At the height of the Husky Energy oil refinery fire Thursday, as a massive plume of smoke billowed into the sky for miles, Superior Mayor Jim Paine was asked by reporters if the air was safe for people.

Yes, the mayor said, later repeating his claim.

While Paine may have been right that the air at his location upwind of the fire was safe, multiple experts say the black plume of smoke from the refinery fire was almost certainly full of toxic fumes and human carcinogens.

In fact, it's the smoke plume, and potentially toxic gas plumes, that spur pre-planned evacuations for miles downwind of refineries - like what happened in Superior Thursday - when a major fire breaks out, said Neil Carman, a former refinery inspector for the Texas Commission on Environmental Quality and now with the Lone Star chapter of the Sierra Club.

Kollin Schade, manager of the refinery, said it was asphalt pouring out of a punctured tank that was causing most of the fire and smoke Thursday. Asphalt is one of the major petroleum products produced at the Superior refinery.

When burned, asphalt not only emits toxic gases and particulate matter, but the smoke can contain small particles that can linger after the smoke dissipates, Carman said.

The U.S. Environmental Protection Agency says burning asphalt 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.

"Everything in there (the smoke) is toxic chemicals. The unburned chemicals in the smoke are full of polycyclic aromatic hydrocarbons ... they have benzene in them,'' Carman said. "They are very nasty chemicals. They are human carcinogens."

The tiny particles can lodge deep in the lungs when inhaled. From there, they can pass directly into a person's bloodstream, Carman said.

"It's microscopic stuff,'' Carman said. "Even where the plume looks like it's dissipating, these little particles are still out there. They are so microscopic that they easily move into the lungs."

How long the toxic danger lasts during and after any fire depends on wind direction, distance from the refinery, weather and other factors, experts said.

David Morrison of the EPA said the agency and Husky Energy employees were monitoring air quality during the fire using equipment on crews at the refinery, most of whom were not in the smoke plume. They also tested air quality at several locations taken in and near Superior - after the fire was out - and found no unsafe levels of particulate matter, hydrogen sulfide or volatile organic compounds, leading city officials to cancel the evacuation notice.

"There were very low, trace levels, well below any health standards, of volatile chemicals and dust and particulates," Morrison said.

But Wilma Subra, a chemist and technical adviser for the Louisiana Environmental Action Network, said polynuclear aromatic hydrocarbons can remain in soil, on grass, on homes and in water long after it has settled out of the air - sometimes for years after events like fires and train derailments.

"People who inhaled it and absorbed through their skin during the fire should have concerns ... But they also are going to keep having contact with all this residual material, as they garden or mow the lawn. ... And two weeks from now they are going to be talking with their neighbors about how sick they are,'' Subra said. "By the time they (EPA) took the air samples, the unburned hydrocarbons were already out of the air. They should be testing the property downwind where the smoke deposited."

Elena Craft, senior health scientist for the Environmental Defense Fund, said monitoring after the fact may not reveal earlier issues. Craft said the smoke plume could have been a serious issue for people with breathing ailments, like COPD or asthma, and for the elderly and children.

"It's one thing to say the air is safe. But, really, did they have any monitoring results from the plume at the time to show that?'' Craft said. "And not only is there concern about the smoke plume you can see, but there is pollution you can't see that can harm you."

Craft said there may be environmental damage lingering long after the fire was put out. While volatile organic compounds dissipate quickly, polyaromatic hydrocarbons can persist in the environment.

"This stuff is going to wash out somewhere," Craft said. "You could see some downstream impacts."

Toxic gas potential

In addition to the toxic smoke from asphalt burning, officials were extremely concerned Thursday that the fire would spread to a tank of hydrogen fluoride, a federally regulated toxic chemical that some public safety groups say is inherently dangerous.

The Superior refinery is one of about 50 nationally that still uses hydrogen fluoride to process high-octane gasoline. An acid catalyst, hydrogen fluoride is one of several federally regulated toxic chemicals at the refinery, such as propane and butane. The refinery can handle about 78,000 pounds of hydrogen fluoride, according to federal EPA records from 2012.

Schade, the refinery manager, would not answer specific questions on hydrogen fluoride Thursday, only saying its presence at the refinery was one reason evacuation precautions were underway. The fire never reached the hydrogen fluoride, and on Friday Schade said the hydrogen fluoride tank was "not compromised whatsoever" by the explosions or fire.

A Superior Fire Department official on Thursday said having the fire spread to the hydrogen fluoride tank would be the worst-case scenario, with other experts saying the fumes could spread a toxic cloud of gas for miles downwind.

"It's a deadly chemical. A lot of things can burn your skin, but this stuff can go through your skin,'' Carman said. "It's the worst-case scenario chemical in every refinery that uses it. It's what sets the parameters for evacuations. It's basically a kill zone that can go out several miles. ... And the thing is, they don't have to use it. There are other options, like sulfuric acid, which is not nearly as deadly. It won't kill you if it's released like hydrogen fluoride will."

According to the Centers for Disease Control and Prevention, hydrogen fluoride is a highly dangerous gas, forming corrosive and penetrating hydrofluoric acid upon contact with moisture. On any exposed skin, it immediately converts to hydrofluoric acid, which is corrosive and toxic and requires immediate medical attention upon exposure, the CDC said. Breathing in hydrogen fluoride at high levels, or in combination with skin contact, can cause death from an irregular heartbeat or from fluid buildup in the lungs. The gas can also cause blindness by rapid destruction of the corneas.

The EPA's fact sheet on hydrogen fluoride says even short-term inhalation "can cause severe respiratory damage in humans, including severe irritation and lung edema." Long-term exposure "has resulted in skeletal fluorosis, a bone disease." In high doses, hydrogen fluoride can cause convulsions and death from irregular heartbeat, according to the agency, and when exposed to water it is "one of the strongest acids known."

An EPA report from 1993 said a vapor release "could pose a significant threat to the public, especially in those instances where hydrogen fluoride is handled at facilities located in densely populated areas." Twenty-five years later, about a third of American refineries still use the compound. The Center for Public Integrity said refineries using hydrogen fluoride put a combined 16 million people at risk.

"It's like chlorine gas. It's an extremely toxic gas cloud that can move for miles downwind," Fred Millar, a Washington, D.C.-based independent consultant and activist on refinery toxicity issues,

told the News Tribune. "If your local officials aren't explaining how concerned they are about that, then they should be. It would be a disaster. That's what the evacuation (distances) should be based on."

News Tribune staff writer Brooks Johnson contributed to this story.