I wouldn't presume to speak for Doug, but I think either Chris or I can answer this from experience- they crack. Specifically, the expensive silver-laden Katadyn ceramic filter elements crack when they freeze. I think they would have to dry outside of the filter to prevent it- or maybe heat?<br><br>This might seem like a disadvantage, but I really don't think ANY sub-micron filter is going to like the formation of ice crystals one little bit- and others might fail in ways that won't let you know they're not working anymore. I'm not sure that's better.<br><br>I've always slept with water bottles in cold weather (foot of the bag), and I've included the filter (inside a plastic bag) when I was carrying one. I suppose in really cold weather, you're going to have to keep it inside your jacket somehow during the day. With my old (white!) Katadyn "Pocket" filter (way too big for a pocket) this would have been a real pain- with my new "mini" it shouldn't be a problem.