A carbon monoxide leak at a Wal-Mart Supercenter in Laurel led to the evacuation of about 250 shoppers and employees Sunday morning, according to Anne Arundel County fire officials.

Emergency personnel first responded to a call around 10:30 a.m. to help an injured employee who had fallen at the store, officials said.

While treating the employee, a carbon monoxide detector on one of the paramedic’s bags sounded an alarm. Subsequent measurements found levels as high as 44 parts per million in the store.

“We start to get concerned any time it’s above 9 parts per million,” said Lt. Russ Davies, a fire department spokesman.

Emergency personnel evacuated the building around noon and aired it out, said Kayla Whaling, a Wal-Mart spokesperson. They identified a heating unit on the roof as the likely source of the leak, she said.

The store reopened Sunday at 1:15 p.m. after fire department officials said it was safe to resume business, she said. The heating unit is being repaired, she said.

Carbon monoxide is an odorless, colorless gas. Exposure can cause flu-like symptoms such as nausea and dizziness or loss of consciousness or death.