Part B: Tyre Wear Particles (TWPs)

Citation: Booth, A.M., Sørensen, L., Maes, T., Hansen, B.H., Nepstad, R., Ruhl, R., Fylakis, G., Blaauw, R., Thoden van Velzen, U., Igartua, A., van Hulst, F., Del Savio, L., Cowan, E., van Leeuwen, J., Vlachogianni, T. (2024). SOS-ZEROPOL2030 Deliverable D4.1 ‘Knowledge and outcomes from T4.1-T4.4 summarised in the integrated analysis section of the individual case study pollutant template. Part B: Tyre Wear Particles (TWPs)’.

Executive Summary

The SOS-ZEROPOL2030 project aims to deliver a stakeholder-led European Seas zero- pollution framework to help achieve the European Union’s long-term ambition of ‘Zero Pollution’ in European seas. The SOS-ZEROPOL2030 project focuses on marine pollution , where (i) per- and polyfluoroalkyl substances (PFAS) and (ii) tyre wear particles (TWPs) were selected as example pollutant case studies for ‘chemical’ and ‘microplastic’ pollution, respectively. It is important to note that these two very complex case study pollutants were intentionally chosen to allow the zero-pollution framework to be stress-tested under the most challenging scenarios. As a part of Work Package 4 (Integrated Case Study Pollutant Assessments) within the SOS-ZEROPOL2030 project, this deliverable report (D4.1. Part B) provides an integrated assessment for TWPs, while a separate report (D4.1 Part A) is available for PFAS. The integrated assessment comprises four primary components: (i) Mapping of primary emission sources along the value chain, (ii) Determination of environmental risk, (iii) Mapping of existing value chain and technological actions and strategies for TWP emission reduction, and (iv) Mapping of current governance strategies/efforts/arrangements for TWP pollution.

Emission sources along the value chain: TWPs are primarily produced through the friction and wear of tyres upon driving vehicles, meaning that urban, highway and rural roads are the main point sources of TWP emissions, with a direct link between increasing traffic volumes and higher TWP emissions. Heavier vehicles emit more particles by distance travelled, while braking, accelerating and turning cause increased emissions. Emissions per km travelled are largest in urban areas (due to increased braking, accelerating and turning), followed by highways and rural areas. In addition to releasing particles, tyres are also the primary sources of many tyre-related chemicals (TWCs), where almost 800 additives and non-intentionally added substances (NIAS) are known to be used and/or present in tyres and TWPs. Roadside soil receives 45-80% of TWPs and surface waters receive 6-26%, while the smaller TWPs can remain airborne for long periods. TWPs and low mobility TWCs will accumulate in marine sediments, while smaller TWPs and mobile TWCs may be transported away from deposition sites. Estuaries and coastal zones are the primary accumulation zones, especially those closest to urban areas that act as a point source of TWPs. Recommendations include:

1. Policy-based mitigation efforts that develop actions to reduce TWP emissions, with the following focus areas: tyre reformulation to reduce emissions, increased driver awareness campaigns regarding the impact of driving style on TWP emissions, improved emission capture technologies and road runoff treatment methods, and consideration of TWP emissions in future road design.
2. Reduction in the emission of hazardous chemicals via leaching into water through reformulation of vehicle tyres, with an emphasis of removing and/or replacing the most hazardous ingredients and those which leach most readily.
3. Emission mitigation actions for the marine environment should target those areas which are considered as hotspots for TWP emissions.

Environmental risk: The TWP risk assessment process also involved a detailed exposure assessment based on reported exposure concentrations and modelled data across different European regional seas and hazard assessment based on database and literature toxicity data and modelling. Owing to the complex nature of the pollutant, the risk assessment was conducted separately for both TWPs and TWCs/tyre wear leachates (TWLs). A method to integrate the exposure and toxicological data into the most comprehensive scenario-based environmental risk assessment for TWPs and TWCs was developed. The outcomes of the work highlighted that there is insufficient empirical TWP. TWL and TWC concentration/exposure data for the marine environment for it to be used for risk assessment purposes and modelling approaches are currently needed. The TWP risk assessment indicated that current particle emission concentrations in natural waters are unlikely to be above critical threshold values, but this does not account for TWP accumulation in sediments, nor does it account for TWCs/TWLs, which are much more toxic. TWLs comprise complex mixtures of TWCs vary in composition and cannot be directly measured in environmental samples, only individual TWCs. Hazard assessment could be conducted for a limited (~10) number of TWCs. However, many TWCs have other sources, attributing individual TWC concentrations in environmental samples is not really possible and they cannot be reliably used in risk assessment. Recommendations include:

1. Improved analytical methods for TWP quantification and identification of tyre-specific TWCs.
2. Generation of significantly more TWP and TWL/TWC toxicity data for use in hazard assessment, including sublethal and chronic toxicity, as well as elucidation of the relative contribution of each to observed toxicity.
3. Inclusion of TWCs into environmental monitoring programs.

Existing value chain and technological actions and strategies for TWP emission reduction: When looking at existing value chain and technological actions and strategies for TWP emission reduction, it is only the fraction of TWPs ending up in wastewater treatment plant sludge that is incinerated that will be permanently removed from the environment. Several technical mitigation strategies are currently in place or in development that reduce the emission of TWPs to waterways and the natural environment, including the use of porous asphalt concrete, capture measures for road runoff, capture of airborne TWPs and reformulation of tyres to reduce emissions and/or the impact of emissions. The generation of TWPs could also be reduce by lowering speed limits, improving road pavement, lowering car weight, and limiting torque at low speeds or limiting braking power where it is safe to do so. However, there are no potential or future technological measures known to reduce the emission of TWPs that have not already been explored and that appear to be technically feasible in the coming years. Recommendations include:

1. A focus on measures that intervene the least with human behaviour while driving as these are in general more effective and acceptable than those that require drivers to adapt their driving behaviour.

2. Technological measures to reduce generation and emission of TWPs and leaching of TWCs into the environment should first be taken at hotspots (e.g. coastal cities and metropolitan areas).

Governance strategies/efforts/arrangements for TWP pollution: Microplastics in general, and TWPs in particular, are a pollutant of emerging concern, and governance and regulation for TWPs is only beginning to emerge. Governance efforts were mapped for TWPs and pollution in the Mediterranean Sea region. Based on existing governance efforts at EU, regional and national levels, as well as industry responses to increased regulatory attention to TWP emissions and pollution, we draw two main conclusions. First, the governance of pollutants of concern currently seems much stronger at the EU level than at the regional sea and national level. Second, the regulatory focus on TWPs at the regional and EU level highlights different pace of agenda-setting and policy making between the Barcelona Convention and EU-level governance of TWPs, where EU-level regulatory developments currently happen rather independently from the regional sea convention. Recommendations include:

1. Governance measures to effectively tackle TWP emissions need to consider sources in addition to solely addressing TWP emissions and release.

2. Enhanced cooperation and exchange between the EU and Barcelona Convention is needed to ensure governance in relation to TWPs in the Mediterranean Sea becomes aligned with EU policy developments.