UPDATE: Spring semester 2021 I taught Air Pollution Control Engineering at CU Boulder in the Mechanical Engineering Department. I love teaching this class. The graduate students in this class do a research project and write a report. One of my students made a Corsi/Rosenthal DIY Air Cleaner and tested it in her apartment. She agreed to share her report and presentation in case it is of interest and helpful. It was a well-done student project!

UPDATE: I gave a seminar on Oct 29 2021 for the International Society of Indoor Air Quality and Climate (ISIAQ) on air cleaners with some updates and discussion of recent studies that had come out over the past year. My slides are available for review and you can watch the webinar if you join ISIAQ or are a member.

UPDATE: an air cleaner sizing tool (downloadable excel spreadsheet) is now available to size portable air cleaners for use in a school classroom, developed by Prof Joe Allen, Dr. Laurent, with input from me. And here is a 5-min video about air cleaners and how to use the air cleaner sizing tool.

Air cleaners (please don’t call them air purifiers, we can’t ever purify the air – what does that even mean…?) are a hot topic right now, due to the potential to be exposed to the SARS-CoV-2 virus by breathing contaminated air. The risk of getting infected is higher in buildings, in situations that are crowded with people that you do not know and that may be infectious, when there is loud talking and vocalizing, and when you are exposed for a longer period of time to higher concentrations, which can happen indoors in spaces without enough ventilation (outside air).

Air cleaners are helpful as an added supplement in buildings that are unable to provide high enough clean (virus-free) air to maintain low aerosol concentrations, which is typically accomplished by supplying some combination of outside air and high-efficiency-particle filtered air. It is best to learn a little about air cleaners before you go out and purchase one (you could but you might be wasting your money). You could end up spending hundreds of dollars on an air cleaner that moves little or no air and generates ozone. Like what happened here with the Sharper Image’s Ionic Breeze (we also tested this air cleaner in this paper and found it did nothing really).

There are a ton of great articles on air cleaners in the peer-reviewed scientific literature. and the work on air cleaners goes way back to the 1990’s at least. This paper was one of my first published, while I was doing my PhD at Berkeley in Civil and Environmental Engineering, and was published in 1996 on air cleaners. One thing I like about this paper is it shows that a 95% efficient fibrous filter insert is just as effective as a HEPA filter insert because an air cleaner works by recirculating the room air though the cleaner many times.

You really do need to educate yourself a bit before you purchase an air cleaner. You have to get one that is efficient, that doesn’t pollute the air with added toxic chemicals (like ozone), and one that moves enough air to clean the volume of the room where you are using it effectively. Because my university asked me to write up a summary of air cleaners and how to purchase them for use in spaces that need supplemental air cleaning during the COVID-19 pandemic, I asked one of my graduate students to write a paper on the topic. The student was taking my class May 2020 on air pollution control engineering remotely. This Air cleaner report is intended to be helpful for others who are trying to get learn more about air cleaners and may want to purchase one of them. It has been reviewed by me and a colleague also an expert on indoor air quality. We provide examples of the kinds of air cleaners that can be purchased and the important parameters to use to make your purchase, but do not endorse any one air cleaner and including those air cleaners in this report are not an endorsement of that manufacturer or air cleaner.

In the report we reference the Association of Home Appliance Manufacturers. They have set up a testing and certification program for air cleaners that can be found here: AHAM Verified. The US EPA also has very good information on air cleaners. The California Air Resources Board has a list of air cleaners that have been certified for sale in CA.