Dense clouds of smoke choked Russia's capital Friday, forcing planes to divert and residents to stay indoors, as raging wildfires continued to spread across the country.
The thick haze limited visibility, descending on Moscow just as the city was being battered by record heat.
Moscow city officials say airborne pollutants like carbon monoxide are at least four times higher than normal. Health officials warned people to limit physical activity and wear surgical masks over their faces if they venture outside to avoid the noxious smog.
The poor visibility also affected activity at Moscow's airports, where several flights were grounded or diverted.
Russia's Emergency Situations Ministry says it is working to prevent the fires from reaching the western Bryansk region, where officials fear they could release into the atmosphere radioactive particles buried in the soil from the 1986 Chernobyl nuclear disaster.
Authorities also have ordered explosives evacuated from military facilities, and are working to control blazes around the country's main nuclear research center in the city of Sarov, about 400 kilometers east of Moscow.
The smoke and smog are the result of more than 500 wildfires burning across parts of central and western Russia, already plagued by drought and record-breaking heat.
Russian officials say the fires have killed at least 52 people and left thousands homeless.
Images from the U.S. space agency, NASA, show the smoke plume from those fires extending about 3,000 kilometers from east to west.
Health officials in Russia compare daily exposure to the air pollution to smoking several packs of cigarettes.
More than 150,000 firefighters, soldiers and others have been working to put out the fires, but there are reports that the dense smoke is seeping into homes in the Russian capital as well as the city's subway system.
Moscow's main meteorological service Friday said temperatures soared to 36 degrees Celsius, breaking the previous record high for the day set in 1920. Officials say it is the fifth temperature record set in Moscow this month.
Some information for this report provided by AP, AFP and Reuters.