Not sure why you're getting down voted. If you're venting air outside, that air must get replaced with other air. Unless you have a fancy heat recovery system,

Not sure why you're getting down voted. If you're venting air outside, that air must get replaced with other air. Unless you have a fancy heat recovery system, pointing a fan out the window on a freezing day is just as bad as pointing one in the window. Maybe it's worth it given the health risks, but let's not pretend the tradeoffs aren't there.