Finland’s cities were swathed in street dust on Friday, as air quality was very poor in Tampere, Seinäjoki and Kuopio.
Poor air quality was expected in Vantaa, Jyväskylä and Vaasa.
"When it’s below freezing but not windy, pollutants circulate close to the ground," said Yle meteorologist Joonas Koskela.
The poor air quality could cause problems for people suffering from asthma or other respiratory conditions. They are advised to avoid busy streets on Friday.
Air quality can be followed via the Meteorological Institute’s website.
Sunny weather over the weekend
Forecasters said on Friday that the weekend was expected to be sunny, with overnight frosts also likely.
In Lapland the temperature was forecast to dip below 10-15 degrees Celsius.
On Sunday a band of snow was predicted to hit Finland, with south and central Finland in the firing line.
The low pressure area should keep temperatures around zero degrees, and the precipitation should also improve air quality.