The images are startling. Billowing black clouds darken the daytime sky as wind-driven grit pelts homes and cars and forces bewildered residents to take cover.
The onslaught, captured in photos and video footage from Detroit and Chicago this year, was caused by the same thing: brisk winds sweeping across huge black piles of petroleum coke, or "petcoke," a powdery byproduct of oil refining that's been accumulating along US Midwest shipping channels and sparking a new wave of health and environmental concerns.
The piles are evidence of a sharp increase in North American oil production particularly crude extracted from oil sands in Canada that has been trapped in the Midwest because of limited pipeline capacity to carry it to the Gulf and West coasts, leading to unprecedented amounts of oil refining and petcoke production here.
In Midwestern neighborhoods near refineries, the growing black mountains have brought outcries from residents and new efforts by lawmakers to control or banish the blowing dust.
"We could barely open the windows this summer because the black dust was so bad," said Susanna Gomez, 37, a mother and grandmother who lives on Chicago's far southeast side, across a set of railroad tracks from a shipping terminal that stockpiles petcoke until it can be loaded on to ships for export.
She said she worries about one of her sons, who's asthmatic, but doesn't have the money to move.
Alan Beemsterboer, whose family owns another nearby site that long has handled slag, asphalt and coal, and now, increasingly, petcoke, said he doesn't understand the controversy.
"This has been an industrial area forever a coke plant used to be there, a steel mill used to be there," Beemsterboer said. "Coal and petcoke are just dirty words now."
Petcoke has been part of the American industrial landscape since the 1930s, when refineries began installing equipment to "cook" residue left over from making petrol and diesel into a solid fuel that could be burned in power plants and cement kilns.
But the sheer volume of petcoke that appeared suddenly in Detroit and Chicago this year almost all of it in open-air piles was unprecedented, and caught residents and public officials off guard.
"With the amount of Canadian oil entering the US increasing almost daily, refineries like Marathon in Detroit, BP in Whiting, Indiana, and Phillips 66 in Roxana, Illinois, have expanded to handle the glut. Even more oil could be on the way if the controversial Keystone XL pipeline is approved, though by then additional domestic pipelines could direct some to refineries in other regions, experts say.
Refineries usually sell the petcoke to other companies, which store it until it can be loaded onto Great Lakes ships for export to places like China. Burning it emits high levels of soot and greenhouse gases, so its use in the US is limited.
In Detroit, petcoke began appearing along the Detroit River in the spring, several months after the Marathon Oil refinery completed a $2.2 billion expansion. But an outcry by residents, who shot video footage of the blowing grit, prompted city officials to order the removal of the piles.
In Chicago, residents became alarmed when the black piles began growing about six months ago, said Tom Shepherd, a member of a neighborhood group. The last straw was when the petcoke went airborne on Aug. 30 and blew into their yards, churches and a Little League field.
Chicago Mayor Rahm Emanuel has ordered the city Health Department to adopt regulations for petcoke, while aldermen introduced competing ordinances to regulate or ban it outright. The city and Illinois Atty. Gen. Lisa Madigan have filed suit against Beemsterboer over the petcoke on his sites.
"You have a byproduct ... that is economically and environmentally degrading a community and the health for our children, and there's nothing on the books that allows Chicago to protect its own citizens," Emanuel said during a news conference.
Although petcoke is not classified as hazardous, it contains heavy metals and inhaling the fine particles can cause respiratory problems.
Shepherd said residents worry because "there's already lot of asthma, cancer and other illnesses around here."
Chicago's southeast side is an area where heavy industry and working-class residents have coexisted for generations one neighborhood is even called Slag Valley. Immigrants came to work in the steel mills along Lake Michigan and many families stayed after the mills began closing.
They now dream of a renewal, including a city plan for a huge park at a former steel mill site, but worry that petcoke will "overtake the entire area and we'll have nothing but black mountains for miles and miles along the river," Shepherd said.
The stage is set for more. Petcoke production at the BP refinery is expected to triple next year, from 700,000 tons a year to 2.2 million tons, after a $4.2 billion upgrade scheduled for completion next month, spokesman Scott Dean said.
The company that handles BP's petcoke storage, KCBX, said it's spending more than $10 million on upgrades, including improved "dust-suppression capabilities."
But Emanuel spokesman Bill McCaffrey said the mayor's is considering requiring that piles be completely or partially enclosed. Some states, such as California, have such requirements.