Study: Indoor air purifiers fail to remove volatile organic compounds | MIT News


Consumer grade air purifiers that promise to reduce indoor levels of volatile organic compound (VOC) pollutants using chemical oxidation may themselves be a source of VOCs, new study by MIT researchers finds .

In addition, the effectiveness of VOC removal varied considerably between the four products examined in the study, found civil and environmental and chemical engineering professor Jesse Kroll and colleagues.

Chemical reactions that were supposed to remove VOCs played a minor role in cleaner operations, with the physical removal of pollutants by cleaner absorbents or filters doing most of the work. In some cases, chemical reactions have led to byproducts, such as formaldehyde, which increased the overall level of pollutants.

“This work shows that for at least some consumer grade portable air purifiers that claim to remove VOCs from indoor air, the removal of VOCs may in fact be minimal, and the supplied air may contain VOCs. additional and / or oxidation by-products, some of which are known to be harmful to human health ”, the researchers write in the review Letters on environmental sciences and technologies.

The popularity of indoor air purifiers has skyrocketed over the past year as most cleaners advertise the ability to remove particles, including those that contain exhaled viruses such as SARS-CoV-2. The MIT researchers did not test how well the cleaners in their study removed particles of all kinds from indoor air.

“During the pandemic, air purifiers have popped up like mushrooms after days of rain, and unfortunately some of these air purifiers can introduce chemicals into indoor air that are of greater concern than chemicals that are ‘they could eliminate,’ says Charles Weschler, an expert on indoor pollution at Rutgers University and the Technical University of Denmark, who was not the author of the MIT study. “The article by Jesse Kroll and his colleagues is an excellent demonstration of this. It is carefully executed and the results are clearly and thoughtfully presented.”

Test the products

VOCs are emitted by thousands of household products including paints, solvents, glues, cleaning products, pesticides, and various cooking and cleaning activities. They are a major source of indoor air pollution, and repeated exposure to certain VOCs can cause long-term health problems such as cancer or damage to the lungs, liver or kidneys.

Most consumer air purifiers contain filters or absorbent materials capable of physically trapping VOCs, but some products also offer chemical methods of destroying VOCs, such as photocatalytic oxidation or ionization using ultraviolet light. , plasma technology or carbon-titanium dioxide filters.

“The oxidation of VOCs is what leads to many important pollutants in our atmosphere, such as ground-level ozone or secondary fine particles,” says Kroll. “So there’s this concern in the atmospheric chemistry community that maybe some of these cleaners that claim to oxidize VOCs are actually generating these harmful byproducts.”

The products are unregulated and there is little data on their VOC removal rates, the researchers note. Kroll and his colleagues measure the oxidation products that form naturally in outdoor air. “So we wanted to bring the same technology to apply to the inner case, because we have the capability,” he says.

Scientists bought four consumer grade air purifiers, priced from $ 65 to $ 400, which advertised a variety of physical and chemical cleaning technologies. They placed these cleaners in a controlled air chamber to observe how quickly they cleaned the air of the high concentrations of two VOCs introduced into the chamber. VOCs included the relatively unreactive toluene VOC (often associated with the smell of paint thinner) and a more reactive one called limonene which gives some cleaning products their citrus scent.

“Wide range” of efficiency

Only two of the cleaners removed both VOCs after 60-90 minutes of operation inside the chamber, while the others removed only the limonene. The research team found that the speed at which machines cleared the air volume of VOCs varied widely. “There was a wide range of effectiveness, with some cleaners virtually incapable of removing toluene,” notes Kroll.

Further experiments confirmed that in the two cleaners that were most successful in removing VOCs, it was the physical or absorbent filters that did most of the successful removal, with oxidation playing a small or negligible role.

When operating inside the chambers, the cleaners themselves produced additional VOCs in two ways. Researchers have detected hundreds of compounds, including formaldehyde and acetone, emitted by the slow “outgassing” of devices.

“We probably shouldn’t have been so surprised,” says Kroll. “

In cases where the oxidation by the cleaner degraded the introduced VOCs, the process also created hundreds of by-products, including formaldehyde and other partially oxidizing VOCs.

To get a better idea of ​​the extent to which cleaners’ emission rates would lead to poor air quality or health issues, he added, “you really would have to fit that into a bigger model. large indoor air… which implies a full house volume, air flow and all sources of VOCs.

Passive VOC production by cleaners is expected to decrease over time, notes Kroll. Of greater concern are by-products created by running machines, as these would likely continue to form throughout the life of the cleaners. “But luckily, because some of the cleaners don’t seem to oxidize VOCs as advertised, they don’t make as many byproducts. Unfortunately, that also means they just don’t work very well, ”he says.

For consumers looking for a way to remove VOCs from their homes and offices, adds Kroll, “cleaning the air using activated carbon filters, a proven technology that doesn’t rely on on chemical reactions, is still the way to go ”.

MIT postdoctoral fellow Qing Ye was the lead author of the article. Co-authors include MIT postdocs Victoria P. Barber and Amy IH Hrdina; MIT graduate students Erik Helstrom, Lesly J. Franco, Matthew B. Goss, and Nadia Tahsini; professor of chemistry and chemical biology at Harvard University Frank N. Keutsch; Harvard graduate students Joshua D. Shutter, Yaowei Li, and Joshua L. Cox; and Aerodyne Research principal scientists Jordan E. Krechmer and Manjula Canagaratna.

The research was funded by the Alfred P. Sloan Foundation and the United States National Science Foundation.

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