The explosion happened 10 days ago at a chemical plant, causing pollutants to leak into the Songhua River, which runs through Harbin, a city of several million people.
China's Environmental Protection Administration says that benzene levels in the river are now 108 times above the national safety levels. Benzene is a toxic industrial solvent.
Russian officials today expressed concern over possible pollution in Khabarovsk, 700 kilometers downriver from Harbin.
Hospitals there have been warned to prepare toxicology wards for possible victims on the Russian side of the border.
Health officials have started checking facilities to ensure they are ready should the contamination reach Russian waterways.