A United Airlines DC-10 en route to Seattle lost cabin pressure Saturday and safely returned to Stapleton International Airport with passengers using oxygen masks.

United spokesman Rob Doughty said there were no injuries.However, four passengers were treated at two Denver hospitals for chest pains, hypertension and lightheadedness, and released by late afternoon, hospital officials said. Also, paramedics treated several passengers at the airport who were shaken up by the incident.

"We have determined the cause was a circuit breaker (on a unit) that controls the pressurization of the cabin," Doughty said. "We have taken that aircraft out of service even though we know the problem. We want to make a thorough investigation."