Earth lose 95,000 tonnes of hydrogen every year

Some studies say that the early Earth had an atmosphere that was about 40% hydrogen.  Today, we lose about 95,000 tons of hydrogen each year.  But don't get too worried, that's only 0.00000000000017%  of the Earth's supply of hydrogen.

Gas molecules bounce around in our atmosphere.  Some of them bounce up.  If they are traveling at or greater than the escape velocity (11.3 km/s), they will leave the Earth and not return.

The kinetic energy of a molecule is:

If we assume that energy is evenly distributed, we can see that a molecule with a lower mass will have a greater velocity and a molecule with a greater mass will have a lower velocity.  So, the lighter molecules will have greater velocity and thus be more likely to attain escape velocity and leave.  Hydrogen, for example, has an average speed of 1930 m/s (6,332 ft/s).  Still not fast enough for the average molecule to escape, but a small amount will be moving fast enough to escape. 

Nitrogen is 14 times heavier than hydrogen.  Oxygen is 16 time heavier than hydrogen.  As we can see, in the below graph, only hydrogen and helium are light enough to attain escape velocity, in significant numbers, on Earth.