These are $38 on Banggood. It measures temperature, humidity and airborne particulates. Laser light scattering technique, the same as in my more expensive Purple Air I wrote about previously. These are display only, two screens, no connectivity but perhaps that’s all you need for something you place on a shelf.
Thanks to my friend Roger for finding these. I bought two and loaned one to our neighbor who promptly used it to check in vs out and verified his AC filters.
Anyway, it’s well made and cheap, so if you want one I say get it.