On Wednesday night, Birmingham emergency responders and hazardous material teams responded to a chemical spill from an 18-wheeler on Interstate 59/20 at the Arkadelphia Road exit.

There are conflicting stories of what caused the chemical spill. Birmingham police spokesman Lt. Sean Edwards said two 18-wheelers were traveling northbound when one of them hit the other and kept driving away from the scene. The trailer that was hit was carrying several hazardous chemicals that were knocked loose in the wreck.

Birmingham Fire and Rescue Battalion Chief C.W. Mardis later said there was only one truck involved and that the driver might have hit something that knocked the chemicals loose. He said the driver, who was uninjured, noticed the smell and discovered a container was leaking.

Edwards said sulfuric acid, hydrogen peroxide and sodium hydroxide were inside the truck, making the spill a public safety hazard. Officials learned that toluene was also among the chemicals. Hazmat teams were called in to contain the spill. The Jefferson County Emergency Management Agency and state authorities also responded.

The truck caught fire sometime after the crash. It burned for more than an hour before Birmingham fire crews got it out. They do not yet know what ignited the fire.

Traffic was backed up on I-59/20 after an 18-wheeler wrecked near the Arkadelphia Road exit on Dec. 9, 2015. Hazardous chemicals were spilled in the wreck. (Jonathan Grass/jgrass@al.com)

The fire had to be contained before hazmat operations could resume.

The area was evacuated for about a half-mile radius around the scene. People were asked to avoid the areas or stay inside.

Edwards said nearby citizens were asked to go to Boutwell Memorial Auditorium.

Authorities believe the cleanup could take several hours.

Officials warn that all of the chemicals involved are dangerous to breathe. Mardis said the chemicals can dissipate in the air but the concentrated levels in the area can create a public safety issue.

I-59/20 is still closed off in both directions, which caused heavy traffic congestion Wednesday night. Northbound traffic is being diverted away to the Arkadelphia Road exit. Southbound traffic is being diverted to Interstate 65.

Fire officials said the cleanup should be completed overnight. I-59/20 should be reopened by Thursday morning.

Birmingham-Southern College reports the campus was not affected.