( CNN ) Passengers aboard a United Airlines flight from Chicago to Denver got sick from a cabin pressure issue that forced a quick landing , according to an airline spokeswoman . United Airlines Flight 1218 took off at 9:01 a.m. , but shortly after it reached 15,000 feet , the pilot took it below 10,000 feet and returned to O'Hare International Airport , according to tracking website FlightAware.com . Below 10,000 feet is a safe altitude for an unpressurized plane . `` The flight landed safely , and we requested paramedics meet the flight to assist a few customers who reported feeling ill , '' said United Airlines spokeswoman Karen May . Seven ambulances met the plane and 175 patients complaining of ear pain or nausea were checked out , according to the Chicago Fire Department . No passengers or crew were taken to the hospital . `` Maintenance is inspecting the aircraft as we accommodate the customers , '' May said in the statement . FAA officials are also investigating the circumstances of the landing . The plane , a Boeing 737-900 , is almost brand new . Records show it was delivered to United in March of last year . As part of the manufacturing process each Boeing aircraft must withstand a two-hour long pressurization test , called a `` high blow . '' The cabin is pressurized to nearly 15 pounds per square inch , well above the normal operating levels , to confirm it properly seals . CNN 's Shawn Nottingham and Thom Patterson contributed to this report .
A United flight returned to Chicago shortly after takeoff . The plane was carrying about 100 people from the University of Illinois at Urbana-Champaign and a group of students who were on their way home for spring break. No one was transported to a hospital .