"The fires have injected an enormous quantity of dust into the atmosphere, (which) is now being detected over Switzerland," the Swiss meteorological authorities said in a report discreetly issued on Tuesday and picked up by Swiss media today.
MeteoSwiss said it had determined that particles observed at the Payerne measurement station in the western canton of Vaud on May 24 and 25 had been emitted from western Canada around May 19-20.
The concentrations measured over Switzerland did not constitute a health risk, it stressed.
But the fires have moved away from populated areas and tracked eastward, allowing Fort McMurray's residents to gradually return to the city.