Short answer
The air after rain is usually clean; pollen and dust are washed down by the rain. This is a good time to air the home. The problem is raindrops coming in while it rains.
The window filter and rain
The tight structure of the nanofiber membrane holds back direct raindrops. With the window ajar, no water gets in during light rain and air keeps flowing.
Made to measure: hook-and-loop or roll

