Carbon monoxide has been detected well outside the territories involved by a NASA satellite equipped with atmospheric instruments, RIA Novosti reported Thursday.
Pollutants from the fires in Russia and Canada have now formed a ring around the planet and are moving north, satellite data shows.
Meanwhile, skies have cleared in Moscow as firefighters have managed to extinguish a number of the forest fires burning near the city, The New York Times reported.
100 workers at the U.S. Embassy were evacuated last week, along with staff from many European embassies, but on Wednesday the European Union said embassies of all its member countries had resumed operations, the Times said.
Forecasters warned that some smoke could periodically waft back over the Russian capital for weeks, even as the fires abate.
Some smoke could return as early as Thursday if the wind dies down as expected, Roman Vilfand, the director of the Russian federal weather bureau, warned.
"The forest fires have not disappeared," he said.
Thousands of Russian emergency workers and military personnel have been working for almost three weeks fighting fires in 22 regions which have so far killed more than 50 people and left over 3,500 homeless.
The immediate economic cost of the fires has been estimated at $15 billion, RIA Novosti reported.