On December 29, 2004, about 1330 central standard time, a Bombardier CL-600-2C10 twin turbo-jet airplane, N506MJ, operated as United Express Flight 7210, by Mesa Airlines, Inc., of Phoenix, Arizona, performed an emergency evacuation after landing at the Austin-Bergstrom International Airport (AUS), near Austin, Texas. The two airline transport rated pilots, two flight attendants, and 55 passengers were not injured. One passenger sustained a serious injury. The airplane was not damaged. Visual meteorological conditions prevailed and an instrument flight rules flight plan had been filed for the scheduled passenger flight conducted under 14 Code of Federal Regulations Part 121. The flight, which was destined for the Washington Dulles International Airport (IAD), had departed AUS approximately ten minutes earlier. According to written statements provided by the flight crew and flight attendants, during the takeoff roll, both flight attendants observed a haze in the cabin. As the airplane rotated and began to climb, the haze thickened. At approximately 500 feet above ground level (agl), the smoke detector in the rear lavatory sounded, the "smoke toilet" message was displayed in the cockpit, and a strong odor of smoke was present. The captain declared an emergency and returned to Austin-Bergstrom International Airport. According to a statement provided by the flight attendants, after a safe and uneventful landing, the captain ordered an emergency evacuation utilizing the airplane's four exits. The flight attendants then assisted the passengers in the emergency evacuation. One Spanish speaking passenger remained, and a flight attendant instructed her to "exit her way." Instead, the passenger exited the airplane through a window exit and fractured a bone in her right foot. Company maintenance personnel examined the airplane and "found that an air conditioning pack that had failed," which would have resulted in smoke filling the cabin. The Liebherr Air Cycle Machine, part number GG670-95009-1 was replaced, and a maintenance operational check was performed by maintenance technicians. No further evidence of smoke was observed in the cabin.