[Pseudoscience-is-malarkey (OP)]

What would it be like?

[Bored chemist]

It would be dense, toxic, flammable, unstable and stinky.
As far as I can tell, the only way it would happen would be an accident in a chemical works.

[alancalverd]

As a gas, it is slightly denser than air so could form localised puddles in zero-wind conditions butwold dispers rapidly n any breeze.

H2S has a remarkably short liquid phase between 188 and 214K, and is thus gaseous at almost all terrestrial altitudes and would not form any kind of visible cloud.

[paul cotter]

My third edition of Cotton&wWilkinson(BC has the first edition) states that hydrogen sulphide is way more toxic than hydrogen cyanide. I dispute this, certainly under conditions of acute exposure. Maybe it is in connection with chronic exposure as the body can metabolise cyanide(via rhodanase if my memory is correct) and possibly h2s can build up?

[Bored chemist]

If this
https://en.wikipedia.org/wiki/Atmospheric_temperature#/media/File:Comparison_US_standard_atmosphere_1962.svg
is to be believed then you only get atmospheric temperatures cold enough to liquefy it either in very cold polar conditions, or most of the way to space.
Obviously, at high altitude the pressure is low so the stuff would boil anyway.
But in principle,  you could get H2S to liquefy, or even freeze...
" Due to the very strong temperature gradient near the surface, these imply near-surface air temperature minima of approximately −94 ?C (−137 ?F; 179 K)."
From
https://en.wikipedia.org/wiki/Lowest_temperature_recorded_on_Earth

So, on the whole, I think it's more accurate to say " gaseous at almost all terrestrial latitudes ".


H2S is a bit more acutely toxic than HCN.
I wouldn't recommend doing the experiment.

[alancalverd]

But it may have been done! Your post suggests that the bunkhouse at Vostok Station contains an ever-growing pool of liquefied  fart gas. Assuming the residents enjoy the traditional Russian cabbage-based diet, this will have a high percentage of H2S. After only 70 years the pool may not have reached the bottom human bunk, but I bet there are a few dead huskies around the place. 

[evan_au]

I have heard of mysterious deaths near a swamp, which some had attributed to the release of H2S as a swamp gas.
- Apparently, the human nose quickly adapts to H2S, so although it is detectable at low concentrations, soon you can't detect it at all.

Apparently, part of its toxicity is it's chemical similarity to water (H2S vs H2O)
- oxygen & Sulphur being in the same column of the periodic table.
- similar enough to fool some body systems, but different enough to be lethal.

https://en.wikipedia.org/wiki/Hydrogen_sulfide#Safety

[alancalverd]

The relative densities of methane, air, H2S and CO2 are about 0.7: 1.2: 1.3 :1.8, so my inclination is towards CO2 being the principal suffocant of swamp gas, since CH4 would dissipate rapidly and H2S would effectively warn folk away from the area, but I guess people might indeed become habituated to it and die from toxicity rather than suffocation or continuous hyperventilation.

Lush vegetation makes swamps attractive.  I wonder if this explains occasional concentrations of dinosaur fossils: a few days of high atmospheric pressure (no wind) increases the concentration of H2S and CO2 in lowlying areas, so the herbivores' feeding ground becomes toxic, then the carnivores attack the corpses and get caught too.

[paul cotter]

The toxicity of h2s is from it's ability to prevent oxygen utilisation similar to the modus operandi of cyanide. I can't remember if this occurs in mitochondrial cytochromes or in haemoblobin but the effect is oxygen deprivation.

[Bored chemist]

How common is it for people to be overcome by gas emissions from swamps?

I know it happens with this sort of thing
https://en.wikipedia.org/wiki/Lake_Nyos_disaster
and even this
https://en.wikipedia.org/wiki/Cave_of_Dogs

I know people get stuck in swamps and die of exposure or drowning.
But I never heard of anyone being gassed.

[paul cotter]

Very interesting links, BC, thank you.