Pollution from peat and forest fires raging around the Russian capital, Moscow, has surged to record levels as wildfires continued to spread across the west of the country.
More than 600 separate blazes were burning on Saturday, raising carbon monoxide levels nearly seven times the safe reading, the highest since Russia’s worst heatwave in living memory began a month ago.
Residents sheltered indoors or fled from Moscow to less polluted climes. Many Muscovites were wearing face masks on the street.
Health experts warned that the best solution is to leave the city for the weekend, leading to a rush for seats on trains and planes out of the capital, news agencies reported.
"The situation is truly extreme. People are in circumstances under which they should not have to live," a member of the League for the Nation's Health group told the Kommersant daily.
At least 52 people have died and 2,000 homes have been destroyed in the fires.
US state department warned its nationals from travelling to affected areas due to the “hazardous levels of pollution", while Germany closed its embassy until further notice and advised citizens against "non-essential" travel.
Dozens of incoming flights were diverted from the city's two main airports as smog brought runway visibility down to 325 metres in Domodedovo airport and 550 metres in Vnukovo airport.
The smog from spreading wildfires has also forced the Russian Football Union (RFU) to move a friendly match against Bulgaria to Saint Petersburg, local media reported.
About 10,000 firefighters have been deployed to battle the blazes, a number Russian officials acknowledge is not enough. Firefighters from Italy, Germany and Bulgaria flew in to help while several other countries have sent firefighting aircraft.
Russian troops excavated an 8km long canal to prevent the flames from advancing to the top secret Sarov nuclear arms facility, located 354km east of Moscow. The emergency ministry has said the situation has"stabilised" as all nuclear materials were transferred to a safe site.
A wildfire last week caused huge damage at a Russian naval air base outside the capital, with Russian media reporting as many as 200 planes may have been destroyed.
Weather forecasters say Russia's worst heatwave in decades would continue for the coming days, with a temperature reaching up to 38 degrees Celsius.