Nearly 50 people, including 42 students, were sent to the hospital after potentially lethal carbon monoxide levels were found at an Atlanta elementary school Monday, prompting some local schools to enhance safety measures.On Monday, firefighters found unsafe levels of carbon monoxide — as high as 1,700 parts per million — near a furnace at Finch Elementary School in southwest Atlanta. Those exposed to the gas can start having medical issues at 50 parts per million, according to Dr. Gaylord Lopez, director of the Georgia Poison Center.“When I talk about carbon monoxide in lectures, I always like to point out how it still remains the No. 1 killer of any poison each year,” he said.Over the last decade, Lopez said, the center has received more than 1,000 calls reporting carbon monoxide poisoning.