Perfectly good tires will lose 2 to 4 psi each month which varies due to heating up and cooling down from use. Yes, the air escapes through the pores of the rubber. If the recommended pressure is 100psi then 90 days from the last time they were inflated properly the tire will be down as much as 12psi or at 88psi. However 100psi isn’t always the best recommended pressure due to irregular wear it will cause when running empty. In short, the best recommended tire pressure must be determined for the application and tire process put in place that periodically monitors and adjusts the air pressure as needed to gain the best fuel economy and maximum tire life.