An orange-brown cloud that hung in the sky above Gillette on Monday afternoon was caused by blasting at Caballo coal mine south of Gillette.
The cloud formed after blasting at 2:20 p.m., said Charlene Murdock, a spokeswoman for mine owner St. Louis-based Peabody Energy Corp.
“Emissions that occasionally arise from blasting normally rise and dissipate, yet in (Monday’s) instance, the emissions lingered east of Gillette before dissipating,” she said. “This is an isolated and rare incident for Peabody Energy and we are working closely with the Wyoming Department of Environmental Quality to investigate its nature and cause.”
The cloud prompted a warning to residents of the Nickelson’s Little Farms and Sleepy Hollow neighborhoods to stay inside. Additionally, residents in Campbell County with sensitive respiratory systems or ailments such as asthma or COPD were told to stay inside and close their doors and windows.
“It’s not poisonous,” said David King, Campbell County emergency management coordinator. “It’s not toxic, but it could have long-term health effects with repeated exposure. I’m not trying to downplay it but I don’t want to give people a heart attack.”
The cloud, which contained nitric oxide, was formed after Caballo mine blasted either dirt or coal, King said.
“It forms normally from incomplete combustion of ammonia nitrate used in blasting,” King said.
Mines blast layers of dirt over the coal seam, called “overburden,” to make it easier to get to the coal.
In the seam, mines will fracture coal through blasting.
The overburden blasts take longer to set up, which can expose the blasting chemicals to moisture and dirt. Most of the nitric oxide clouds are from overburden blasts, but it’s unclear whether Monday’s blast was an overburden blast.
The calls to the public began at 3 p.m.
The cloud moved north but by sunset it had either dissipated or could no longer be seen, King said.
Such clouds, which occasionally appear above Campbell County due to mining, typically sit low. Monday’s lack of wind kept the cloud in tact for more than an hour.
“Ultimately, it’s one of those chemical combinations that is heavier than air,” King said.
The Wyoming DEQ is looking into the incident, agency spokesman Keith Guille said.
“We do our own investigations any time this happens,” he said.