Helium is seen to be "lighter" than air only when it is confined in a balloon or similar so as to be able to displace the heavier stuff around it. Released loose, the atoms seem to mix with the air around very swiftly. I mean about 50 metres in 2 or 3 seconds, and then it seems to hang about steadily getting more dispersed, but still enough to make a vacuum leak detector howl, for maybe 20 minutes. Forgive the very casual approximate descriptions - this was not a real experiment! There have been recent alleged "concerns" that helium is a non-renewable resource, countered by other accusations that this is hype intended to affect the share price and profits of helium-mining companies. As I understand it, Earth Helium is mostly derived from Radon gas decay. http://amarillo.com/news/local-news/2010-09-13/helium-prices-jump-curb-debt" http://www.mnn.com/earth-matters/wilderness-resources/stories/helium-crisis-spreads-around-the-world So the question(s) .. What exactly happens to released helium, party balloon or otherwise? Does it drift up to "float "above the atmosphere, and get blown off into space? Does it remain a "mixture" with air?