We know, from earlier research, that tyres emit lots of particles, both coarser and the more potentially dangerous ultrafines. To put this in context, the levels are less than from exhausts of many older diesel vehicles without filters, but orders of magnitude greater than from the exhausts of modern internal combustion engine vehicles with the latest filters. But, where do these particles go, and can they be found in the environment?
As a consequence of the size distribution of particles in tyre wear, as set out in our earlier newsletter, it is reasonable to believe that the particles go to air, soil and water. Some particles will be directly deposited on the road verge or in rivers near roads, but the smaller ones will settle further away after a period of time. The tyre particles contain on average over 400 organic compounds, plus a range of metals. Together, this makes for a complex product that is emitted into the environment in many different ways.
Compare this with tailpipe emissions. While there are many volatile organic compounds in exhaust fumes, the pollution is dominated by carbon dioxide (CO2) and nitrogen oxides in modern vehicles, with some carbon monoxide and ultrafine particles. Therefore, the environmental impact is dominated by a small number of compounds almost all of which are suspended in air for an extended period.
The differences between tyre and tailpipe emissions presents an interesting paradox: as tyres are more complex, they may leave a more easily identifiable fingerprint in the environment. If we take an air sample and observe some CO2, it is impossible to ascribe that to a source, whether vehicular or from human exhalation. Even with NOx, it is impossible to say whether that comes from a car, truck, home heating or an industrial source. In contrast, if we find some benzene‚ 1‚2‚4-trimethyl- in river water, for example, there is a fair chance it originated from tyres. Seeing multiple compounds in the environment that we know can come from tyres only increases that confidence. The very complex nature of tyres means such a fingerprint can be left. In contrast, for tailpipe emissions, we have to fall back on constructing ‘inventories’ – look-up tables containing average values – to characterise what emissions come from different types of vehicle, derived from testing those vehicles, and often combined with activity data.
Although it took many years of work, researchers in the US were eventually able to link the death of significant numbers of coho salmon, and latterly also trout, to the chemical preservative 6PPD in tyres. This was covered in our earlier newsletter, 'Fishy'. Therefore, it is complex, but possible, to determine the original source of pollutants or causes deleterious effects observed in the environment. How can this approach be generalised?
Emissions Analytics has compiled a database of organic compound profiles of hundreds of different tyre models, drawn from over 40 different brands. To achieve this, we have developed a highly optimised analytical pyrolysis method to understand as closely as possible the compounds in the original tyre. This method uses a two-dimensional chromatography system to separate the compounds, which are then identified and quantified using a time-of-flight mass spectrometer. From this, the chemical fingerprint of an ‘average’ tyre can be determined by taking the mean concentration of each compound across all the tyres analysed.
Taking a real sample, we can see how this fingerprinting might work. A water sample was taken from an undisclosed body of water that was believed potentially to contain contaminants or leachates from tyres. It was analysed by ‘solid-phase microextraction’, which essentially involves dipping a thin fibre into the water, which extracts the compounds within. A blank sample of water should show no organic compounds on the chromatogram. Analysis of the sample in fact identified 115 organic compounds, many at the parts-per-billion level. The chromatogram is below, which shows compounds across a wide area and, consequently, many different functional groups.
From this we can conclude that there is very likely to be contamination in this water sample. However, how confident can we be that it comes from tyres?
To assess this, we can aggregate the individual compounds represented by the peaks on the chromatogram into functional groups based on their chemical properties: acids, alcohols, aldehydes, and so on. From an environmental and health perspective, the aromatics group is the most concerning as they are often carcinogenic. The esters and terpenes functional groups represent the least concerning compounds, and are most commonly fragrances and flavours. To estimate the prevalence of each group, the area under the peak on each compound is taken, and expressed as a percentage of the total peak area across the whole chromatogram. This gives a chemical profile of the water sample. This can then be compared with the concentrations of chemicals in the Emissions Analytics’ database, averaged across all the tyres tested, in nanograms of target chemical per milligram of sample. The chart below then compares these measures of prevalence between the water sample and the reference database.
The most striking element is the peak of aromatics, which gives good evidence that it is chemicals from tyres that are present in the water. The biggest difference is in the terpenes, principally limonene, which are generally not water soluble and therefore float, so are likely to have been under-sampled. As well as the similar aromatic peaks, there are similar absences of acids and esters between the two samples. The presence of some compounds from the aldehyde and alkane groups suggests the presence of a low level of non-tyre pollution as well in this water sample. Overall, this is a simplified version of what is possible, as more granular functional groups, and even individual compounds, can be used in the fingerprinting.
The same essential approach can be applied to identify tyre wear compounds in soil. Previous research typically identified a small number of organic ‘tracer’ compounds and then used one-dimensional gas chromatography and mass spectrometry (GC-MS) to measure those tracers in the environmental sample. In a recent paper in Chemosphere, ‘Determination of tire wear markers in soil samples and their distribution in roadside soil’, styrene-butadiene rubber (SBR) was used as the tracer, together with thermal desorption and GC-MS, to quantify the distribution of tyre wear at different distances from the roadside. In a further paper in Critical Reviews in Environmental Science and Technology from 2022, ‘Tire wear particles: An emerging threat to soil health’, the use of traditional tracers such as 2-(4-morpholinyl) benzothiazole and hydrogenated resin acids was mentioned, but indicated the need for new and better markers that do not easily leach into water, and are resistant to heat and light exposure. The approach to fingerprinting water samples using two-dimensional gas chromatography and a fingerprinting database may provide that way forward.
Less has been done so far on looking for ultrafine tyre particles in air. Almost by definition, there will be low mass concentrations of tyre particles in air, due to their small size. This is likely to underestimate the potential health effects of such particles, due to their large relative surface area, and the potential for transporting other pollutants, such as VOCs, deep into the human body. This remains, however, work in progress without definitive conclusions. For now, Emissions Analytics is collecting particles as they are shed from tyres in real driving environments. Inevitably, such collection gathers some proportion of non-tyre particles, such as from brakes, road wear and resuspension. The same essential fingerprinting process is being used to estimate what that proportion of non-tyre ‘interference’ in a sample is.
In short, while tyres are highly complex products, containing hundreds of different chemical compounds, the latest analytical techniques present the opportunity for more sophisticated fingerprinting techniques compared to traditional tracer analysis. The tyre tracks can now be followed to understand the ultimate fate of tyre wear in the air, soil and water, and indirectly the effect on human and animal health.