usually dental dams are used in root canals.
Gut’s link is a good one. My father is a dentist, and using rubber dams long before the more detailed rules and procedures and such came around due to the whole AIDS deal.
He used them on things even as simple as fillings and sealants. He used them when he did sealants on my teeth when I was a kid. Typically, they help keep the tooth dry and isolated while the dentist works on it – and keep debris from the throat. They are perhaps a little uncomfortable at first, but not terribly so. But I wouldn’t want to wear one on my face all the time… 
I’m curious enough about this dam/no dam use that I have emailed a friend who is a practicing dentist. Meanwhile,
Badmana kinda answered that:
In my experience as a patient, just as Badmana says, it was a piece of latex stretched over a metal frame with a hole in the latex just big enough for the tooth/teeth, which protrude thru the opening while the latex covers everything else.
I think if I had to use it for sex, I might just celebrate the joys of celibacy.
From a university website:
Use non-microwaveable saran wrap–the microwaveable saran wrap has large enough “pores” to allow viruses and bacteria through therefore STI’s as well)–which can be doubled up if wanted.
Here’s the reply from my dentist friend, who shall remain anonymous for now, but he actively practices in the U.S. I have edited his reply a little, as he writes more in “stream of consciousness” than complete thoughts. I consider it literary license!
It sounds like, in the U.S. at least, use or non-use is up to the particular practitioner.
Well I am in the mood to perform today…
You were watching your stopwatch? Now that’s dedication!
Pah… stopwatches are for amateurs. I have a special pair of sunglasses designed for, er… pie eating, with a projection on the lenses showing time, ambient temperature, and current decibel level, among other things…
Okay, so we’ve got a link and plenty of explaination as to how they’re used at the dentists office, but the OP is about their use for performing oral sex!
So, how exactly does it work? Is there supposed to be a slit in it to allow one to still contact the clitoris but avoid the vagina and all the subsequent fluids, or is it a solid piece of rubber that covers the whole thing? If it leaves space for the clitoris, doesn’t that still allow oportunity for the transmission of STDs? And if you go with ceran wrap, how much do you use (are we talking a few inches to cover the area, or enough to make a special little pair of panties?
"Me being the big butch dyke that I am, I feel I should not leave the stage without saying a few fast words about lesbian safe sex. IT SUCKS. I fuckin hate those dental dams! And now I know why you guys have been so pissed off these last few years, it’s like going down on the Michelin Man, isn’t it?! It’s a disgusting thing! You might as well eat somebody through a magazine! I picked up a chick last night and she said ‘I’ve got a dental dam,’ I said, ‘Oh I’ve got a better idea, why don’t you go in the next room, I’LL LICK THE WALL!’
“And you can’t even find a goddamn dental dam anywhere! You go to these gay gatherings and they throw condoms at you and draw you a PICTURE of a dental dam. And they say, ‘Oh, if you can’t find a dental dam, use saran wrap!’ Why? Cos what you don’t finish you can wrap and save for later?!”
- Lea Delaria
With saran wrap, I used about 12 square inches, and folded it over once… she was a tiny little thing; bigger girls will need more… uh… coverage.
A dab of KY jelly here and there underneath won’t hurt.
I don’t know jackSHIT about oral sex and dental dams (wouldn’t it just be a female condom type things?), but I HAVE had them occassionally at the dentist. When I was little, I had two extractions and later on, when they did my root canal.
(I don’t recall if they used them when I had my wisdom teeth out-because I was asleep the whole time)
I hate those things-I was practically gagging on it.
A female condom is quite different. While a male condom hugs and is supposed to stay with the penis, a female condom hugs the vagina and stays there. In normal use, the penis would thrust in and out while the condom would not move much. Exactly how they do that, I don’t know; I have seen them but not used them.
What is being discussed here is basically a flat barrier sheet. Saran wrap can obviously be flat, but cutting a standard condom lengthwise works, too. And a dental dam is mostly flat to start with, I think.
Dental dams aren’t just for using on women… some people also use them for rimming.
all this safe-sex stuff is just a pain. I think I’d rather wait until I know the girl doesn’t have something that will break out on my face a few days later.
I can’t imagine someone going down on me with plastic wrap and it would totally kill the mood! I like the feeling of the tongue, especially in my nether regions, and I guess I’d have to really want it to say: “saran wrap please!”
Besides, wouldn’t it block finger entry? I can’t have that.