Helium balloons naturally deflate over a few days. Here's the scientific explanation for why they stop floating so quickly. read more
The answer has to do with the nature of helium and the balloon material. read more
The balloon material is not perfectly opaque to gases passing through it, so all balloons eventually deflate. But helium atoms are small and mobile, and they diffuse through the material much faster than air does, so helium balloons deflate more quickly than air-filled balloons. read more