Moscow residents were advised to stay indoors after parts of the Russian capital were blanketed in what officials said was a foul-smelling and potentially harmful haze of hydrogen sulfide and sulfur dioxide. Inhaled at high concentrations, these substances can have a range of effects from eye irritation to nearly instant death. But while many Moscovites took to Facebook and Twitter on Monday to complain of minor discomfort — as well as the overpowering whiff of rotten eggs — no serious injuries were reported. "Is it just me smelling sulfur, or has a portal to hell opened in Moscow?" @ping_f wrote in one of the many online reactions to the fog, according to a translation by the Moscow Times.
The State Department issued an emergency message for Americans living in the city to stay themselves indoors and immediately report any health concerns. Levels returned to normal by Tuesday, according to city environmental watchdog Mosekomonitoring. Russia's First Deputy Emergency Situations Minister Sergei Shlyakov told the state-run Interfax news agency that the cloud was believed to be caused by a leak at an oil refinery unit.