Residents of parts of the Russian capital Moscow have been urged by the emergencies ministry to stay indoors because of a noxious gas that is spreading through the city.
The source of the gas is unclear – no accidents have been reported at any of Moscow’s chemical factories.
Media reports said that the gas was hydrogen sulphide, which can be highly toxic and smells like rotten eggs.
It has been smelt in central, eastern and south-eastern parts of the city.
The gas was also detected in the capital’s main shopping areas and around the parliament building, reports said.
Russia’s emergencies ministry blamed the problem on a failure at a Moscow oil refinery, Interfax reported. However, the refinery’s owner Gazprom Neft said there had been no accident and the levels of hydrogen sulphide at the plant were not excessive.
An earlier report suggested that the foul-smelling gas was coming from a network of facilities treating urban wastewater.
Natalya Gorelova, a 25-year-old resident of north-west Moscow, told the BBC she first noticed the gas on her way to her work in the south-west of the city during the morning.
“I have smelt the toxic gas all day. I’m at home now and I closed the windows. We are sitting at home, but I have a headache.”
Exposure even to low concentrations of hydrogen sulphide can lead to headaches, dizziness and nausea, experts say.
“Since 11:00 this morning I have sensed a bad smell everywhere I go,” one Muscovite tweeted, “whether at home, in the street or at work. “I thought I was getting sick and having [a] hallucination.”
Weather experts quoted in the Russian media say that the effects of the gas have been made worse because the current conditions in Moscow are “not conducive to the rapid dispersion of pollutants in the air”.