The plane was diverted to Minneapolis with a dozen people struggling to breath.
Four crew members were hospitalised and another six were examined after the landing.
Two passengers were also examined at the airport, the airline confirmed in a statement.
The source of the smell on the Boeing 737-900, which had 144 people on board, has not been determined.
Alaska Airlines said: “Alaska Airlines Flight 51 departed Chicago O’Hare for Seattle at 5:48 p.m. local time. During the flight, the crew reported a cabin odour.
“As a precaution, the aircraft diverted to Minneapolis-St Paul International landing at 7:17 p.m. local time.
“Six crew members received a medical evaluation at the airport and four were transported to the hospital for further evaluation. Two passengers were also evaluated at the airport. The cause of the odour is being investigated and the plane has been removed from service pending an inspection.
“The flight has cancelled and passengers will be rebooked on a different flight to Seattle.”