What causes helium ballons to lose their lift after a day or two?

★ wanayoo — archive 1999 http://howstuffworks.com/question10.htm

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Question
Scenario : A helium baloon is sitting on the ceiling one day, and the next day, it's on the floor. Does the helium leak out of the balloon, or do the helium molecules slow down due to the fact that they are no longer under enough pressure, and this subsequently causes the baloon to lose its buoyancy, and fall to the ground. Please enlighten us!

Answer
The helium does leak out. Helium is a very small atom and latex is pretty porous at the scale of a helium atom (if you ever go looking to buy balloons, you will see that there are "helium-grade" balloons - which try to be thicker and less porous). The weight balance that keeps a balloon afloat does not leave a lot of room for leakage, so once a little leaks out the balloon falls.

See the HSW page on helium balloons for complete details on the weight that a helium balloon can lift!

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