The household appliance that promises to improve health may actually be harming you

Air purifiers – owned by one in four Americans – are rarely tested on humans and could actually do more harm than good, a new study suggests.  

Researchers from the University of Colorado Anschutz Medical Campus reviewed nearly 700 studies on air-cleaning technologies intended to clean indoor air and prevent viruses and other disease-causing pathogens from spreading.

They discovered that only eight percent of these studies actually tested the effectiveness of the devices on people, while over 90 percent tested the devices in unoccupied spaces.

When the devices were tested, the vast majority were lab-based using guinea pigs or mice or air samples were taken to determine whether the devices reduced the number of small particles or microbes in the air.

What’s more, the researchers found that some of these devices produce harmful byproducts like ozone or formaldehyde, that can lead to complications such as permanent lung damage, asthma and cancer.

Air purifiers make ozone either intentionally, in ozone generators designed to use ozone gas to neutralize pollutants, or unintentionally as a by-product. Formaldehyde can be a byproduct when the ozone or ionizers, reacts with other chemicals in the air.

Of 112 studies looking at air purifiers that used chemicals to kill microbes, only 14 of these tested for harmful byproducts. 

‘This is a stark contrast to pharmaceutical research, where safety testing is standard practice,’ the researchers state.  

Ozone is a lung irritant and not a healthy component of indoor air, so the California Air Resources Board advises against using ozone generators and recommends opting for purifiers with HEPA filters instead.

Meanwhile, ozone has been found to react with existing chemicals in the air to create additional toxic pollutants, most notably formaldehyde – a known carcinogen – and ultrafine particles (UFPs).

UFPs are microscopic airborne pollutants that can deeply penetrate the lungs, enter the bloodstream, and travel to all organs, leading to inflammation, cardiovascular diseases, and increased mortality. 

Formaldehyde is a toxic gas found indoors and outdoors that causes irritation of the eyes, nose, and throat, coughing, and wheezing at higher concentrations. 

The health effects of formaldehyde can be worsened by ultrafine particles, which can carry the chemical and other toxic compounds deeper into the body.

The University of Colorado researchers worked with colleagues across three academic institutions and two government science agencies to analyze every research study evaluating the effectiveness of air purifiers published from the 1920s through 2023. 

This totaled 672 investigations.

They said they found ‘substantial variation across different technologies’. 

For example, 44 studies examined an air cleaning process called photocatalytic oxidation, which produces chemicals that kill microbes, but only one of those tested whether the technology prevented infections in people. 

Another 35 studies evaluated plasma-based technologies for killing microbes, and none involved human participants. 

They also found 43 studies on filters incorporating ‘nanomaterials’ designed to both capture and kill microbes – again, none included human testing.

Off the back of their findings, the researchers suggest that consumers should proceed with caution when investing in air cleaning devices.

Lead researchers, Amiran Baduashvili and Lisa Bero, conclude: ‘The gap between marketing claims and evidence of effectiveness [of air purifiers] might not be surprising, but there is more at stake here. 

‘The safety of these products should be the baseline requirement before they are widely deployed.

‘The idea is that cleaner air should mean lower chances of infection. But when it comes to air cleaning, researchers don’t yet know how strongly these air measurements reflect actual reduction in infections for people.

‘More real-world, human-centered testing is needed to identify which devices are both effective and safe for schools, hospitals, and workplaces.’

The US air purifier market size was estimated at $4.5 billion in 2024 and is projected to grow 7.2 percent from 2025 to 2030. 

Analysts from Grand View Research say the rising demand for air purifiers in the US can be attributed to the ‘rising health awareness and the increasing importance of indoor air quality in residential, commercial, and industrial facilities’. 

The Covid-19 pandemic also caused a surge in air purifier demand as consumers looked to protect indoor air quality against airborne viruses.