LANZHOU - The results of a preliminary investigation released Thursday show that the lead poisoning which has affected over 50 workers in a plant in Northwest China's Gansu province was caused by their contact with lead-bearing raw materials.

The local work safety bureau in Jiayuguan said the incident, which occurred in the city's Tongjia Renewable Resources Processing Co Ltd, was the result of workers inhaling and absorbing through their skin the steam from high-temperature lead-bearing raw materials.

Bureau officials said they received a telephone report around 9 pm on Wednesday that more than 100 people working for the company were suffering from lead poisoning.

So far, over 50 people have been diagnosed as having excessive lead in their blood and they are suffering from moderate or higher levels of lead poisoning.

Medical tests showed that 57 people had between 400 and 800 micrograms of lead per liter of blood. "Normal" levels are below 100 micrograms of lead per liter of blood, according to national diagnostic standards.

Founded in 2007, Tongjia Renewable Resources Processing Co Ltd mainly focuses on the smelting and processing of pig iron.

The city government said further investigation into the incident is underway.