Nothing to do with humidity Kev. It negatively charges airborne particles with a high voltage (low amps). Its like the effect you get when you've been indoors for sometime then suddenly go outside on a fresh day. Lot of negative ions outside - mostly positive inside. Very pleasant to sit near one.
The two probes have tiny carbon filaments on them. Quite weird as it feels like its blowing air at you if you bring your hand near them. I had one many years ago and loved it. Perfect for anyone that suffers from hayfever too. In a camper, particularly when you're camped for the night and its all warm, it does get stuffy. Works even better if you blow a small PC fan across it.
I've got the 12v one in the Smart (comes on with ignition) and one at home thats on 24/7. You do notice it when you enter the room.
Theres a lot of rubbish spouted about negative ions (mostly sales pitch) but it does reduce airborne dust/pollen etc:
https://www.youtube.com/watch?v=toHa16cDeIg" onclick="window.open(this.href);return false; - breathing in a sandalwood jostick in a confined box isn't exactly the same
