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Is radon heavier than air?

By Radon411 Editorial TeamUpdated How we research

Short answer

Yes: radon is about 7.8 times as dense as nitrogen and 6.8 times as dense as oxygen, and it was the heaviest known gas when first measured in 1908. Even so, it does not stay in the basement: a home's lower air pressure draws it in, and it reaches upper floors too.

Next step: a test is the only way to know your home's level. How to test your home

Radon was isolated in 1908 by Ramsay and Gray, who measured its density and found it to be the heaviest known gas. Its density is 0.00973, against 0.0012506 for nitrogen and 0.001429 for oxygen in the same units: about 7.8 times nitrogen's and 6.8 times oxygen's.

Being heavy does not keep radon in the basement. Radon levels are usually higher in basements, cellars and living spaces in contact with the ground, but considerable radon concentration can also be found above the ground floor. Radon moves up from the ground into buildings through openings in floors or walls that are in contact with the ground.

That is why EPA and the Surgeon General recommend testing all homes below the third floor, with the test placed in the lowest lived-in level of the home.

What to do next: test the lowest lived-in level of your home, and see the radon element guide for why radon's properties matter indoors.

How much heavier radon is than air

Radon is denser than every other noble gas with a measured density and, when it was first measured, it was the heaviest gas known. Its density is 0.00973 grams per cubic centimeter. Nitrogen comes in at 0.0012506 in the same units, and oxygen at 0.001429, so radon is roughly seven to eight times as dense as either.

Radon against nitrogen, oxygen and the other noble gases
GasDensity (g/cm3)Compared with radon
Radon0.00973The baseline
Xenon0.005887Radon is about 1.7 times as dense
Krypton0.003733About 2.6 times
Argon0.0017837About 5.5 times
Oxygen0.001429About 6.8 times
Nitrogen0.0012506About 7.8 times

The ratios are our arithmetic on those densities. Radon was isolated in 1908 by Ramsay and Gray, who measured its density and found it to be the heaviest known gas; its atomic mass, about 222, is well above xenon's 131.29.

Is radon lighter than air?

No. Radon is several times denser than nitrogen and oxygen, so it does not float up the way a light gas would. Yet it still turns up on upper floors. What brings it indoors is not its weight but the building's own air pressure.

Once inside, radon spreads with the indoor air. Levels are usually higher in basements, cellars and living spaces in contact with the ground, but considerable radon concentration can also be found above the ground floor. See does radon gas rise.

What radon's weight means for testing

Because radon enters from the ground, the lowest lived-in level is where a test goes. EPA and the Surgeon General recommend testing all homes below the third floor, which covers ground-floor apartments as well as houses.

Where to test, given how radon behaves
Your homeWhere to test
Basement used regularlyIn the basement
Basement not used, or no basementOn the first floor
Apartment below the third floorIn the unit
Any homeIn a room used regularly, not the kitchen or bathroom, at least 20 inches above the floor

If you start living on a lower floor later, retest on that floor. Full placement rules are on where a radon test should be placed.

Radon's other physical properties

Radon is a gas at ordinary temperatures, colorless and odorless. It turns liquid and solid only far below freezing.

Radon's physical properties
PropertyValue
Standard stateGas
Density0.00973 g/cm3
Melting point202 K (-71 °C, -96 °F)
Boiling point211.45 K (-61.7 °C, -79.1 °F)
Atomic mass222.01758
Color and smellColorless and odorless

When cooled to a solid, radon glows yellow, turning orange-red as the temperature drops further. None of that happens in a home; what matters there is that radon is a heavy, invisible gas that comes up from the ground. The radon element guide covers the rest.

Questions about radon's weight

Is radon lighter than air?

No. Radon's density is about 7.8 times that of nitrogen and about 6.8 times that of oxygen.

Does radon sink to the floor?

Levels are usually highest in basements, cellars and living spaces in contact with the ground, the floors nearest the soil it enters from. But considerable radon can also be found above the ground floor, so its weight alone does not keep it low.

Does radon stay in the basement?

No. Considerable radon concentration can be found above the ground floor, which is why EPA recommends testing all homes below the third floor.

Can radon be on the second floor?

Yes. Levels are usually higher in basements and rooms in contact with the ground, but considerable radon can be found above the ground floor. Test the lowest lived-in level first.

Is radon the heaviest gas?

When Ramsay and Gray measured it in 1908, it was the heaviest known gas, and today it is still denser than every other noble gas with a measured density.

Is radon denser than xenon?

Yes. Xenon's density is 0.005887 grams per cubic centimeter, against radon's 0.00973, so radon is about 1.7 times as dense as the next-heaviest noble gas.

Why does a heavy gas get into houses?

Because the house pulls it in. Indoor air pressure is usually lower than the pressure in the soil, so the home draws soil gas, radon included, through cracks and openings in the foundation.

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