sozobe wrote: I'd still like to pin down whether steadily cold but nowhere near hypothermia-inducing temperatures have any harmful impact on the immune system... if the answer is definitely no, I'll relax already. (Seems likely, but there's that wisdom-of-the-grandmothers thing Dag mentions...)
I can't find the source, but about a year ago I read an article about "evidence based medicine", an enterprise that has set out to test "wisdom of the grandmothers" claims. As best I remember, "cold temperatures cause colds" was one hypothesis that was not confirmed. As you said earlier, it is the dryness of the inside air that gets your respitory system.
Since Ossobucco and littlek raised the point, maybe I should say something about relative humidity vs. absolute humidity. 'Absolute humidity' measures the mass of of water in any given volume of air. The warmer the air, the greater the mass of water it can hold. (Note, "can hold". Just because the air can hold more, that doesn't necessarily mean it does.) 'Relative humidity' measures a percentage of water in the air, where 100% represents the maximum amount of water the air can hold at this temperature.
Outside air and inside air tend to have similar absolute humidity because it tends to be the same air. But they have different relative humidity because their temperature is different. In winter, the outside air is cold so can only retain a small absulute amount of water. By contrast, the inside air is warmer, so what is 100% relative humidity for the outside air is less than 50% for the inside air. Our body feels humidity in terms of how easily water evaporates from its surface, which corresponds to relative humidity, not absolute humidity. It is that drop in relative humidity in the air inside that our respitory tracts react badly to. (That's what I read anyway. On a gut level, I still don't quite buy that temperature has nothing to do with it.)