The accident happened yesterday at a chemical factory in Yinchuan, capital of the Ningxia Hui autonomous region, bordering Inner Mongolia.
''Police received calls late Sunday night about people poisoned in a factory in Xixia district, where 210,000 residents live,'' the agency said.
Gas leaks are not uncommon in China. In March, a leak at a gas well in southwest China prompted the emergency evacuation of 15,000 people.
In December 2003, in one of China's worst industrial accidents, 243 people died when a leak in the same area sent out a cloud of toxic gas that killed residents in their sleep.
REUTERS KD KN1534