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 high and unsafe levels of carbon monoxide near a furnace at the school, which is 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 generally a person starts feeling better once he or she is moved to an area with fresh air, Hon said.
Hon said it was fortunate the children do not appear to have suffered severe symptoms and said that was likely due to a short exposure time and perhaps the location of 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 if 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.
A total of 42 students and six adults were taken to hospitals, Atlanta Public Schools said on its official Twitter site.
Other students from the school were taken to Brown Middle School for the rest of the day, and were being released to their parents, Atlanta Public Schools said in a statement on its official Twitter site.
Meanwhile, firefighters were ventilating the school in an attempt to reduce the amount of carbon monoxide, McDaniel said.
In Baltimore last year, school officials vowed to put carbon monoxide detectors in all of the school system's approximately 200 schools after two carbon monoxide leaks within a week's time at one of the schools.
City officials in Baltimore said the battery-powered detectors cost $15 each wholesale.