Three children in Pingliang, Gansu, have died and 36 others have fallen ill from nitrite poisoning after drinking milk bought direct from farmers.
Pingliang's No 2 People's Hospital recorded the first food-poisoning death around 9am on Thursday and another hospital recorded two similar deaths shortly afterwards.
'The three dead children were all under three years old. The rest of the patients were mostly children under 14 years old,' a Pingliang government spokesman said.
Of the 36 patients receiving hospital treatment, one had been discharged and one, previously in critical condition, was now in serious condition, the city government said in a statement. The others were in stable condition and getting better.
It said the results of an initial investigation showed that the patients all fell ill after drinking milk bought direct from farmers, but the source of the nitrite was still uncertain.
Nitrite, widely used to cure meat, looks and tastes like salt. Nitrite poisoning can result in nausea, dizziness, respiratory distress, loss of consciousness and seizures. Consumption of a third to half a gram of nitrite can cause poisoning or death.
The two farms involved were sealed off and their managers are being investigated,
A Pingliang health bureau official said local police were also considering the possibility that the milk had been deliberately poisoned.
'It makes no sense to add nitrite to milk and it even affects the taste,' the official said. 'Police are also looking into whether someone deliberately poisoned the milk.'
The district where the tragedy occurred was a rural area where villagers commonly bought milk directly from dairy farmers, the official said.
The city government is going from door to door to check if anyone has drunk suspect milk, and is asking residents who appeared to show poisoning symptoms to report to hospitals.
It is also conducting an emergency inspection of local milk farms.
The food sector is regularly tarnished by poisonings and toxic contamination scandals, and mainlanders have little confidence in the safety of food. The fast-growing but fragmented dairy sector has been at the heart of such concerns.
In 2008, at least six children died and nearly 300,000 fell ill from baby formula contaminated with melamine, an industrial chemical added to fool inspectors by giving artificially high readings for protein levels.
The authorities said at the time that all suspect milk powder had been confiscated or destroyed. But last July, 25,000 tonnes of tainted milk powder that should have been destroyed was found.
The quality inspection agency last week ordered nearly half the mainland's dairy firms out of the market because they failed to prove they could produce safe products.
Last month, the country's largest meat processor, Shuanghui, was forced to apologise after a television programme reported that one of its branches bought clenbuterol-tainted pigs and that the pork had found its way onto the market. The illegal additive, fed to pigs to make them leaner, can lead to dizziness, heart palpitations and profuse sweating.