The incident occurred around 8:40 p.m. local time [Friday, September 29] on U.S. Highway 40 less than one mile east of the village of Teutopolis, Illinois, authorities said. A semi-truck transporting anhydrous ammonia rolled over, causing a "large plume cloud of anhydrous ammonia on the roadway that caused terribly dangerous air conditions in the northeast area," Effingham County Sheriff Paul Kuhns told reporters during a Saturday press briefing. About 500 people have been evacuated in the wake of the crash, with the evacuation zone approximately 2 square miles on the east side of Teutopolis, authorities said.