5.2 ppm

The air consist for just 5.2 ppm (parts per million), so 0.00052% of helium. If you look at the weight, the portion is even smaller, because Helium is very light. Actually, even with the additional Helium created by the natural radioactive decay of heavy radioactive elements, the amount of it is decreasing, due to it escaping into space.
My entry is the outlet of a 22.5 litre can/tank of Helium, used to fill balloons.
Thanks to freespiral for hosting the Tiny Tuesday challenge.