Why Do Helium Balloons Deflate?

Why Helium Balloons Deflate So Quickly

Helium balloons deflate because the helium gas atoms are small enough to pass through the mylar balloon material.
Helium balloons deflate because the helium gas atoms are small enough to pass through the mylar balloon material. andresr / Getty Images

Helium balloons deflate after a few days, even though ordinary latex balloons filled with air may hold their shape for weeks. Why do helium balloons lose their gas and their lift so quickly? The answer has to do with the nature of helium and the balloon material.

Helium Versus Air in Balloons

Helium is a noble gas, which means each helium atom has a full valence electron shell. Because helium atoms are stable on their own, they don't form chemical bonds with other atoms.

So, helium balloons are filled with lots of tiny helium atoms. Regular balloons are filled with air, which is mostly nitrogen and oxygen. Single nitrogen and oxygen atoms are already much larger and more massive than helium atoms, plus these atoms bond together to form N2 and O2 molecules. Since helium is much less massive than nitrogen and oxygen in air, helium balloons float. However, the smaller size also explains why helium balloons deflate so quickly.

The helium atoms are very tiny -- so tiny the random motion of the atoms eventually lets them find their way through the material of the balloon through a process called diffusion. Some helium even finds its way through the knot that ties off the balloon.

Neither helium nor air balloons deflate completely. At some point, the pressure of gases on both the inside and outside of the balloon becomes the same and the balloon reaches equilibrium.

Gases are still exchanged across the wall of the balloon, but it doesn't shrink any further.

Why Helium Balloons Are Foil or Mylar

Air slowly diffuses through regular latex balloons, but the gaps between latex molecules are small enough that it takes a long time for enough air to leak out to really matter.

If you put helium into a latex balloon, it diffuses out so quickly your balloon would deflate in next to no time. Also, when you inflate a latex balloon, you fill the balloon with gas and put pressure on the inside surface of its material. A 5-inch radius balloon has roughly 1000 pounds of force exerted on its surface! You can inflate a balloon by blowing air into it because the force per unit area of the membrane isn't that much. It's still enough pressure to force helium through the wall of the balloon, much like how water drips through a paper towel.

So, helium balloons are thin foil or Mylar because these balloons hold their shape without the need for a lot of pressure and because the pores between the molecules are smaller.

Hydrogen Versus Helium

What deflates faster than a helium balloon? A hydrogen balloon! Even though hydrogen atoms form chemical bonds with each other to become H2 gas, each hydrogen molecule is still smaller than a single helium atom. This is because normal hydrogen atoms lack neutrons, while each helium atom has two neutrons.

Factor That Affect How Quickly a Helium Balloon Deflates

You already know the balloon material affects how well it holds helium. Foil and Mylar work better than latex or paper or other porous materials.

 There are other factors that influence how long a helium balloon remains inflated and floats.

  • Coatings on the inside of the balloon affect how long it lasts. Some helium balloons are treated with a gel that helps hold the gas inside the balloon longer.
  • Temperature influences how long a balloon lasts. At higher temperature, the motion of molecules increases, so the rate of diffusion (and rate of deflation) increases. Increases the temperature also increases the pressure the gas exerts on the wall of the balloon. If the balloon is latex, it can expand to accommodate the increased pressure, but this also increases the gaps between latex molecules, so the gas can escape more quickly. A foil balloon can't expand, so the increased pressure can cause the balloon to burst. If the balloon doesn't pop, the pressure means helium atoms interact more often with the balloon material, leaking out faster.