A couple of approaches that haven't been mentioned:
Depending on the meeting, it might make sense to interrupt it long enough to note what's happening, that if it gets worse you may have to step out for an indeterminate amount of time, and even ask if you staying will disturb anyone. Don't necessarily assume everyone (anyone!) will be grossed out.
Leaving without letting someone know what's happening is a bad idea - the others in the meeting may get worried. If you happen to have a friend in the meeting who knows about the problem, ask them to explain as appropriate if you simply have to leave immediately. If someone else is leading them meeting, you might approach them sometime beforehand, mention the possibility of this happening, and see if they have a suggestion/preference regarding whether you step out or try to deal with it in place. If you do leave, they'll know why, and can manage things better.
How critical your participation is also makes a difference. If (for example) you're working together on a school project, where everyone is expected to participate, trying to stick around if everyone is OK with it might make sense. If it's a staff meeting at your place of work (teenagers can have jobs too!), unless you explicitly have a role to play, then stepping out may make more sense (especially if you work around food, and are in a food-preparation area). If you're leading the meeting, you should definitely have someone ready to back you up if you do need to step out, or step aside for a few minutes.
If you've been able to feel out the group, or a group leader, as to their preferences regarding what you should do, then let that (and the severity of the situation) guide your actions if it happens.
If you've got an explicit reason to expect that an incident is likely (for instance, the air in the meeting place is particularly dry and you know this is a trigger after 30 minutes or so), then you should definitely bring it up beforehand somehow. As mentioned above, this might be mentioning it to whoever is leading the meeting (especially in a larger group).
In a smaller group, especially one you'll be working with regularly, mentioning it once to the group as a whole means you don't have to worry about it; you can assume it will happen eventually, but you've already dealt with it. In that case, getting feedback from the group as to what they'd like you to do is reasonable as well.
To digress for a moment, I'm a type-2 diabetic, and need to check my blood sugar and take some insulin before I eat a meal. I usually do this at the table (as it's almost always cleaner than the bathroom!) If I'm eating with one or more people I haven't eaten with before, I mention that I need to do this, and ask if anyone minds. I've had one person who did mind in the 15 years I've been doing this, and I always made sure I did this before leaving for lunch (or in the bathroom) when eating with her.