λ = kB·T / ( 2½·π·σ²·P )
where kB is Boltzmann’s constant (kB = 1,381·10⁻²³J/K).
The relation shows that the mean free path strongly depends on the sigma value (because it is squared). In other words it depends on the size and therefore the type of atom.
Taking as reference the smallest atom existing in nature, the hydrogen, and adopting a sigma value equal to twice its Van der Waals’s radius (2·120pm = 240·10⁻¹²m), a temperature of 300K (about 27°C or 80°F) and a pressure of 1bar=100˙000Pa the mean free path is equal to
λH,300K,100000Pa = 1,6·10⁻⁷m = 1,6·10⁻⁴mm = 0,16μm
Since the mean free path is inversely proportional to the pressure, if the pressure decreases by 10 times, the mean free path increases tenfold. Of course, when the pressure assumes the 1Pa=0,01mbar the mean free path increases by 100˙000 times and it becomes equal to
λH,300K,1Pa = 0,016m = 16mm