PHOTO: AFP
Sparks from a pipe being welded at Qilu Tianhe Huishi Pharmaceutical Co in eastern Shandong province caused a substance to catch fire and give off the smoke, state news agency Xinhua reported.
Twelve rescue workers were hurt but were not in life-threatening condition, it added.
Qilu Tianhe has invested 1 billion yuan ($150 million) in its massive pharmaceutical plant covering 280,000 square meters in the city of Jinan.
It exports its medicines that fight infections, cancer, and cardiovascular problems to Europe, North and South America, and the Middle East, the company's website said.
The accident is under investigation, according to Xinhua.
Deadly industrial accidents are common in China.
A chemical blast in eastern Jiangsu province last month killed 78 and left hundreds injured.
In November, a gas leak at a plant in the northern Chinese city of Zhangjiakou -- which will host the 2022 Winter Olympics -- killed 24 people and injured 21 others.
In 2015, China saw one of its worst industrial accidents when giant chemical blasts in the northern port city of Tianjin killed at least 165 people.
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