<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by cnk:
Under 9,000 ft isn't air pressure equalized between cabin and outside? Does that mean that at low altitudes nothing keeps someone from opening the doors?</font>
No. The airplane begins to pressurize either during taxi, or immediately after takeoff, depending on the aircraft.
The idea is to keep the rate of change and maximum cabin altitude both as low as possible to minimize discomfort.