By Margaret Anderson

What’s a traveler’s worst nightmare? Is it a sudden drop in cabin pressure with oxygen masks falling like dangling jellyfish above the passengers’ heads, or is it no masks at all?

That’s just what happened Tuesday morning when a United flight made an emergency landing at O’Hare International Airport after a pressurization issue sickened passengers.

The plane’s oxygen masks did not deploy, said Karen May, a public relations manager for United Airlines.

Will Knight, a spokesman for the Chicago Fire Department, said 176 people were checked after complaining of ear pain and nausea. No passengers or crew were hospitalized.

United 1218 departed O’Hare for Denver at 8:40 a.m. with 174 passengers and seven crew members on board. The plane, a Boeing 737-900, was forced to return to Chicago an hour later after passengers began to feel sick, May said.

Paramedics boarded the airplane within five minutes of landing and no passengers or crew were taken to the hospital, Knight said.

Air pressure inside an airplane cabin is controlled by mixing outside air with recirculated air from the cabin, a system that also regulates airflow and temperature, according to Boeing.

Like soda bubbles appearing when a bottle is opened, when airplane cabin pressure drops, nitrogen, normally dissolved in bodily fluids, turns back into a gas. The phenomenon can cause ear pain, dizziness and what is commonly referred to as “the bends,” according to the Federal Aviation Administration.

According to May, the United flight left again for Denver a few hours late after paramedics checked passengers’ health and United Airlines maintenance inspected the aircraft.