You are supposed to check your tires every month because of temperture changes.

Higher temperature = more pressure
Colder temperature = less pressure

90% of drivers drive with under-inflated tires, more so in Winter.
A tire of today "looks" normal with 50% inflation pressure.
You can't tell pressure by looking at them.
A tire is still drivable with 20% of normal pressure. ( but not safe )

Tires loose 1 psi per month on average, just due to useage.*
Tires loose 1 psi per 10'F temperature drop, on average.*

75% of drivers wash their car once per month.*
14% of drivers check their tire pressures once per month.*
60% of drivers rotate their tires.*

[quote]*
"Vehicle manufacturers list recommended tire pressures for original vehicle tires in the owner's manual or on a placard on the end of the driver's side door or in the glove box.
For continuous high speed driving, tire pressures should be increased by 3 to 5psi above the normal cold inflation recommended.

However, for passenger tires, never exceed the maximum inflation pressure molded on the sidewall. The inflation pressure for light truck tires may exceed that molded on the tire by 10psi. Any recommended front to rear pressure differential should be maintained."


*per Firestone / Bridgestone

FYI
Normal air is 78% Nitrogen, 21% Oxygen, 0.935% Argon, 0.035% Carbon Dioxide. Those 4 alone = 99.98% of air. Not much room for anything else.
Everything else is a few parts per million or parts per billion.
The Myth that certain molecules leak out of the tire at different rates is preposterous. Contrary to some claims, nitrogen does not diffuse through tire rubber more slowly than air. The nitrogen molecules are smaller. All else being equal, smaller molecules diffuse through porous substances more quickly. In real life, oxygen and nitrogen molecules are so similar in size compared to the rubber that there is no net difference in diffusion rates.