People in the area affected by the smoke said it was difficult to breathe, burned their eyes and stung their noses and throats when they inhaled it, and burned any exposed wet skin as the cloud intermittently blew over the area depending on winds.

Sulfur dioxide from burning stocks is highly toxic and can be lethal. It can also quickly cause shortness of breath and coughing, according to a chemical fact sheet from Sultran, a Canadian rail company that ships large quantities of sulfur.

Militants set the residue alight at the Mishraq sulfur plant on Thursday as a tactical measure to slow Iraqi military advances in the offensive to recapture Mosul Islamic State’s last remaining stronghold in Iraq.

The toxic cloud mixed with choking black smoke already filling the air from oil well fires started two months ago and still burning in the town of Qayara.

The US-led coalition provided approximately 24,000 gas masks to Iraqi and Peshmerga forces in the run up to the Mosul operation, according to a statement from the coalition.

“The enemy has used chemical weapons in the past,” said Maj. Gen.Gary J. Volesky, a top general in Iraq. “Force protection is my number one priority here.”

On Saturday afternoon, Iraqi state television reported that specialized teams put the fire out, though some smoke was still coming from the stocks. U.S. military officials said the fires had not yet been extinguished, a process that could take two to three days.

ISIS set alight a handful of oil wells in Qayara two months ago and they have been burning steadily since then. The town now lives under a black cloud and a slurry like liquid runs in the streets.

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