A chemical spill injured dozens of people, some seriously, and forced hundreds to evacuate from a western Oklahoma city on Wednesday night, according to local officials. Evacuation orders were lifted Thursday morning.

The incident occurred around 10 p.m. local time in Weatherford, Oklahoma, where an 8,500-gallon tanker truck began leaking toxic ammonia, Weatherford Police Chief Angelo Orefice told CBS News on Thursday morning. Responding crews stopped the leak, he said, and the airborne chemical had dissipated to a level deemed safe for people to return.

The leaking truck was parked in the lot outside of a Holiday Inn Express hotel, said Weatherford Emergency Manager Mike Karlin at a news conference streamed by CBS affiliate KWTV. When first responders arrived at the scene, they found people in respiratory distress and exiting the hotel, according to the emergency manager.

"As the event unfolded, we had a large plume of anhydrous ammonia gas spread over a pretty large area," Karlin said, estimating that between 500 or 600 people were displaced and staying in makeshift shelters, after being evacuated from the hotel as well as their personal residences. Orefice said the number of evacuees was slightly lower, between 300 and 500, although he acknowledged that the situation at that time remained in flux.

Both the police chief and emergency manager said approximately 36 people sought medical care at hospitals in the aftermath of the leak. Orefice told CBS News that responders transported 11 people to a regional hospital and flew four whose conditions were considered critical, to a hospital in Oklahoma City.

Southwest Oklahoma State University, where some evacuees were staying, announced that in-person classes would be canceled Thursday because of the ammonia leak.

"First responders on scene have recommended that all students in SWOSU on-campus housing shelter in place due to the fumes. Students in residence halls should remain indoors, keep windows and doors closed," the university said in a statement.  Weatherford Public Schools announced closures throughout the district as well.

A public safety notice issued by the Weatherford Police Department on Thursday morning asked businesses to remain closed and residents of certain neighborhoods to shelter in place until further notice.

Ammonia is a clear, foul-smelling gas or liquid that occurs naturally but is also commonly used in fertilizers and pharmaceutical products, according to the U.S. Centers for Disease Control and Prevention. Exposure to normal amounts of ammonia do not harm humans, although higher levels can hurt the eyes, skin, throat and lungs, potentially causing coughing and burns, the health agency says.

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