real-world settings (i.e., in environments where humans live).

In addition, it will be necessary to standardize measurement protocols. Only then can we hope to collect evidence-based data that will enable us to make the right decisions to improve air quality in homes, schools, healthcare facilities, etc.