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EDMONTON — Four people were rushed to hospital in critical condition and another person with non-life threatening injuries after they were all exposed to high amounts of carbon monoxide Sunday afternoon, AHS said.

Emergency responders were called to the home near 37A Avenue and 23 Street at around 4 p.m. Sunday, after a vehicle was left idling in the home’s garage.

The garage door was half closed as the car was left idling for about 40 minutes.

“They started the car in the garage and boosted the car,” said the homeowner, who didn’t want to be identified.

“CO [carbon monoxide] maybe entered into the house which they did not realize.” Tweet This

READ MORE: What you should know about carbon monoxide poisoning

A call was made to emergency services after five people inside the home began showing symptoms of carbon monoxide poisoning.

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“They were watching TV and sitting on the sofa. They felt little headaches,” the homeowner said. “One [person] was sleeping upstairs. They wanted to wake him. Then, they noticed he was unconscious. Then they got scared.”

A scary reminder of the dangers of carbon monoxide. 4 ppl in critical condition after leaving car idling in garage. pic.twitter.com/sQoI1hGJFT — Erin Chalmers (@ChalmersErin) December 14, 2015

One neighbour who was home when the emergency crews arrived saw two men being taken out of the house by EMS.

“The guy I saw on the stretcher is like white,” Juvy Romero said. “It’s very scary. It’s my first time hearing about this.”

According to the homeowner, four men live in the home, all between the ages of 22 and 27. She said three of them have since been released from the hospital and the other two remain in hospital as a precaution.

Officials have not provided an update on their condition.

READ MORE: Carbon monoxide: 5 things to know to protect your home

Firefighters described this call as one involving “significant carbon monoxide exposure.”

Platoon Chief Craig Menard said the case shows the importance of “common sense” safety.

“You should always remove the vehicle from the garage if you’re going to let it idle.” Tweet This

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This is especially true in attached garages. Even if the large garage door is open, gas can find its way into homes.

Home inspector Doug Fleshman said he often sees leakage problems in attached garages. Homeowners put holes into drywall or remove attached shelving units. The gaps left make it easier for all gases to enter the home,” he said.

“You definitely need to make sure walls attached to the home are drywalled, taped and mudded – at least one coat,” Fleshman said.

Carbon monoxide can be particularly dangerous because it’s a clear, tasteless and odorless gas.

Both Fleshman and firefighters said homeowners should ensure carbon monoxide detectors are installed in their homes.

With files from Fletcher Kent, Global News