Are tires the main source of ocean microplastics?

The quantity of tire and road wear particles (TRWP) present in oceans is often significantly  overestimated. Scientific research has suggested a much lower proportion of TRWP reach the oceans than claimed by some organizations. This may be attributed to the specific characteristics of TRWP that make them different to plastics found in the environment. 

Scientifically validated (peer reviewed) research that verified computational modeling with field sampling found that 2-5% of all TRWP emitted make it to a marine estuary (Unice et al., 2019a, b).  

The Pew and IUCN reports, that claim that tires are responsible for 78 and 28% respectively of global releases of primary microplastics to oceans, ignored known fate and transport mechanisms, drew conclusions based on an oversimplification of the fate and transport of tire wear particles in the environment, and did not validate model assumptions with field sampling. In both cases this resulted in significant overestimation of the percentage of TRWP that reaches the ocean. Importantly, the Pew report indicates that there is high uncertainty associated with its modeled estimates, and only assessed 4 of 20 microplastic land-based sources; therefore, reliance on this modeled estimate in not advised. 

Other recent research found that paint is the largest source of microplastic leakage into the ocean and waterways, “outweighing all other sources of microplastic leakage including textile fibers and tire dust” (Earth Action, 2022). The report estimated ~37% of marine microplastics are from paint, versus ~8% from tires. 

Unice et al., 2019 found that most TRWP finds its way to roadside soils or is removed from the road surface as runoff. A small fraction is released to the air.  

See also: Are TRWP a form of microplastic?