MUNI Etiquette Question

Let's say you're on the N Judah coming home from work in the evening. You're lucky enough to have a seat, because you get on before most of the thundering herds. As more and more people crowd on there are several who are standing directly in front of you - in close proximity to your face. When, if at all, is it appropriate for you to tell the gentleman in front of you that his fly is open?