The emergency has been caused by forest fires and burning peat bogs in countryside around the city, which have continued unabated for most of the summer.
Incoming flights at Moscow's airports were suspended for a few hours on Thursday, but on Friday, the air was noticeably clearer.
"I am so relieved. My 12-year-old daughter had an asthma attack last night. I don't know what I would have done if it went on like that," said a young Russian mother, Natasha.
Carbon monoxide
Russian authorities have announced plans to step up efforts to douse the forest fires, doubling the number of firemen in the operation to 4,000.
The largest firefighting plane in the world, an Ilyushin Il-76, is already in action.
In total 188 separate fires have been identified in the Moscow region, affecting more than 500 hectares (1,300 acres) of land.
On Thursday, BBC correspondent Steve Rosenberg reported that the air in the city was suffocating, causing sore throats and watering eyes.
People were wearing masks not only on the street, but inside too, as levels of carbon monoxide and nitrogen oxides soared above safety limits.
Visibility
Visibility was reduced on Thursday to just 100 metres (300 feet) in some parts of the city.
The Itar-Tass news agency said the smog was the worst for 30 years.
In 1972, peat bogs around the town of Shatura, 100 kilometres (62 miles) from the city, were also burning for most of the summer.
As well as advising pregnant women and people with asthma and heart diseases to leave the city, doctors said children should stay indoors.
The smog has usually been worst in the morning, tending to disperse later in the day.
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