Up to 50 litres of the toxic element bromine leaked Thursday at a rail station in the Russian Urals, releasing clouds of yellow smoke and forcing dozens to seek medical help.
Authorities in the industrial city of Chelyabisk, where the containers leaked, sought to downplay the scale of the problem and urged the media not to "sow panic."
Moscow-based investigators said the fuming red-brown liquid was released into the air as a result of violation of safety procedures in Chelyabinsk, nearly 2,000 kilometres east of Moscow.
"Liquid bromine spilled at the station as a result of car coupling," investigators said, citing preliminary information.
Russia's state consumer watchdog Rospotrebnadzor estimated the amount of bromine released as between 40 and 50 litres. Investigators earlier put the amount at some 24 litres.
The freight train car carrying a total of 2,267 two-litre glass containers of bromine was later hauled outside the city, the emergencies ministry said.
It said that the liquid with an "aggressive odour" was released into the air when several glass containers broke.
Television footage showed clouds of yellow smoke, police in gas masks cordoning off the area and residents wearing blue gauze masks.
Bromine is poisonous to humans and can damage internal organs and the nervous system.
The city administration admitted that bromine could exacerbate existing health problems like bronchial allergy but insisted the city was not in danger.
"To calm down the population, I wanted to say that bromine is used for medical purposes to sedate people," Alexander Galichin, the region's minister of radiation and ecological security, said in televised remarks.
"Albeit not in such quantities, of course."
Health officials did not say how many people sought medical help by the end of the day. The ITAR-Tass news agency quoted an unnamed official in the regional health ministry as saying that 38 people were hospitalised.
A total of 143 people complained of health problem and sought medical help, the news agency said.
Earlier investigators had said that only 31 people sought medical help, while local authorities had said that just eight people were hospitalised.
Andrei, a resident of Chelyabinsk, told the Echo of Moscow radio that the bromine leak had interrupted the first day of school and several schools made a decision not to assemble pupils and teachers in school yards.
He estimated around 200,000 people might have experienced breathing difficulties.
"It's impossible to breathe," another local resident said on television.
Later Thursday the authorities said they had the leak under control.
"Many witnesses were simply frightened by the brown cloud -- that's how the fumes of bromine doused in water look," the regional government said.
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