A dirt road is made up of several layers. Above the terass surface, the reinforcement layer comes and then a bearing layer. It is important that these layers can drain the gravel road while they can withstand high weights. At the top there is a wear layer which is only 5-9 cm thick. It is this layer that essentially gives a good or bad gravel road. The wear layer contains stone material in several different grain sizes and the proportion of each size is very important. Normally, a gravel road contains little fine material and coarse material, ie an excess of the medium sized grains. The below diagram shows the ideal grain size distribution of the top layer.

Particles of all sizes have been found to damage health, especially in sensitive populations. Particularly sensitive are asthmatics, young children, elderly people and people suffering from coronary and vascular diseases. In addition to health risks, the street dust degrades the well-being and dirty the surroundings.

The street dust is especially visible in the spring, but some of the particles are so small that you cannot see them with the naked eye. These particles with a diameter of less than 10 microns are called inhalable (PM₁₀). They end up in the lower airways.

A number of cities have made attempts at dust binding in the spring before all gravel and other dirt are swept up. Helsinki is constantly working on systems with warning signals on small particles and established routes that are dust-bound with calcium chloride.