[nether]

The downside is that these filters sell out in places where there are fires. So the time to buy them is now.

[rootusrootus]

I agree. When we had wildfire smoke descend on our area, the stores sold out within hours. If you live in an area which routinely experiences wildfire smoke, it's worth stocking a few filters, both 20x20x4 for box fans and whatever size your furnace uses. It made a huge difference to have fresh filters to swap out every few days.