The incident was reported around 10:30 a.m. inside a business in the 300 block of Market Street.
Authorities say a welding crew was working in one of the buildings. Their welding equipment was powered by a gas generator that was inside the building.
That generator was letting out the carbon monoxide but it couldn't escape due to poor ventilation.
The workers shut down the generator.
Firefighters arrived and found the CO levels were below dangerous levels.
Three apartments were evacuated but those residents are now allowed back into their homes.