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en.wikipedia.org/wiki/Lighter_than_air

Only certain lighter than air gases are suitable as lifting gases. Dry air has a density of about 1.29 g/L (gram per liter) at standard conditions for temperature and pressure (STP) and an average molecular mass of 28.97 g/mol, and so lighter than air gases have a density lower than this.

www.quora.com/Which-gases-are-lighter-than-air

At normal pressures (e.g. air), gases follow the ideal gas law and all gases have the same number of particles (molecules) per unit of volume. Air is 80% nitrogen (MW 28) and 20% oxygen (MW 32), for an average MW of about 29. So any gas with a MW ...

www.reference.com/science/list-gases-lighter-air-2ef84f0075c64098

Gases that are lighter than air include water vapor, methane, hot air, hydrogen, neon, nitrogen, ammonia and helium. These gases have a lower density than air, which causes them to rise and float in the atmosphere. Many lighter-than-air gases provide lifting power for airborne objects.

www.answers.com/Q/What_Gases_are_lighter_than_air

Helium rises because it is less dense than the air. If it is less dense then it will rise above the air until it meets a gas lighter than it. There are other gases lighter than air.

www.esabna.com/euweb/oxy_handbook/589oxy1_13.htm

Be sure to provide enough ventilation to disperse the lighter than air vapors. Propylene, butane, propane, and MAPP being heavier than air may spread to poorly ventilated areas along the floor or beneath equipment. Also, the heavier gases tend to diffuse in air slowly, increasing the hazard of any accumulations.

www.chem.hawaii.edu/uham/lift.html

Natural gas or Methane is also lighter than air, not because it contains helium, which most of it does not, but because it is just less dense than air. It can be used to fill balloons, but it suffers from the same flammability and explosion hazard as hydrogen. Methane gas is roughly half the weight of air and provides anemic, but useful lift.

www.reference.com/science/gas-lighter-air-6b391099a5ae4c1d

Hydrogen, helium, ammonia, methane, neon, coal gas and pure nitrogen are all lighter than air. A pocket of air can be made lighter than the surrounding air by heating it. According to the Underwater Robotics Research Group, gases that are lighter than air have densities that are less than 1.2 kilograms per cubic meter or 1.2 grams per liter.

www.firehouse.com/rescue/article/10495996/evolution-of-an-hazmat-acronym

The new acronym was complete! "4H MEDIC ANNA" was born and could now easily be remembered as the 13 gases that were lighter than air. That bolt of enlightenment, that new paradigm, ...

scienceline.ucsb.edu/getkey.php?key=924

Air is a mixture of gases, mostly nitrogen and oxygen 78.1% nitrogen 20.9% oxygen 0.9% argon 0.1% other gases (like carbon dioxide) Air is well mixed, so we do not usually talk about oxygen being lighter or heavier than air. Helium, on the other hand, is much lighter than air (nitrogen and oxygen), so that is why a helium-filled balloon goes up.

ask.metafilter.com/32595/Household-gas-rises-or-falls

Household gas: rises or falls? February 13, ... Short answer: yes methane is lighter than air. You won't get pooling at the bottom of your stairwell. Because of the way residential units are built, it's not likely to become a risk. The major risk with methane is explosion. Suffocation is a risk, but not likely int he situation you describe.