Although nailed winter tyres have generally been thought of as a major source of particulate matter in the atmosphere, this may not be the case after all. A recent Finnish study has shown that unnailed tyres churn up more particulates than do nailed tyres, except in a few instances.
The Finnish researchers transformed a car into a research laboratory on wheels. It was driven on the streets of Helsinki and particulates in the atmosphere were measured as the car was driven. The results showed that the main source of particulates came from sand, which is blasted onto streets to deal with icy road conditions. The other components of particular matter came mainly from road works, house foundations, industry and soil particles that are carried a considerable distance by wind. In calm weather this material settles on the streets and is churned up by traffic, regardless of whether nailed tyres or unnailed tyres are used on vehicles. It is only on streets that are extremely clean that nailed tyres come out slightly worse than unnailed tyres. Heikki Tervahattu, who led the research, said that it is a great oversimplification to believe that banning nailed winter tyres would lead to a cleaner atmosphere.
Indeed, in Norway measurements have been made before and after a ban on nailed tyres was imposed in Oslo. The results showed that there is little direct connection between particulate matter and the use of nailed tyres and a much better correlation with weather and wet weather spells.
The soft surface and fine pattern on unnailed tyres is thought to grip the road surface better, which means that they churn up more sand, particles and steam than nailed tyres do. Nailed tyres are generally harder and have a rougher pattern than do unnailed tyres, and drivers trust in the nails to grip the road surface in icy conditions whereas it is the softness of unnailed tyres that makes the car grip the road. The width of the tyres also makes a difference, with broader tyres churning up much more particulates than narrow tyres.
The Finnish researchers believe that instead of concentrating only on nailed or unnailed tyres, more emphasis should be laid on keeping streets clean, using machines designed for the job. Thorough cleaning should be done as early as possible in spring. They say it might also be sensible to reduce maximum speeds on certain roads if particulate matter readings are too high.