According to a new study led by MIT researchers, consumer-grade air purifiers that promise to use chemical oxidation to reduce indoor volatile organic compound (VOC) pollutant levels may themselves be a source of VOC.

In addition, Jesse Kroll, a professor of civil and environmental engineering and chemical engineering, and his colleagues found that the VOC removal effects of the four products examined in the study vary greatly. 

The chemical reaction that is supposed to remove VOC plays only a minor role in the cleaner's operation, and most of the work is done by physically removing contaminants through the absorbent or filter of the cleaner. In some cases, chemical reactions can produce by-products, such as formaldehyde, which increases overall pollutant levels.

"This work shows that, at least for some consumer-grade portable air purifiers that claim to remove VOCs from indoor air, the VOC removal may actually be minimal, and the delivered air may contain additional VOCs and/or oxidation. By-products, some of which are known to be harmful to human health," the researchers wrote in the journal Environmental Science and Technology Letters. The popularity of indoor air purifiers has skyrocketed in the past year because most cleaners advertise their ability to remove particles, including those that contain exhaled viruses (such as SARS-CoV-2). Researchers at the Massachusetts Institute of Technology did not test the effectiveness of the cleaner in their study to remove any kind of particles from indoor air.

Expert Charles Weschler said: "During the pandemic, after a few days of rain, air purifiers look like mushrooms. Sadly, some of these air purifiers introduce chemicals into the indoor air. These chemicals are more worrying than the chemicals they may remove." He is not the author of the MIT study on indoor pollution at Rutgers University and the Technical University of Denmark. "The paper by Jesse Kroll and colleagues is a good proof of this fact. It was executed carefully, and the results were obvious and well thought out."

Thousands of household products emit VOCs, including paints, solvents, glues, cleaning supplies, pesticides, and various cooking and cleaning activities. They are an important source of indoor air pollution, and repeated exposure to certain VOCs can cause long-term health problems such as cancer or lung, liver or kidney damage. Most consumer-grade air purifiers contain filters or adsorbent materials that can physically capture VOCs, but some products also provide chemical methods to destroy VOCs, such as photocatalytic oxidation or ionization using ultraviolet light, plasma technology, or carbon dioxide titanium filters. "The oxidation of volatile organic compounds is responsible for many important pollutants in our atmosphere, such as ground-level ozone or secondary fine particulate matter," Kroll explained. “Therefore, there is a concern in the atmospheric chemistry community that some of these cleaners that claim to be able to oxidize VOCs are actually producing these harmful by-products.” The researchers pointed out that these products are not regulated, with regard to their VOC removal rate. The data is scarce. Kroll and his colleagues measure the oxidation products that naturally form in outdoor air, "so we want to apply the same technology to indoor cases because we have the ability," he said. Scientists purchased four consumer-grade air purifiers, with prices ranging from US$65 to US$400, advertising various physical and chemical cleaning technologies. They placed these cleaners in a controlled air chamber to observe the speed at which they cleaned the air with increasing concentrations of two volatile organic compounds introduced into the air chamber. Volatile organic compounds include the relatively unreactive volatile organic compound toluene (usually related to the smell of paint thinner) and a more reactive volatile organic compound called limonene, which gives some cleaning products a citrus scent.

After running indoors for 60 to 90 minutes, only two cleaners removed two VOCs, while the other cleaners only removed limonene. The research team found that the speed at which the machine cleans the amount of VOC air varies greatly. "The range of effects is so wide that some cleaners can't remove toluene at all," Kroll points out. Further experiments confirmed that of the two cleaning agents that do the best in removing VOC, the physical or adsorbent filter successfully removes most of them, and the oxidation effect is small or negligible. When they operate indoors, the cleaner itself generates additional VOC in two ways. Researchers have detected hundreds of compounds, including formaldehyde and acetone, which are slowly "vented" by the device. "We probably shouldn't be so surprised," Kroll said. "Because for all consumer electronic products, you take them out of the box, tear off the plastic, and then emit an odor. This is the gas released by the VOC." In the case that the detergent oxidation does degrade the introduced VOC, The process also produces hundreds of by-products, including formaldehyde and other partially oxidized VOCs. He added that in order to better understand the extent to which the emission rate of cleaners can cause poor air quality or health problems, “people really need to put it into a larger indoor air model...volume , Airflow, and all VOC sources.” Kroll pointed out that over time, the passive VOC generated by cleaners may decrease. The by-products produced by running machines are even more disturbing, because these by-products may continue to form throughout the life cycle of the cleaner. "But fortunately, because some cleaners don't seem to oxidize VOCs as advertised, they don't produce as many by-products. Unfortunately, this also means they don't work well," he said. For consumers who are looking for ways to remove VOCs in their homes and offices, Kroll added, “Use activated carbon filters to clean the air. This is a reliable technology that does not rely on chemical reactions and is still a viable method.” MIT Qingye, a postdoctoral fellow, is the first author of the paper. Co-authors include MIT postdoctoral fellows Victoria P. Barber and Amy IH 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; Aerodyne Research chief scientists Jordan E. Krechmer and Manjula Canagaratna. The research was funded by the Alfred P. Sloan Foundation and the National Science Foundation.

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