“We should get an air filter but can’t do now. So we’re breathing smoke...”
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Get any decent size square or rectangular air filter you can find, a box window fan, and a roll of duct tape.
Make one.
~Easy
Yes, the furnace filter taped to the box fan is fairly good. Low speed only, I would say. Also, like any filter, you’ll need one that is rated for smoke. generally, the smaller the particles that it filters, for this purpose, the better. But also the more expensive.