Forty two people were hospitalized in and around the Urals city of Chelyabinsk on Thursday following a bromine leak, the health ministry said.
A total of 103 people sought medical assistance after several bromine containers leaked in a rail car at 7:35 am (1:35 GMT), forcing an evacuation of the Chelyabinsk railway terminal. The cause of the leak has yet to be established.
State television showed pictures of a cloud of dirty yellow smog billowing over the station. The smog drifted some 20kms, state media said. Schools were closed down as a precautionary measure.
People in the city have been warned to stay indoors and close their windows, or wear masks if they need to leave their homes.
Regional officials said there was no serious risk to people's health. Independent experts were, however, less optimistic.
"Bromine is a strong poison, not just an irritant. It really is dangerous," Moscow-based chemist Alexei Korunov was quoted as saying on the state-run Channel One TV station.
Chelyabinsk has a history of environmental problems and was the tenth most polluted city in Russia last year.