I would not be in a crowded elevator for even a short period right now (especially, if there is a high prevalence of the virus in my area). When you do use them, minimize touching high contact surfaces and sanitize hands after. If you have to be in them with others, invest in a n95 and learn how to fit it properly.
Depends on the ventilation.
How do you learn how to fit an N-95 or N 100 mask properly?
If it fogs your glasses, try again.