It's made from wood. I'm not afraid of moisture, because the temp inside the case is always about 15 °C higher than the outside temperature.

I was afraid, though, that little droplets might get sucked in when there's heavy rain and winds, so I put 3 layers of insect-stopping gaze at the air-intake (bottom), spaced about 1 cm apart. It survived a couple of windy nights with heavy rain, so I'm not worried any more.

Also: there's very little dust buildup. I live in a city, but not near a big road or anything that has high traffic volume (so no diesel shit). This surprised me, but it makes sense: of course the inside of a room with people and carpets and all kinds of shit in it will have more dust than the outside.

In spring some very fine pollen got inside the case, don't know if that does any harm, guess not.