I've been running one of these for a few months. The filter has slowly turned darker brown - which suggests it's working (not anywhere near forest fires here!).

I've been running one of these for a few months. The filter has slowly turned darker brown - which suggests it's working (not anywhere near forest fires here!). It has a trickle mode which is completely silent and uses about 1W of power. Something you won't get on a DIY fan.