It was not the first time Chenjiagang Chemical Park had serious accidents. In late 2007, an explosion ripped through Jiangsu Lianhua Technology Co in the park, killing eight people and wounding dozens. In November 2010, chlorine leaked at another company in the park, Jiangsu Dahe Chlor-alkali Chemistry Co, and poisoned over 30 people nearby, according to media reports.

Li Chunhua, director-general of Green Stone Environmental Protection Center, an environmental NGO based in Nanjing, Jiangsu, said on Friday that they had started to look into the park in 2015 and found that not only Jiangsu Tianjiayi Chemical Co but also many other enterprises in the park had safety hazards and pollution issues including wastewater and exhaust gas emissions.

In 2018, the entire park was ordered to conduct a thorough overhaul and operations of all chemical companies were suspended over safety and pollution problems, Li said. After eight months, the problems were considered to be corrected and the factories were allowed to reopen.

Xinhua contributed to this story.