Tires lose air for a couple of reasons. One is underinflation of the tires and another is through a natural process called permeation. Tires should be checked briefly every week to ensure proper air fill.
Underinflated tires become hotter when driven or ridden on than properly filled ones, which may lead to tire failure. It also lets the air out much faster than with normal pressure, both while active and while resting.
Tires are made of rubber, which has pores in its material like anything else. According to writer Peter du Pre, "On the average, a tire will lose one or two pounds of air per month in cool weather and more during the warmer months." This is because of the way air flows between different temperatures.
Check pressure of tires weekly with a reliable pressure gauge. If you're checking the tires on a car, also make sure the spare is filled up. Also make sure to check the tires before you ride or drive, when they're cold or normal temperature.
