Consumer-grade air cleaners that promise to reduce indoor levels of volatile organic compound (VOC) pollutants using chemical oxidation can be a source of VOCs themselves, according to a new study led by MIT researchers.
What’s more, the effectiveness of VOC removal varied considerably among the four products examined in the study, professor of civil and environmental engineering and chemical engineering Jesse Kroll and colleagues found.
The chemical reactions that were supposed to remove VOCs played a minor role in the cleaners’ operations, with physical removal of the pollutants through the cleaner’s absorbents or filters doing most of the work. In some cases, the chemical reactions led to byproducts, such as formaldehyde, that added to the overall pollutant level.
“This work shows that, for at least some consumer-grade portable air cleaners that claim to remove VOCs from indoor air, VOC removal may actually be minimal, and the air delivered may contain additional VOCs and/or oxidation byproducts, some of which are known to be harmful to human health,” the researchers write in the journal Environmental Science and Technology Letters.
The popularity of indoor air cleaners has soared in 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 any kind from indoor air.
"During the pandemic, air cleaners have appeared like mushrooms after days of rain, and sadly, some of these air cleaners can introduce chemicals to indoor air that are of greater concern than the chemicals that they might remove,” says Charles Weschler, an expert on indoor pollution at Rutgers University and Technical University of Denmark, who was not an author of the MIT study. “The paper by Jesse Kroll and co-workers is an excellent demonstration of this fact. It is carefully executed, and the results are clearly and thoughtfully presented."
Testing the products
VOCs are emitted by thousands of household products, including paints, solvents, glues, cleaning supplies, pesticides, and a variety of cooking and cleaning activities. They are a significant source of indoor air pollution, and repeated exposure to some VOCs can cause long-term health problems such as cancer or lung, liver, or kidney damage.
Most consumer-grade air cleaners contain filters or sorbent materials that can physically trap VOCs, but some products also offer chemical methods of destroying VOCs, such photocatalytic oxidation or ionization using ultraviolet light, plasma technology, or carbon-titanium-dioxide filters.
“Oxidation of VOCs is what leads to a lot of important pollutants in our atmosphere, such as ground-level ozone or secondary fine particulate matter,” Kroll explains. “So there’s this concern in the atmospheric chemistry community that maybe some of these cleaners that claim to be oxidizing away the VOCs are actually generating these harmful byproducts.”
The products are not regulated, and there are few data on their VOC removal rates, the researchers note. Kroll and his colleagues measure oxidation products that form naturally in outdoor air, “so we wanted to bring the same technology to apply to the indoor case, since we have the capability,” he says.
The scientists bought four consumer-grade air cleaners, ranging in price from $65 to $400, that advertised a variety of physical and chemical cleaning technologies. They placed these cleaners in a controlled air chamber to observe the rate at which they cleaned the air of elevated concentrations of two VOCs introduced to the chamber. The VOCs included the relatively nonreactive VOC toluene (often associated with the smell of paint thinners) and a more reactive one called limonene that gives some cleaning products their citrus scent.
“Huge range” in efficacy
Only two of the cleaners removed both VOCs after 60 to 90 minutes running inside the chamber, while the others removed only limonene. The rate at which the machines cleaned the volume of air of the VOCs varied substantially, the research team found. “There was a huge range in efficacy, with some cleaners essentially unable to remove the toluene at all,” Kroll notes.
Further experiments confirmed that in the two cleaners that did the best at removing VOCs, it was the physical or sorbent filters that did the bulk of the successful removal, with oxidation playing a small or negligible role.
As they operated inside the chambers, the cleaners themselves produced extra VOCs in two ways. The researchers detected hundreds of compounds, including formaldehyde and acetone, emitted by slow “outgassing” of the devices.
“We probably shouldn’t have been that surprised,” Kroll says. “Because with all consumer electronics, you take them out of the box, rip off the plastic, and then there’s that smell, which is from the VOCs outgassing.”
In the cases where oxidation by the cleaner did degrade the introduced VOCs, the process also created hundreds of byproducts, including formaldehyde and other partially oxidizing VOCs.
To get a better idea of the extent to which the rates of emissions from the cleaners would lead to poor air quality or health problems, he added, “one would really need to put this into a larger model of indoor air … that involves full house volume, air flow, and all sources of VOCs.”
Passive VOC production by the cleaners is likely to lessen over time, Kroll notes. The byproducts created by the machines in operation are more troubling, since those would probably continue to be formed over the whole life of the cleaners. “But luckily, because some of the cleaners don’t appear to oxidize the VOCs away as advertised, they don’t make that many byproducts. Unfortunately, that also means that they just don’t work that well,” he says.
For consumers looking for a way to remove VOCs in their homes and offices, Kroll adds, “air cleaning using activated carbon filters, a tried-and-true technology that doesn't rely on chemical reactions, is still the way to go.”
MIT postdoc Qing Ye was the lead author on the paper. Co-authors include MIT postdocs Victoria P. Barber and Amy I. H. Hrdina; MIT graduate students Erik Helstrom, Lesly J. Franco, Matthew B. Goss, and Nadia Tahsini; Harvard University professor of chemistry and chemical biology 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 U.S. National Science Foundation.
