Moscow's air pollution is expected to reach especially hazardous levels on Wednesday as temperatures in the city climb to 32 degrees Celsius (90 Fahrenheit).
The warning was triggered after the morning temperature exceeded the expected norm by about 5 C, said the federal agency in charge of monitoring the environment, RosHydroMet.
An unexpectedly high concentration of pollutants is being recorded in the city, with the pollution at its worst in the early hours of the morning and evening, state news outlet ITAR-Tass reported, citing RosHydroMet.
Children, pregnant women and the elderly, as well as sufferers of respiratory and cardiovascular diseases, are advised to avoid going outside in the late afternoon, between 3 and 6 p.m.
The air should be cleaner on Thursday as a wind blows in from the northwest, the report said. Temperatures are expected to drop slightly on Thursday to between 27 and 29 C.