Summit Fire & EMS says it responded to two calls this week where carbon monoxide created life-threatening situations — including one call that resulted in five people being sent to the hospital.

“In both instances, we were extremely fortunate that those involved called 9-1-1, or our community may have suffered a horrible tragedy,” Summit Fire & EMS wrote in a Facebook post.

The first call came in Sunday, April 6, after a resident in a Silverthorne condominium complex awoke just before 5 a.m. to what they initially thought was a fire alarm, according to Summit Fire & EMS. Firefighters determined that the building’s fire alarms were not activated, and therefore the alarms did not trigger an automated 911 call, the post states. However, multiple carbon monoxide alarms, which do not automatically alert 911, had reportedly been triggered.

Some other residents in the condo complex who had also been woken up were trying to turn off their alarms, thinking that the alarms were not working properly, according to Summit Fire & EMS. But when firefighters used gas detectors they found potentially lethal levels of carbon monoxide and evacuated the entire building, the post states.

First responders transported three people to St. Anthony Summit Hospital in an ambulance and two others followed in a passenger vehicle, all suffering from mild symptoms of carbon monoxide poisoning, according to Summit Fire & EMS. The fire protection district said it determined that a rented hybrid vehicle that had been left in a powered mode in the underground parking garage was the likely cause of the carbon monoxide incident.

Then, on Monday, April 7, an employee at Arapahoe Basin Ski Area heard an alarm begin to sound as he was starting his usual process in the kitchen, the post states. The employee reportedly confirmed that it was a carbon monoxide alarm, turned on some of the hood fans in the kitchen to ventilate the space, advised everyone to get out of the building and then called 911.

Summit Fire & EMS said it determined that a malfunctioning cooking appliance had been generating 2,000 parts per million of carbon monoxide, “a lethal dose,” within seconds of being turned on.

“The overriding lesson: It is imperative that every building have working carbon-monoxide alarms that are tested regularly, and that the occupants heed the alarms and immediately get out of any place where one is going off,” Summit Fire & EMS wrote. “Carbon monoxide is a colorless, odorless gas that is produced by incomplete combustion — furnaces, stoves, vehicles and the like — and it can kill without warning by glomming onto red blood cells and blocking the transmission of oxygen to the body.”