Not since the Chevrolet Corvette Stingray or perhaps, more recently, the Citroen Nemo, have cars and fish been so closely associated. But this time it is not branding, but rather the pollution of one by the other. That laws are not being broken should be stated up front, and hopefully this will spare us the term ‘Trout-gate’, but nevertheless we need to consider the rapidly emerging evidence linking tyre wear emissions to serious effects on marine wildlife.
Emissions Analytics, in previous newsletters, has evidenced the magnitude of tyre wear emissions, in both mass and number. Many have rightly asked why, if these emissions are so high, has it not been an issue before – which is a good question. An answer is that, until very recently, tailpipe emissions have been high – especially for nitrogen oxides, of Dieselgate fame – and manufacturers have been pursued by authorities and car owners in many states around the world. There are few rules regarding tyre wear emissions, but it is the move towards battery electric vehicles (BEVs), which are typically around 40% heavier than standard internal combustion engine (ICE) vehicles, and which can lead to significantly higher tyre wear, that has brought the issue into focus.
While that has been going on, academic research has been able to link the effects of a chemical in tyres to certain fish populations. A ground-breaking article in Science in 2020, linked a common preservative in tyres – N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine, commonly known as 6PPD – to unexplained acute mortality when adult salmon migrate to urban creeks to reproduce. Adding 6PPD to tyres prevents hardening and cracking – which can impair durability and safety in operation. 6PPD reacts with ozone in the air to produce 6PPD-quinone, which is the specific compound measured in waters off the US West Coast. However, it was noted, this is unlikely to be an affect unique to coho salmon, or indeed this geographical location.
A more recent study, published in 2022, looked at species of trout, char and sturgeon, and concluded that 6PPD also has toxic effects on both rainbow and brook trout, while the other species were apparently unaffected. More concerning is the potential reinforcing effects when combined with other ambient chemicals, as analysed in this 2022 paper in Science of the Total Environment. The study looked at the effects on population growth of 6PPD and salt on the Brachionus calyciflorus freshwater herbivore. The presence of salt, which could come from road treatment, was found to amplify the effects of 6PPD on these organisms.
These effects matter not just because of the direct impact on wildlife, but also due to the economic effects. Declining populations of salmon and trout may well have detrimental effects on fishing industries. In 2019, commercial landings of coho salmon in the US totalled approximately 12 million kilograms and were valued at more than $30 million, according to NOAA Fisheries. Around 95% of the US coho salmon harvest comes from Alaska, where the impact of declining fish stocks on the livelihoods of the local population could be significant. Rainbow trout production for food in the US is around 27 million kilograms, with about three quarters coming from Idaho. Revenues generated are typically over $70-90 million per year.
As the academic evidence has accumulated, Emissions Analytics has been building a detailed database of the organic compounds in hundreds of new tyres on the market. The test equipment and method has been described in previous newsletters. As a result, we are now able to start quantifying the potential release of such chemicals, which can then provide a baseline from which policy could target their reduction. Before this can happen, however, it is necessary to evolve the traditional association of vehicle emissions with air quality, to a wider concept of the effect of vehicle emissions on the environment more generally – most importantly on marine and soil environments. This is essential if we are not to underestimate the effects of tyres.
While the most common in tyres, 6PPD is not the only preservative available. Potential other compounds are 7PPD (N-(1,4-dimethylpentyl)-N'-phenyl-p-phenylenediamine) and IPPD (N-Isopropyl-N′-phenyl-1,4-phenylenediamine). Across the more than 200 tyres tested so far, we can see the prevalence of each of these compounds, as set out in the table below.
The concentration is the amount of the compound – on a toluene equivalent basis of quantification – as a proportion of the tyre sample mass, and is measured using two-dimensional gas chromatography and time-of-flight mass spectrometry on samples taken directly from new tyres. Although the method is highly sensitive – picking up compounds at concentrations as low as parts-per-trillion – there is a lower limit of detection and, therefore, where the compounds are reported as not present, the concentration may just be below that lower limit.
These results show that a small minority of tyres currently contain IPPD, whereas almost all contain 6PPD. While it is a reasonable hypothesis that 6PPD and IPPD are substitutes for one another, the correlation between their concentrations in this dataset is weak, apparently because other factors determine each brand’s approach to the use of preservatives.
Drilling down to the individual tyres and their manufacturers, we can see how concentrations and compound mixes vary between the five manufacturers using the smallest amount of IPPD as a proportion of their total use of preservatives, compared to the five using the most, as shown in the table below.
If the average concentration of 6PPD is multiplied by the average lifetime wear rate of a set of tyres of 37 mg/km, that is approximately 33 µg/km of 6PPD released by an average car. When that is applied across the roughly 250 million cars on the roads of Europe and an average distance of about 16,000 km per year, the total 6PPD potentially released annually into the environment is approximately 130 tonnes in Europe.
As a cross-reference for the plausibility of significant 6PPD being released in this way, another study from 2022 assessed the concentration and leachability of 6PPD and 6PPD-quinone in road dust collected in Tokyo, Japan. The widespread presence of these compounds was confirmed by their presence in all the samples collected. Concentrations were higher typically where there was more traffic volume, suggesting a strong dependency, along with a weaker dependency on seasonal conditions.
In the analysis above we have considered the environmental impacts of just two compounds in tyres. On average, across all the tyres tested, 410 organic compounds are identified per tyre, across functional groups such as alkanes, aldehydes, aromatics and polycyclic aromatic hydrocarbons. Therefore, there is significant further research to do to understand the potential effects of the multitude of other compounds in tyres. Furthermore, less than one quarter of the compounds are identified by the standard spectral libraries with a sufficient level of confidence, and consequently there is a significant number of compounds unknown to all except perhaps the tyre manufacturer itself.
Authorities and regulators and now looking at this area actively. The Department of Toxic Substances Control (DTSC) in California is proposing a regulation to list Motor Vehicle Tires containing 6PPD as a ‘Priority Product’. DTSC has determined that there is potential for exposure to 6PPD from these products and for that exposure, “to cause or contribute to significant or widespread adverse impacts.” A public consultation is ongoing. The UNECE together with the EU are now actively working on a standardised method for measuring tyre wear rates, which would be a natural precursor to wider consideration of the chemical composition of tyres beyond the few compounds currently limited under REACH.
In summary, with the growing ability to identify compounds both inherent in tyres and mixed into samples collected in the environment, it is possible that we will progressively find further links between tyre ingredients or derivative compounds and deleterious environmental effects. Emissions Analytics is continuing its work to test a wide range of tyres around the world and resolve the currently unidentified compounds. What we can be certain of today is that tyres – while all looking very similar – are made with recipes that vary significantly between products, and that the total material released each year is large, dwarfing the particulate matter released from the tailpipes of modern combustion engines, which Emissions Analytics also continues to track through its EQUA programme.