Updated 02/03/15 – 12:04 p.m.

CHICAGO (CBS) — A flight from Chicago to Denver was forced to return to O’Hare International Airport on Tuesday, after multiple passengers became ill due to pressurization issues.

United Airlines Flight 1218 left O’Hare around 8:40 a.m., headed to Denver, but was diverted back to O’Hare, and landed around 9:40 a.m.

United representatives said a pressurization issue caused some crew and passengers to experience discomfort. Chicago Fire Department paramedics treated the passengers after the flight returned to O’Hare. United said initial reports indicated three or four passengers complained of discomfort.

United spokeswoman Karen May said there were 174 customers and seven crew members on the flight. Five crew members and two passengers complained of ear pain and nausea from the pressurization, according to the Fire Department.

Paramedics checked the sick passengers, while the five crew members were examined at an airport clinic. None of the passengers or crew needed to go to a hospital.

Maintenance crews were inspecting the plane, a Boeing 737-900, while United was making accommodations for the passengers to get to Denver on other flights.