1) It doesn't have an on/off switch, but the worse it is out the faster it gets saturated, and you don't want to wear it too long because you breathe a higher percentage of CO2 when you have them on, but IME they last more like 8 hours (I would not wear more than 2-3 at a time). I'm not a doctor but I spend a lot of time around them.
Edit: Get the yellow rectangular ones with two straps instead of the cup-style ones because they fit better, and the material is better. Occasionally blue but Kaiser and Sutter seem to both use the yellow ones.
2) The wind was better today, earlier but not now, worse tomorrow, better Sunday, and by next Wednesday we should have rain.