An explosion and asphalt fire at a Wisconsin oil refinery sent huge plumes of smoke into the air that pollution experts said almost certainly contained large amounts of toxins, posing a serious health risk to those living downwind.

Asphalt is a petroleum product that when burned emits chemicals in gaseous form and small particles that can linger long after the smoke dissipates, said Wilma Subra, a chemist with the Louisiana Environmental Action Network who has examined past refinery accidents.

The gases include so-called volatile organic compounds, or VOCs, which can cause symptoms ranging from dizziness, breathing problems and nausea to liver damage and cancer, depending on the level and length of exposure, according to the U.S. Environmental Protection Agency.

Also present in asphalt smoke are microscopic particles of chemicals that stick together as visible smoke.

Those particles carry cancer-causing benzene and other contaminants that can lodge deep in the lungs when inhaled. From there, they can pass directly into a person's bloodstream, said Neil Carman, a former refinery inspector for the Texas Commission on Environmental Quality, now with the Sierra Club.

"Anybody breathing that stuff should be very concerned about what's getting into deep tissue, into the bloodstream," Carman said. "When you see that kind of smoke, it means you're getting a lot of unburned hydrocarbons. ... Those particles are loaded with carcinogens."

Officials ordered an evacuation of a wide area around the Husky Energy refinery to reduce the public's exposure to the plume. The fire Thursday was later put out, and residents were told they could return to their homes. But then authorities announced the evacuation order would remain and be re-evaluated throughout the night.

Government agencies planned to conduct air monitoring tests to gauge the hazard, refinery manager Kollin Schade said during an evening press conference.

The duration and extent of the toxic hazard depends on a variety of factors, such as wind direction and speed, proximity to the refinery and weather events that can trap pollution close to the ground, said Elena Craft, senior health scientist for the Environmental Defense Fund.

Federal officials did not immediately respond to questions about the health risks from the smoke. A spokeswoman for the Wisconsin Department of Health Services referred questions to local officials in Douglas County, who could not be reached for comment.