What are T(R)WP
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It is surprisingly difficult to answer this question accurately because of the complexity of tire composition.The most simplistic answer, offered by the tire industry, is that they are cigar-shaped agglomerations of materials that range in size from 100µm, up to around 2mm in length comprising roughly a 50-50 mix of tire material and road materials. According to the tire industry, they have a density of around 1.8.
The evidence shows this characterisation is highly over-simplified and potentially misleading. In reality, the particles typically vary in size from around 3mm down to ultra-fine particles of 5nm or less. In some special circumstances – such as race tracks – these tire ‘particles’ can be up to 50mm in size, when they become known as ‘marbles’.
While there is some difficulty in collecting and identifying particles less than about 5nm because of the availability of suitable filtration media, there is every reason to believe that particles smaller than 5nm are produced. When we look at the distribution plots, there is no evidence of any reduction in the number of particles produced as we approach the limits of the filtration systems. On the contrary, the evidence shows that when we look for smaller particles, we find very large numbers of them.
In terms of distribution, the plot by number of particles shows very many ultra-fine particles and relatively few in the millimetre size range. Thus, the vast majority of particles (of any size) are in the ultra-fine range below 100nm.
The distribution by mass, however, shows that the vast majority of the total mass of these particles is concentrated in the larger particles.
Some commentators have focussed on these larger particles. This enables them to claim that by working with 95% of the total mass contained in these larger particles, it is possible to ignore the smaller mass fraction of ultra-fine particles.
While the ultra-fine particles make up less than 5% of the total mass, they should not be ignored, as they are the ones that are breathed in and can easily be ingested and find their way into lungs, and from there into the bloodstream.
Evidence shows they also find their way into the gut and from there into human urine and fecal matter. From fields, they find they way into agricultural produce.
Furthermore, these ultra-fine particles have the highest specific surface (ratio of surface to volume) and thus the ones where potentially harmful chemicals will migrate most quickly to the surface, compared with the larger particles.
In terms of composition, the particles are made up of varying fractions of tire material and minerals from the road surface. There is no clear preferred fraction of these two types of material, there is a full spectrum from 100% tire material to 100% mineral materials. The materials with a higher proportion of tire material have a lower density – from 1.2 upwards. The particles with a high proportion of minerals tend to have a higher density. There is no value that can be put on density to characterise all such particles. Assigning a single value to the density of these particles tends to be misleading.
