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Honestly though, is the Earth considered air-tight in the sense that its gases don't escape?

I'm sure every physicist who reads this is going to tear their hairs out, but the extent of my knowledge in this area is that you need to travel a certain speed to break Earth's gravitational pull and that has me wondering how gases could escape.

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What do the plastic bags have to do with your question? If you intended them as a joke, I find it simply distracting. – Mark Eichenlaub Jan 20 '12 at 1:36

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Molecules of an ideal gas will have velocities that follow the Maxwell-Boltzmann distribution. Some fraction of the molecules will have a velocity greater than Earth's escape velocity, and so will be able to escape into space. However, this fraction is insignificantly tiny for most gasses with the exception of hydrogen and helium.

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That is to say that small amounts helium and hydrogen may escape from the atmosphere? – Walkerneo Jan 20 '12 at 1:42
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It means that the time scale goes as $\exp{(m/T)}$ for $m$ the mass of the molecule in question and $T$ the temperature. To first order the time scale for losing $\mathrm{O}_2$ is $\approx\exp{(8)}$ times that for losing $\mathrm{H}_2$. – dmckee♦ Jan 20 '12 at 1:55
...and if the Earth's surface hydrogen was in the form of $\mathrm{H_2}$ gas instead of mostly water it would all have escaped long ago. – Nathaniel Jan 20 '12 at 10:52
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The Earth is not air-tight the other way, either.... there is a serious scientific theory that the water in the atmosphere and oceans is largely due to capture of frozen ice snowballs from outer space...so if we're not water-tight, obviously we're not airtight either... see also Estimating hydrogen loss by Jeans escape for hydrogen escape

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