Author: Becky Ham, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology November 12, 2021

A comparison of four oxidation-based cleaning agents shows that these devices produce their own pollutants, and the effects are different.

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, 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 should remove VOCs only plays a minor role in the operation of cleaners, and most of the work is done by physically removing contaminants through cleaner absorbents or filters. In some cases, chemical reactions can produce by-products, such as formaldehyde, which can increase 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 removal of VOCs 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.

"During the pandemic, air purifiers become like mushrooms after a few days of rain. Sadly, some of them introduce chemicals into the indoor air. These chemicals are more harmful than the chemicals they might remove. People are worried," said expert Charles Weschler about indoor pollution at Rutgers University and the Technical University of Denmark. He is not the author of the MIT study. "The paper by Jesse Kroll and colleagues is a good proof of this fact. It was executed carefully and the results were presented clearly and thoughtfully."

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 adsorbents 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 oxidize VOCs are actually producing these harmful by-products."

The researchers pointed out that these products are not regulated and there is little data on their VOC removal rate. 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 how quickly they cleaned the air with elevated concentrations of two volatile organic compounds entering 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, which is the gas released by the VOC."

In the case that the detergent oxidation does degrade the introduced VOC, the process will also produce 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.

Kroll added that for consumers who are looking for ways to remove VOCs in their homes and offices, “using activated carbon filters to clean the air, which is a tried-and-tested technology that does not rely on chemical reactions, is still a viable method. "

Reference: "Real-time laboratory measurement of VOC emission, removal rate and by-product formation of consumer-grade oxidizing air purifiers", Authors: Qing Ye, Jordan E. Krechmer, Joshua D. Shutter, Victoria P. Barber, Yaowei Li, Erik Helstrom, Lesly J. Franco, Joshua L. Cox, Amy IH Hrdina, Matthew B. Goss, Nadia Tahsini, Manjula Canagaratna, Frank N. Keutsch, and Jesse H. Kroll, October 27, 2021, Environmental Science and Technology Letters. DOI: 10.1021/acs.estlett.1c00773

Ye Qing, a postdoctoral fellow at MIT, 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.

It is helpful to name the product so that I can know if my product is good or bad.

Guess they are not so confident in their research because they don't have the courage to name the product.

Yes, this will be more helpful because I recently purchased 4 air purifiers, 3 Levoit and 1 Germ Gaurdian.

They worry about responsibility and have no confidence in their work, so they didn't name the product. Worthless

When research is bad for their products, researchers are often sued by manufacturers. This may be the reason why the product name is not mentioned.

I also hope to identify air purifiers that score high in this study.

I am currently buying these in the market, can I find the detailed information of the research, namely the model and its efficacy?

The carbon filter has been tried and tested. This is what I read.

You don’t necessarily need to know the name of the product. If it uses an activated carbon filter, then you are fine. If it uses anything else, it may not be good.

Does my air purifier claim to remove VOC? I bought it to remove dust, dander, cat hair, etc. In other words, Hepa-level particles...and judge that my filter works well.

Thanks! It's worth considering, even if you don't know the specific name, because I believe this applies to many brands that were not included in the study.

No need to name a brand... If your air purifier/purifier has an ionizer or other form of "oxidation" mechanism, it may add toxins to your indoor air. For this reason, I have three that use only carbon filters. The important thing is the purification method.

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