Why Do Helium Balloons Deflate and Sink?

Why Helium Balloons Deflate and Sink
Helium balloons deflate when helium atoms escape through pores in the balloon material. When enough gas escapes, the balloon sinks.

Have you ever wondered why helium balloons deflate so quickly and sink? Meanwhile, ordinary latex balloons filled with air keep their shape for weeks. Here’s the explanation for why helium balloons deflate and what you can do to revive a balloon after it sinks.

  • Helium balloons float because helium is less dense than air.
  • Helium balloons deflate because helium atoms are small enough to leak through spaces in the balloon’s material and escape.
  • Foil helium balloons don’t deflate as easily because they aren’t under as much pressure and because there is less space between the metal atoms.

Why Helium Balloons Float

To understand why helium balloons deflate, it helps to first understand why they float. Helium is lighter or less dense than air. What this means is that a balloon filled with helium has less mass than the same balloon filled with air. Less mass per volume means lower density. A helium balloon and an air-filled balloon displace the same volume of air, but the air-filled balloon sinks because the balloon plus the air make it heavier than the atmosphere. Meanwhile, the balloon plus the helium is still less dense than the air it displaces.

The reason helium is less dense than air is because helium atoms have a very low mass, while air consists mostly of nitrogen and oxygen. Not only are nitrogen and oxygen atoms much more massive than helium, but they exist as molecules (N2, O2) in air. Helium has a stable outer electron shell, so it occurs as single (monatomic) He atoms and not as molecules.

Why Helium Balloons Deflate

So, helium atoms are very small. The reason helium balloons deflate so quickly is because helium escapes a balloon faster than air can enter it. In fact, helium diffuses through a solid (like a balloon) three times faster than air.

Foil helium balloons don’t deflate as quickly as latex balloons for two reasons. First, it’s much harder for helium to navigate through the aluminum that coats the plastic of this kind of balloon. Second, foil or Mylar balloons aren’t under pressure like latex balloons. A latex balloon expands when it’s blown up. This stretches the latex and leaves more space between plastic molecules for helium to escape, while at the same time the pressure pushes the helium out. There isn’t as much pressure within a foil balloon, plus they are more rigid, so they don’t shrink when empty.

Hydrogen Versus Helium Balloons

Hydrogen balloons are even lighter than helium balloons. Even though hydrogen gas contains hydrogen molecules (H2), it’s still less dense than monatomic helium (He). The hydrogen molecule is also smaller than the helium atom, so hydrogen balloons deflate even more quickly than helium balloons.

How to Revive a Deflated Helium Balloon

A deflated helium balloon still contains helium, so it’s possible to revive it and make it float again. Warm the balloon by placing it in a hot location or gently heating it with a hair dryer. The heat increases the kinetic energy of the helium atoms. As the atoms gain energy, they strike the walls of the balloon faster and more often, increasing gas pressure. This expands the balloon and makes it lighter than air again. The same principle explains how hot air balloons rise and descend.


  • Considine, Glenn D., ed. (2005). “Helium”. Van Nostrand’s Encyclopedia of Chemistry. Wiley-Interscience. pp. 764–765. ISBN 978-0-471-61525-5.
  • Grummer, Arnold E. (1987). The Great Balloon Game Book and More Balloon Activities. Greg Markim, Inc.:Appleton, Wisconsin. ISBN 0-938251-00-7.
  • Shuen-Chen Hwang, Robert D. Lein, Daniel A. Morgan (2005). “Noble Gases”. Kirk Othmer Encyclopedia of Chemical Technology. Wiley. pp. 343–383. doi:10.1002/0471238961.0701190508230114.a01
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 978-0-8493-0464-4.