'Pressurization issue' causes United flight to return to O'Hare

A "pressurization issue" reportedly sickened half a dozen passengers and caused a Denver-bound flight from Chicago's O'Hare International Airport to turn back on Tuesday afternoon.

United Flight 1218 departed at 8:41 a.m. central time, but returned to O'Hare about an hour later after the issue was discovered, spokeswoman Karen May told She said maintenance crews were investigating Tuesday evening.

The Boeing 737-900 carried 174 passengers and seven crew members. No one aboard was hospitalized, the airline said in a statement, through The Chicago Tribune reported that some passengers were treated for ear pain and nausea.

May said the passengers were placed on another Denver-bound flight.'s Karl de Vries contributed to this report.