Here in the Iraqi town of Qayyarah, in the desert south of Mosul, the sun is an orange orb burning through a screen of black and gray clouds that cover the sky. The sky at midafternoon is the color of dusk, and the air is painful to breathe. Retreating ISIS fighters have set fire to oil wells as well as a sulfur plant in the area, sending up plumes of toxic black and white smoke that blanket the sky and burn the lungs. Some of the oil fires have burned since last summer. On Friday, Islamic State fighters reportedly torched the Mishraq sulfur plant, north of Qayyarah. At least two civilians died from the effects of the gas. The attack on the sulfur facility came days after Iraqi forces launched an offensive to retake Mosul, threatening the largest city under Islamic State control. The toxic pall south of the city underscores the jihadists’ ability to render portions of Iraq unlivable even as their self-proclaimed caliphate contracts.