ISIS militants set ablaze sulfur stocks at a factory south of Mosul, the U.S. military said Saturday.

With noxious fumes brought by the wind, U.S. forces at Iraq’s Qayyara West airfield near Mosul were forced to wear protective masks as a precaution, U.S. military officials added.

The Qayyara West airfield is the main U.S. hub to support Iraqi-led operations to retake the city of Mosul from Islamic State. There are about 5,000 U.S. troops in Iraq but the U.S. military has not disclosed the number of personnel at the airfield.

“The winds have actually shifted south, so, as a precautionary measure, the troops at Qayyara West have donned their personal protective equipment – continuing their operations at this point in time,” an official said, speaking on condition of anonymity.

The main concern was avoiding potential respiratory ailments resulting from inhaling the fumes. Air samples from Qayyara West were sent to the U.S. Defense Threat Reduction Agency for analysis as to “what, if any concerns, may result,” the second official said.

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.

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.

The officials said ISIS set the sulfur plant ablaze two days ago during fighting around al-Mishraq, which is south of Mosul. Iraqi security forces estimate it will take 2-3 days to put out the blaze, the U.S. official said.