How do people get lighter-than-air fumes out of the house?

And not have it collect in the ceiling?

Open all the windows. Exhaust fans in the windows/doors. With adequate ventilation, normal air currents will mix with the fumes and it shouldn’t accumulate to badly.

Great username/post combo…

The answer is in good ventilation and your user name.!

Lighter than air fumes have smaller molecules. Smaller molecules usually means higher velocity of the molecules at room temperature and hence higher diffusion. So it mixes readily within the house and doesn’t accumulate near the roof.

If it’s a gas, it will eventually mix with the rest of the air. The question is how long it will take?

As noted it’s not all the long, especially if you have any sort of air circulation going on.

So forget it’s lighter-than-air and do what you normally do to air out your place.

How often to people have to worry about the amount of hydrogen and helium fumes in their houses?

Open every door and window and turn a box fan on high. Move it room to room.

Part of the problem with lingering odours is once you’ve detected it initially, your sniffer becomes a titch hypersensitive to that particular aroma. So YOU can continue to detect it long past when others are saying it’s now mostly gone.

The solution, after a very thorough airing is to use a little air freshener, or insense, or even heavily scented flowers introduced to the room. That should mask the trace amounts your nose is still detecting, and you’ll feel much better!

Good Luck!

I thought it was the other way around. We tend to adapt and tune out stimuli when we are exposed to them on a steady basis, which would explain why people can’t seem to detect their own body odor or bad breath, and why they are able to tune out background noise at night like passing cars.

Odours on your body are a different kettle of fish entirely, to those in your external environment. If it’s on you, you are never away from it, you just get accustomed to it. When you smell an odour in a room, wait for it to dissipate, re enter the area and are certain you can still detect it, when others may not, is what I am speaking of here. You, having been previously exposed, are now a titch hypersensitive to it, where those not previously exposed, detect nothing. Apples and oranges.

There aren’t very many gases lighter than air. Aside from hydrogen and helium, the only one I can think of offhand is CO, which of course is very dangerous. Hydrogen sulfide is also quite poisonous, but you will know if you have that and it is slightly heavier than air. All other odorous gases are heavier than air too. I don’t think the CO will accumulate at the ceiling (it is about the same as nitrogen actually), but in any case you want to get rid of it fast. Ventilation is what you need. Notice that although oxygen is significantly heaver than nitrogen (32 v. 28), they do not spontaneously separate.

There are a few gases less dense than air, but it’s a short list:

-water vapor (not generally an issue at room temperature, since it quickly condenses)

Ammonia is the only one that’s really noxious in small quantities, but you tend to use it at ground level, and it’s close enough in density to air that you’re more likely to get the fumes well mixed/diffused before they accumulate in a homogenous cloud at the ceiling.

Neon, hydrogen and helium are indeed rarely found in households in quantities that would compel the occupant to take action.

If methane is present, it’s most likely as a component of natural gas, which is deliberately tainted with an odorant so the average person can detect it (and take action) before the concentration is high enough to be dangerous.

Bottom line, most people don’t worry about fumes collecting near the ceiling. The far bigger hazard is heavier-than-air fumes, which are often flammable VOCs: these accumulate near the floor and tend to spread in a way similar to pouring a bucket of water on the floor. People messing with paint or solvents in their basement are often caught off-guard when the fumes spread over to the water heater, where the pilot light is conveniently located just a few inches above the floor and ignites these fumes.

In other strange news, a HEAVIER-than-air gas (methylene chloride) from my DOWNSTAIR neighbor’s apartment has infiltrated my apartment. That seems to defy physics.

This is what am77494 pointed out … if we allow these gas molecules to mix, then they will … it’s only when we confine helium in a sealed balloon that we get lift … or confine methylene chloride so it sinks …

If you have a fireplace, open the flue.

Isn’t there a phenomenon called “flash over” when gases near the ceiling–generated by an intense room fire in process–ignite? (Not OP, obviously.)

The gases are thoroughly mixed … it’s temperature that separates them … the fuel and oxygen reaches it’s kindling temperature at the ceiling first …

Do you have a cite for the idea that people become more sensitive to smells that they have been exposed to?

Regarding molecular weight and speed of diffusion, I remember an experiment from HS freshman Science with long glass tubes, like neon lights but clear. Hmm, lots of YouTube videos of this. Here’s one.

Graham’s Law is the formula. Ratio of the square root of molecular masses. So really heavy gases diffuse slowly. But ones near normal air mass are fairly fast. Outside a lab YMMV.

Generall, lighter-than-air gases diffuse fast enough that they don’t significantly accumulate near the ceiling if you’re making any effort to air out things.