As winter deepens and pollution spikes, many households across cities like Delhi or Mumbai place faith in air purifiers as a shield against dust, smoke, and harmful particles. In bedrooms, living rooms, and even kitchens, these devices hum away for hours. Some people keep them running almost continuously, assuming the cleaner, the better. But could running an air purifier nonstop be harmful? Might it over-clean the air, dry it out, or create new problems?
We turned to science for answers.
What exactly is an air purifier?
Air purifiers pull dirty indoor air through one or more filters before pushing clean air out. Most home purifiers use HEPA filters, engineered to trap extremely small particles - smoke, dust, pollen, microbes, and traffic pollution.
But are they effective when used continuously? And can too much purification go wrong?
What do studies show?
A closer look at scientific research reveals a far more complicated picture, one where some devices can help, some can harm, and others require much more evidence before they can be called safe.
The California Air Resources Board (CARB) makes one of the strongest warnings. “Not all air-cleaning devices are appropriate for home use; some are known to be harmful to human and animal health,” the agency notes.
The biggest concern, according to the agency, is ozone-generating air cleaners, devices that intentionally create ozone as a by-product of purification.
“Ozone generators, which are portable or in-duct air cleaners that intentionally create ozone, can produce levels that are much higher than health-based standards,” the CARB says. The agency adds that “there are companies that try to sell the idea that breathing ozone is actually healthy, but there are 40 years of scientific studies that show even relatively low concentrations of ozone are harmful to biological tissue in plants and animals, including humans.”
The effects are not mild. CARB explains that “it is well-documented that ozone can cause respiratory tract irritation and inflammation, serious breathing difficulty including asthma, permanent lung damage, and cardiovascular effects.” As the American Lung Association concluded, breathing even low levels of ozone increases the risk of premature death.
Because of these dangers, California strictly regulates air cleaners. CARB states: “In 2008, CARB enacted an air cleaner regulation to limit the ozone emissions from indoor air cleaning devices. The regulation requires all indoor air cleaners sold in California to be certified by CARB, including air cleaners sold online.” Even then, the agency warns that ozone-generating devices should not be used at home: “CARB strongly advises against the use of ozone generators at home because of the risk to you and your family's health from exposure to ozone.”
However, this is a concern that can easily be addressed. Air purifiers with HEPA filters use a “mechanical” filtration process and do not produce Ozone.
But ozone generators are only one category of devices under scrutiny. A growing body of research is examining ionization-based purifiers- machines that release negative ions to clump particles together and remove them from the air. These devices promise low noise and energy efficiency. However, as one scientific team in Beijing notes, “its health effects remain unclear.”
In a randomised, double-blind crossover study involving school children, researchers found both benefits and possible risks. The study reports that “ionization air purifiers could reduce indoor particulate pollution substantially,” noting that PM2.5, PM10, PM0.5 and black carbon dropped by “48%, 44%, 34% and 50%, respectively.” The purification also yielded measurable improvements in breathing: “Real purification was associated with a 4.4% increase in forced exhaled volume in 1 s (FEV1) and a 14.7% decrease in fractional exhaled nitrogen oxide (FeNO).”
But the same study cautions that not all effects were positive, finding that “ionization air purifiers might have a negative effect on cardiac autonomic function.” In particular, “heart rate variability (HRV) was altered negatively,” and the impact was greater when negative ion concentrations were high. The authors conclude that “an ionization air purifier could bring substantial respiratory benefits; however, the potential negative effects on HRV need further investigation.”
A third study, published in the International Journal of Environmental Research and Public Health, tries to understand why negative ions may have mixed effects. The researchers used “a full-scale experiment using an in vitro airway model connected to a breathing simulator to mimic inhalation,” replicating airflow inside human airways. Their findings show that negative ion purifiers do change how particles behave. As they explain, “NIAP effectively reduced the small airway exposure dose by 20.3% compared to the filtration purifier with a similar CADR.” The study adds that “increasing the negative ion concentration helped reduce the small airway exposure risk.”
However, the researchers also found differences in particle size and distribution. They write that the ionizer “reduced the dose delivered to the small airway by coagulating fine particles into coarse particles,” while the filtration purifier, by contrast, produced “smaller” particles that “reached the small airway” more easily. The authors emphasize that while NIAPs “are an energy-efficient air purification intervention,” the way they change particle characteristics may connect to the physiological effects seen in the second study—particularly the unexpected cardiac responses.
Together, these three studies offer a nuanced picture. Ozone-generating air purifiers are clearly harmful and should not be used at home under any circumstances. Ionization-based purifiers appear to improve certain respiratory measures and reduce particulate matter, but they also raise new questions about cardiac impacts and long-term safety. Even devices that reduce airborne particles may not be fully safe if they introduce other physiological risks.
This story is done in collaboration with First Check, which is the health journalism vertical of DataLEADS.
