On December 20, 2008, about 0747 Alaska standard time, the cabin crew of Alaska Airlines Flight 73, N771AS, a Boeing 737, reported to the captain that they became nauseated after departing from Sitka, Alaska. The flight was being operated by Alaska Airlines, Seattle, Washington, as an instrument flight rules (IFR) passenger flight under Title 14, CFR Part 121, when the event occurred. The flight departed Sitka for Juneau, Alaska about 0710, and an instrument flight plan was filed and activated. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on December 21, the acting director of flight safety for the operator said he requested a written report from the Captain, and that he was en route to Sitka to investigate the event. The IIC discussed with the acting director testing the external heater for leaks, and the placement of other equipment near the heaters air intake. He said he would report back to the IIC after assessing the situation. In a written statement submitted to the NTSB on December 22, the captain reported that on the morning of the event the crew met early, as the departure from Sitka was the first flight of the day. The airplane had been sitting on the ramp overnight and was connected to an external heating unit. The captain said the first officer and cabin crew went to prepare the airplane while he remained in the operations center to complete the required paperwork for the day's flights. He said he overheard the first officer request the ground crew shutdown the external heater because of exhaust fumes in the airplane's cabin and the jetway. The captain wrote that the first officer remained on the flightdeck long enough to start the airplane's auxiliary power unit (APU) to provide power and heat to the airplane. The first officer and cabin crew returned to the operations center. The captain said about 15 minutes later he went to the airplane to evaluate the condition of the airplane. He said he tried to be cognizant of unusual smells/odors, and wiped his hand across seats and surfaces to detect contamination. Finding none, he felt it was safe to continue boarding for the flight. He reported that the boarding process was normal, and none the crew or passengers reported any unusual smells/odors. After departure the captain reported that the cabin crew reported feeling nauseated. The captain said he consulted with the cabin crew and first officer, and determined that since they were 20 minutes from their destination (Juneau) he would continue the flight rather than turn back to Sitka. The captain reported that he and the first officer were symptom free, and that the first officer remained on oxygen for the remainder of the flight as a precaution. Upon arrival at Juneau the flight crew and one passenger, an off-duty airline employee, were taken to the local hospital to be tested for carbon monoxide (CO). The captain said he was told that the normal levels for carbon monoxide in the blood are 0.0-0.08 percent. He reported that the crew's levels were from 1.8 percent to 4.7 percent. According to the Center for Disease Control (CDC), elevated levels of carbon monoxide in the blood of 2.0 percent for non-smokers and 9.0 percent for smokers, can support a diagnosis of carbon monoxide poisoning. About an hour after landing the cabin of the airplane was examined by the fire department, and no traces of carbon monoxide were found. On February 27, 2009, the acting director submitted his findings to the IIC. Tests of the heating unit found no anomalies, and the heat output hose was tested for the presences of CO, with negative results. He said he believed the contamination occurred when the morning ground crew started the push-back unit in the proximity of the heater's fresh air intake. , According to the acting director, as a result of this event, the heating unit is now repositioned to the opposite side of the aircraft away form other equipment, and a 20-foot exhaust hose is attached. No other operating equipment is allowed in the vicinity of the heater. Carbon monoxide detectors are now placed in the operator's aircraft during extended external heater usage.