ATLANTA - Potentially lethal carbon monoxide levels in an Atlanta elementary school sent 42 students and six adults to hospitals Monday amid the evacuation of about 500 students, authorities said.

A teacher and a cafeteria worker were among patients treated after firefighters responded to Finch Elementary School shortly after 8 a.m., Atlanta Fire Capt. Marian McDaniel told the Associated Press.

Firefighters were initially told that people were unconscious at the school, but none were when fire crews arrived. All of the patients were conscious and alert as they were being taken to hospitals, McDaniel said.

Video on WSB-TV showed rescue crews carrying young children strapped to gurneys with oxygen masks on their faces.

Firefighters detected unsafe levels of carbon monoxide near a furnace at the school, on the city's southwest side, McDaniel said. She called the reading - 1,700 parts per million - extremely high.

"It was one of the highest we've seen," she said.

The colorless, odorless gas can be deadly at that level, one expert said.

"Seventeen hundred parts per million is potentially lethal if exposed to it for a period of time," said Stephanie Hon, assistant director of the Georgia Poison Center.

Children could be affected faster than adults and are generally affected at lower parts per million, Hon said.

She said it's easy for initial symptoms of carbon monoxide poisoning to be confused with the flu since both include malaise, headache, nausea, and vomiting. A few key differences: Carbon monoxide poisoning generally does not cause a fever, and a person usually starts feeling better once he or she has been moved to an area with fresh air, Hon said.

Hon said it was fortunate that the children did not appear to have suffered severe symptoms and said that was likely due to a short exposure time and perhaps the leak being some distance from where the children were.

"The good news is that they sound like mild to moderate symptoms," Hon said. "Luckily those kinds of exposures do not carry significant long-term health risks, especially with the children involved."

It was not immediately known whether the school had a carbon monoxide detector. If it didn't, Hon said, "a carbon monoxide detector, if appropriately used and installed and checked often, could have very well been a warning for this school system."

An Atlanta Public Schools spokesman did not immediately return calls for comment.