About a week ago, when I read wildfire smoke would soon sweep through Michigan, I went to our basement and brought up a box I hadn’t used in a while.
In summer 2023, when wildfire smoke moved through our southeast Michigan area, I built our own air purifier from a box fan, furnace filters, cardboard, and duct tape.
Known as a Corsi-Rosenthal box, my do-it-yourself air purifier is something anyone can build for less than $100 (US dollars) in materials.
It became popular during the COVID pandemic, to keep air clean in classrooms, homes, and other places.
What I liked: it worked better than portable air purifiers in cleaning air inside our house.
And it cost a lot less!
After that summer in 2023, I moved the Corsi-Rosenthal box to the basement and covered it to keep it clean and safe from any dust.
Last week, I was thankful for our homemade air purifier when I read nearby Detroit, Michigan had the worst air quality index (AQI) for major cities in the world, due to wildfire smoke.
For two consecutive days on July 16 and 17, 2026.
Our Corsi-Rosenthal box worked well on those days.
Overnight readings of bad air quality (based on Airnow) in our area ranged from 450-680, which is considered hazardous by Airnow.
On the first day of bad air quality, our community had air quality of 455 in the morning.
When air quality is hazardous, Airnow AQI Basics describes it as:
Health warning of emergency conditions: everyone is more likely to be affected.
Inside our home, our air quality monitor averaged below 50, which represents good air quality by Airnow.
For those two days of hazardous air quality, our homemade air purifier kept inside air clean throughout our home.
Outside, the sky looked gray, sometimes dull orange in color. The air was grayish, at times we could see plumes slowly moving through yards.
Few birds came to our bird feeders.
We saw little of our neighbors, other than when they would leave their garages in their vehicles.
After two days of hazardous air quaility, a cold front came through our area and moved out the wildfire smoke.
Air quality improved outside to the good rating and we saw many neighbors out and about in their yards.
We could see blue skies again!
I visited one of my favorite local locations where I met up with one of my birding friends. Of course, we talked about birds and wildfire smoke.
When they told me they’ve been trying to find air purifiers at local stores, but found they were all sold out, I asked if they were handy.
They replied they were.
I shared my photo with them of my Corsi-Rosenthal box and explained how they could make it themselves for a lot less than buying one.
And that they could make a big air purifier like mine or a smaller one with a nine-inch box fan.
They seemed interested!
