An unemployed worker poisoned a reservoir, sickening 64 people, because he wanted to boost sales for water purifiers, the official Xinhua News Agency reported Monday.
Police detained Cao Qian, 27, in Henan province's Ruyang County after the reservoir that provides drinking water to homes in the area was found tainted by poison Wednesday.
No deaths were reported, but 42 of those were poisoned badly enough to be hospitalized, the report said.
It said Cao "claimed to poison the water in a bid to have his water purifying devices sell well." No details were given.
Such crimes are not unheard of in China. Last year, at least 38 people died in the eastern city of Nanjing after eating snacks that a rival snack shop owner laced with the poison.
Authorities earlier this month threatened penalties including execution for people who illegally make or sell illegal rat poison.
Last month, dozens of elementary school students and teachers in central China were hospitalized after ingesting rat poison with their school breakfasts. Authorities said the poisoning appeared to be deliberate, but no arrests have been announced.