Methane outgassing in Russia's capital? Mystery fog, 'toxic' sulfur odor covers Moscow

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A massive cloud smelling like 'hell' sparked a widespread panic in Moscow. Residents were afraid of a toxic gas after warnings to stay indoors.

The source of the rotten eggs smell turned out to be a leak from a sulfur dioxide processing facility at an oil refinery in Moscow, the Emergencies Ministry confirmed.

The city's ecology watchdog and the Emergencies Ministry said that the level of hydrogen sulfide was over the permitted level for a short period of time, while the levels of sulfur dioxide remained within the norm. The two gases are byproducts of oil processing.

The smell created a wave of panic on local social media.

"Guys, who knows what the reason for such a stink in Moscow? There has been a smell of hydrogen-sulfide in many parts of the city," wrote Twitter user Sergey Krasilnikov.

"The smell even travels through [closed] windows," wrote Twitter user Asya Zateeva.

Residents of the southeast of Moscow saw the buildings covered in smog in the morning.

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Many MP's in the Russian parliament, whose building is situated right next to the Kremlin, also complained of the odor since Monday morning, but their work was not disrupted.

The Emergencies Ministry checked air samples in the southeast, east and center of Moscow following an avalanche of complaints from local residents.

A representative of the Russian Hydro-Meteorological Service, which monitors environmental pollution, told Tass that the cloud first appeared in Moscow's south-east Lublino region, where the concentration of hydrogen sulfide has exceeded normal levels.