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'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 FoxNews.com. 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.

FoxNews.com's Karl de Vries contributed to this report.