Smog thickens in Moscow, people told to stay inside
Schoolchildren were kept inside and officials warned that pregnant women and people with heart conditions should take extra precautions as the fires raged in peat bogs and forests around the rim of the capital.
"We are concerned about children and pregnant women as they are the most sensitive to this," Russia's chief medical officer, Gennady Onishchenko, told NTV television.
"People who smoke and drink are also more at risk," he said, adding that those with heart or respiratory problems were advised to stay at home.
The smoky haze, which has cloaked the capital for several days, shrouded the morning sun and crept into homes, offices and metro stations as firefighters battled to control the blazes.
City officials recommended hanging damp sheets over the windows to absorb the smog, but said there was no need to panic.
Yevgenia Syemutnikova of the state ecological monitoring body said the concentration of carbon dioxide in the air was two to three times the norm, but said for normal, healthy people "there would be no (lasting) effect".
A cargo plane and helicopters have been drafted in to help fight up to 200 fires, which have raged through one of the hottest summers on record.
In St Petersburg a similar haze hung over the city as peat bogs burned around Russia's second city.