[JP]

Couldn't find Argon in the top few hits, but since breathing in a dense gas makes your voice sound funny, you can find tons of youtube videos of it.  Here's a couple of the more reputable ones talking about it.  They talk about it pooling in one of them, and note that a few breaths is enough to clear the lungs of Xenon, due to the fact that the lungs are quite good at mixing gasses.

[Bored chemist]

A guy saying that it pools is trumped by an MRI image that shows that it doesn't, even if the bloke has a PhD.
If it pooled then you would hear the effect  of that pooling.
His first bar of a Modern Major General would be normal pitch, as because he's be exhaling the air floating on top of the Xe, and then the pitch would drop as he got to the Xe.
Also, if the gases didn't  mix then the cut off of the effect would be sudden, rather thann gradual.

This "pooling in the lungs" has become a factioid.
http://en.wikipedia.org/wiki/Factoid

[damocles]

 [:I]I must begin with an apology to bored chemist and to the forum in general. I did not read the material in the linked image properly -- confused MRI with other radiological imaging techniques.

Just stop being wrong.
"The linked image that you show of the distribution of Xenon in the lung proves nothing without the full text of the scientific article. It is much more likely that the patient inhaled air with a trace of radioactive xenon incorporated than that s/he breathed xenon in high concentration.
"

For the record, the Xe-129 isotope referred to is a stable isotope of xenon, not a radioactive one. And MRI does require quite a large sample of the atom being monitored to produce its images -- which means either high concentrations or long exposure times or both.

However, an image showing a uniform distribution of xenon in a lung, presented without a good amount of background detail, is not evidence that xenon does not pool. Before bored chemist has a fit of apoplexy I will stress here that I am talking here about the evidential value of these images, not the issue of pooling of itself.
It has no evidential value because, whether efficient mixing occurs or not, an "inhaled" gas will reach a steady state of uniform distribution in any container after several "breaths" if any mixing at all occurs. This can refer (again for the sake of bc's blood pressure) to an open cylinder in which a piston is raised and lowered to take in 70% of new gas on each stroke.

According to this
http://books.google.co.uk/books?id=AqGQN1-JMuYC&pg=PA60&lpg=PA60&dq=MRI+imaging+lungs+%22xenon+concentration%22&source=bl&ots=Sx_yquoZhE&sig=D5yKEedw9A93EYqRc2FCFyyWqfw&hl=en&ei=iZu0TvfWIYSWOsWhhO8B&sa=X&oi=book_result&ct=result&resnum=5&ved=0CFAQ6AEwBA#v=onepage&q&f=false
The xenon concentration mat well be about 70%.

I genuinely thank Bored chemist for providing this link, because it fills in a lot of the background information about the technique that was used to provide the image in question. In fact it gives everything except the specific details of the conditions under which the particular images were obtained -- the important ones being
(1) actual gas composition used
(2) exposure time of patient to xenon gas mix (or number of breaths)
(3) image exposure time, i.e. started immediately patient was exposed to xenon or after a few breaths; concluded immediately normal air breathing resumed or after a few breaths.
Here I mean important in considering the image as evidence of thorough mixing, not clinically important.

To very briefly summarize the linked material on these issues: Patient exposure appears to be tens of breaths. The gas mix is limited by a protocol of less than 70% xenon (to avoid possible narcosis/anaesthesia) and at least 20% oxygen (to ensure continuance of normal respiration).

The nature of the MRI technique suggests that xenon content and number of breaths were unlikely to be far below these levels.

Gases mix as they enter the lungs. To pool as you said they do requires them to unmix. if they did that we would drown in the 1% of argon in the air.
I'm not the one backing down here.
You said that argon pooled in the lungs. It doesn't.
I pointed out that, if it pooled in lungs it would pool in general, notably the atmosphere.
if it did that we would all be dead.
We are not dead.
Argon doesn't unmix from the air and pool.
It doesn't unmix in general.
It doesn't unmix in lungs.
It doesn't pool in lungs.

This summary shows an attitude of "black and white" to pooling and mixing. They are not switches, they are matters of degree. Both are very real facts about the behaviour of gases, and each is seen in many context.

Gases mix thoroughly and in all proportions. There is only ever one gas phase. Gases do not unmix. The fact that the diverse mixture of gases in air do not layer out under gravity is clear evidence of this.

but

Gases also pool. Lake Nyos and the various demonstrations of carbon dioxide flowing to extinguish candles are clear evidence of this. A heavier gas (or gas mix), on introduction to a lighter gas (or gas mix), will move downward under gravity and make pools in low lying vessels or hollows.

The extent and duration of pooling (which is not a yes/no switch) is governed by the efficiency of mixing in the environment/context that the heavy gas encounters.

Gases mix in the lungs.
They clearly do not mix much on entry because they are in a strong inward flow, in the wake of whatever gases are already present in mouth and windpipe. How efficient is the mixing once they have already reached the lungs? 50%? 90%? 99%?

Pooling (a real effect with major implications in many contexts) has nothing to do with unmixing (which never happens at near-atmospheric pressure).

The rest is a "yes it is/no it isn't argument". The point I was trying, and failing, to make is that pooling is a real effect governed by real physical laws, and as such it must occur in the lungs as in any other context. Apparently the extent of pooling is very small, trivially small even, due to the efficiency of lungs as gas mixers.

This will be my last post to this thread, as I am sure that bc will want to have the last word, and we are already generating more heat than light.

[Bored chemist]

Well, I guess I will let you have the last word, and it's this sentence concerning the pooling of gases in the lungs and the OP's question. We both agree that the evidence tells us that:
"Apparently the extent of pooling is very small, trivially small even, due to the efficiency of lungs as gas mixers."

[Bill Natan]

Argon won't kill you, the lack of oxygen will, VERY bad idea to breath in pure argon.