Story highlights

A United flight returned to Chicago shortly after takeoff

175 passengers complaining of ear pain and nausea were checked out by medics

No one was transported to a hospital

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

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.