I have to say I'm guilty of this at times because I never learned to blow my nose until I was 16, so it's not really an instinct for me to take care of the free-roaming snot by blowing it (and besides, I have heard that blowing it doesn't really help much and can make your sickness last longer). My roomie's boyfriend does it all the time, and he does it really loud. I can handle people sniffing if they've got a runny nose, but when it's loud and long it makes me a little queasy.
Allergies suck. There's no better way to put it.
I take my regular allergy pills every morning, and they're supposed to last 24 hours. For normal situations, they
do last 24 hours, the exceptions being really cold days (whose nose doesn't run then?) or when I've been exposed to more allergen than normal doses of antihistamines can handle--the best example being
perfume. I'm to the point now where I feel that if a person is wearing enough of any scent that moving away from them in a restaurant lobby doesn't clear it from my head, it's assault. At that point there is nothing I can do. Either the plumbing stops up completely and I can't breathe or it springs a seemingly neverending leak. Sure, I can add some diphenhydramine, but that takes time to work and will only knock me out in the middle of my meal.
Stopping into the bathroom for a quick nose-blowing doesn't do more than very temporarily alleviate this. I could get up every five minutes to blow, but that's insane. So I'm sometimes stuck being that offending sniffer.
It's not fair to judge on that, though, since I was FINE when I left the house and became sick when someone polluted the lobby. I'd love to say this happens rarely, but it's far more frequent than it would be if people understood that their scents should be noticeable ONLY within a foot or so of their person, rather than following the example of the old Jean Nate "Splash" commercials, where the model bathed in the stuff.