The monthlong heat wave, with record temperatures reaching 104 degrees Fahrenheit, has caused a number of fires, including peat fires, creating heavy smog throughout the region, RIA Novosti reported Wednesday.
Doctors are advising that the elderly and the very young should remain indoors in the worst affected areas.
Most of the fires have been extinguished, and experts say the pollution should eventually diminish.
"Southeasterly winds are dissipating the smoke that hit Moscow's atmosphere during the fires. If there are no more fires, the smoke will also gradually disappear," a state environmental agency spokesman said.
"The situation with the smoke will continue for another couple of days, but by the evening of Thursday or Friday the air could be cleansed with rain," a deputy director from the Russian Meteorological Office said. "A thunderstorm is expected from the west and may cleanse the air, making it easier to breathe."