Most air cleaning technologies lack evidence of real-world effectiveness

A new study led by researchers from the University of Colorado Anschutz Medical Campus and the National Institute for Occupational Safety and Health (NIOSH) through the Centers for Disease Control and Prevention (CDC) finds that although many technologies claim to clean indoor air and prevent the spread of viruses like COVID-19 and the flu, most have not been tested on people and their potential risks are not yet fully understood.

Published today in the Annals of Internal Medicine, the research analyzed nearly 700 studies between 1929 and 2024 on engineering controls such as HEPA filters, UV light, ionizers and advanced ventilation systems designed to reduce infection transmission through indoor air. While these technologies are commonly found in homes, schools and public buildings, researchers found just 9% of studies examined whether they reduce illness in humans.

We were surprised to find that most of the research tested air cleaning devices in lab chambers, not in real-world settings where people live, work or go to school. We need stronger studies that look at actual health outcomes, whether people are actually exposed to fewer pathogens or get sick less often, and not just measurements of particles in the air."

Lisa Bero, PhD, professor of internal medicine at the University of Colorado School of Medicine and co-author of the study

Most studies in the review focused on indirect measurements such as tracer gases, dust particles or harmless microbes rather than actual viruses or bacteria that cause illness. Very few tracked whether people experienced fewer infections when using air-cleaning technologies.

"Many of these technologies look promising on paper but we just don't know if they work in the real world," said Amiran Baduashvili, MD, associate professor of medicine at the University of Colorado School of Medicine and the paper's first author. "People are buying and installing these systems in homes and schools hoping to protect themselves and their families but the science hasn't caught up to the marketing."

The study also raises concerns about potential health risks. Only a small number of papers examined harmful byproducts such as ozone, which can irritate the lungs and worsen respiratory conditions. Many air cleaning technologies, such as ionizers, plasma-based devices, and certain ultraviolet light systems, can produce ozone, yet few studies evaluate the safety of their long-term operation in homes and workplaces.

"Ozone and other chemicals created by some air-cleaning devices can actually harm the respiratory system, especially in children or people with chronic respiratory illnesses," said Louis Leslie, research services senior professional in the department of ophthalmology at the University of Colorado School of Medicine and co-author of the study.

While the risks vary depending on the type of technology and how it is used, the researchers stress that more attention should be paid to possible unintended consequences.

"It may be worth checking whether the manufacturer provides data on any potentially harmful emissions from a device and what can be done to minimize them," said Bero, who is also with the Colorado School of Public Health. "Being more aware of possible risks is an important part of making informed decisions especially as more people and organizations spend money on these technologies and bring them into clinics, schools and homes."

The researchers are calling for a new generation of studies that evaluate these technologies in real-world environments such as classrooms and hospitals and track actual infections rather than relying on indirect measures like air particle counts. They also emphasize the importance of assessing potential side effects, environmental impact, cost, and accessibility, including how feasible these solutions are across different types of settings.

In addition, they recommend the development of a standard set of health-related outcomes for future research. This would make findings more comparable and more useful for informing public health policy.

"Public health decisions should be based on solid, independent evidence," said Bero. "We're not saying these technologies don't work, we're saying we don't know enough yet. Some of these studies are funded by the companies that make the technologies being evaluated, which creates a conflict of interest. Until we do know more, the public deserves clear and transparent information."

For those buying an air purifier or installing a new ventilation system to reduce illness in homes, schools or workplaces, the researchers recommend choosing technologies that have been independently tested in real-world environments. They also advise avoiding products that produce harmful byproducts like ozone. Most importantly they emphasize that dependable practices such as improving ventilation, opening windows and regular cleaning remain effective ways to keep indoor spaces healthier.

"This study highlights the urgent need for better science to guide how we make our indoor environments safer especially as respiratory infections continue to pose a serious public health threat," said Bero.