Imperial Oil is investigating what caused a diesel spill at its Sarnia plant.

According to a company spokesperson, diesel product leaked as "a fine mist" outside a plant early Thursday morning.

The diesel then fell onto the nearby roadways.

"This event ... occurred during turnaround activities which are maintenance activities that are done regularly on the refinery," Imperial Oil's Pius Rolheiser said from Calgary. "It is estimated that one barrel of diesel product was released in the form of a mist."

The incident occurred between 1 a.m. and 2 a.m.

"From our perspective, the incident was quickly controlled and isolated," Rolheiser said. "The health and safety of the public was never jeopardized," Rolheiser said.

Sarnia Police were dispatched because the roadways were unsafe for travel, Sarnia Police spokesperson Const. Heather Emmons said.

Rolheiser says downwind air monitoring results showed nothing to indicate any health risk to the public or employees.

"We regret when incidents like this happen," Rolheiser said.