CALGARY -- Seven people were transported to the hospital Thursday afternoon after a carbon monoxide leak was discovered at a commercial building in northwest Calgary.

A technician responded to a call about 1:30 p.m. at a building in the 2500 block of Kensington Road N.W.  after an alarm was activated in the basement mechanical room.

The worker discovered it was a carbon monoxide alarm and immediately called 911.

Emergency medical services confirmed three people felt ill and left before fire crews arrived at the scene. They were all told to seek medical advice.

When Calgary firefighters arrived, they evacuated the building.

The CO readings from the gas detection device used by fire crews showed levels as high as 440 parts-per-million.

ATCO was called to the scene, and fire crews started ventilating the building to bring the carbon monoxide (CO) levels down.

According to fire officials, levels that high can become life-threatening after approximately three hours.

Officials believe the source of CO is from a problem with the ventilation system and furnace.

The Calgary Fire Department is reminding Calgarians to be aware of CO poisoning risks and to have a functioning carbon monoxide alarm in their place of residence.

Officials recommend testing CO alarms once-a-month and if your alarm uses batteries, be sure to replace the batteries once a year.

Symptoms of carbon monoxide poisoning include headaches, nausea and dizziness, and loss of consciousness.

Additional information about carbon monoxide safety is available on the Calgary Fire Departments website.