Images shared online showed parts of Hengshanqiao—a town of around 100,000 people in Changzhou, Jiangsu—shrouded in mist at roughly 7 a.m. local time, following what some described as a “chemical accident.”

Unsuspecting residents were left driving or walking through the substance, which local media reports said caused shortness of breath and stinging in the eyes. Meanwhile, uniformed firefighters in gas masks were pictured arriving to assist in the aftermath.

The local government later said in a statement that workers at a “derelict plant” had accidentally damaged a tank containing “disinfectant” while operating in an area belonging to utility company Changzhou CGE Water.

Communist Party-owned People’s Daily said the “chlorine leak” happened across a canal from Hengshanqiao Central Elementary School, the teachers and pupils of which were quickly sent home or to a nearby hospital.

“The situation has been dealt with. Those with discomfort were taken to hospital,” a government spokesperson told Zhengzhou’s online news service Zhengguan, without elaborating.

Wang Shijun, a member of the Changzhou fire department, told the state-owned newspaper People’s Daily that the leak can come from a “metallic container” roughly 100 liters (26 gallons) in volume. However, it was unclear how much chlorine was left in the disused tank at the time of said accident.

Wang said the incident had caused “yellow-green” chlorine gas to spew out into the air. First responders buried the tank and sprayed the surrounding area with a neutralizing agent, he added.

On social media application Hualongxiang, local residents posted images of the chlorine cloud as it engulfed parts of town. Parents said their children complained of “stinging in the eyes.”

Videos posted to Weibo, China’s equivalent of Twitter, showed pupils vomiting inside classrooms on the morning of the incident.

Zhang Rongqing, one of the elementary school’s administrative staff, told Nanjing-based Yangtse Evening Post that he was among the last to leave the area. He had stayed to ensure children and faculty members got out, he told the newspaper.

Zhang, who was given oxygen treatment at Hengshanqiao Health Center, said he suffered from coughing and discomfort in the throat.

According to local media reports, the scare appeared to be over by noon. However, it was unclear how many residents were given medical attention or whether there were any serious injuries.