First off, you have dust from the gravel roads. 75% of the road network is gravel. The dust pollutant is known as PM10.
In Sweden, about 20 percent of the 100,000 km national road network consists of gravel roads (Vägverket, 2005a). In total, the Swedish gravel road network is roughly estimated to be 300,000 km, or about 75 percent of the total road network (Enkell, 2003). A considerable problem on these roads is dust generation from vehicle passage, most commonly observed as a dust cloud behind vehicles
travelling on the road.
Most particle measurements have focused on particulate matter of sizes up to 10 μm in diameter (PM10) because of the health hazard posed by these particles in the atmosphere (Addo and Sanders, 1995). Gottschalk (1994) reports that PM10 can be retained in the human respiratory system. Increased PM10 concentrations, regardless of particle composition, have through several medical studies been shown to increase the mortality rate from heart and lung diseases (Gustafsson, 2005). Particle pollution has been linked to decreased lung function, aggravated asthma, development of chronic bronchitis, and irregular heartbeat (Kuennen, 2006). Concerns have been raised whether the maximum allowed concentration of particulate matter (PM10), according to the European Council Directive 1999/30/EC, is violated in the vicinity of a gravel road, which could create a threat to inhabitants of buildings adjacent to the road.
As if this is not bad enough, it has been postulated that the studded tires traveling on stone mastic asphalt are forming particulates smaller even than PM10, nano particulate. There are 1000 nanometers in a micron.
Airborne particulate matter varies widely in its physical and chemical composition, source and particle size. PM10 particles (the fraction of particulates in air of very small size (<10 µm)) and PM2.5 particles (<2.5 µm) are of major current concern, as they are small enough to penetrate deep into the lungs and so potentially pose significant health risks. Larger particles meanwhile, are not readily inhaled, and are removed relatively efficiently from the air by sedimentation. The principal source of airborne PM10 and PM2.5 matter in European cities is road traffic emissions, particularly from diesel vehicles. The limit values are very often exceeded in European cities.