LITTLETON, Colo. (CBS4) – Carbon Monoxide filled a Littleton home Tuesday, sending 10 people to the hospital. Firefighters say if it had happened at night they could have all been dead.

Five adults and five elementary school aged children were sent to St. Anthony’s hospital where they are reportedly all doing well and recovering.

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Firefighters tell CBS4 the levels of carbon monoxide found inside the home were high enough to be fatal.

“It looks like it was grandparents mom and dad and a sleepover with the girls,” said neighbor and family friend, Heidi O’Leary. “Everyone was sick.”

Just after 11 a.m. 10 people inside the home slowly started to get sick.

“One of the little girls called her dad this morning and said she was getting sick and she didn’t feel well,” adds O’Leary. “Her dad came over and he came in the door it sounded like everyone was throwing up. They kind of put two and two together and said maybe something else is going on here.”

Headache, nausea and dizziness are all classic symptoms of carbon monoxide poisoning.

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“We definitely had elevated readings within the home,” says Travis Hopwood of the West Metro Fire Department. “We were over 250 parts per million which is definitely fatal.”

The Finnigan family did have a detector in their home but it was their symptoms that alerted them to the poisoning. Their detector was broken and the alarm did not sound.

“If that had been like an overnight we could have had fatalities,” adds Hopwood.

“It sounds like everyone is having their blood rechecked and making sure everybody is okay at the hospital,” O’Leary says.

Neither firefighters nor Xcel Energy have found the source of the leak. Until they do the Finnigan family will likely have to find somewhere else to sleep.

Firefighters say to make sure you have a working carbon monoxide detector on each floor of your home and replace each detector every seven years.

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– Written by Nina Sparano for