Release of Microplastics from Shoe Outsoles into the Environment by Walking and Jogging, and Ingestion of Shoe Outsole Microplastics by Goldfish

The migration of terrestrial microplastics (MPs) into aquatic environments is considered to have negatively affected aquatic ecosystems. However, quantifying terrestrial pathways is challenging because of the diverse sources of MPs. A poorly understood MP source is shoe outsoles. Therefore, this study determined whether shoes released MPs by abrasion and conducted a series of five experiments to study the potential impacts on aquatic life. The first and second experiments examined the relationship between the distance walked or jogged and the weight loss of shoes. The weight of the shoes decreased significantly as the moving distance increased; however, the values did not differ significantly between walking and jogging. The third experiment tested the size and number of MPs originating from shoe outsole under simulated conditions using sandpaper, which imitated the surface of an asphaltic pavement. The number of MPs increased as the number of contacts with the sandpaper increased. The fourth and fifth experiments examined whether goldfish, Carassius auratus, ingested rubber tips (RTs) derived from the shoe outsoles. When goldfish were given sinking RT or floating RT, they ingested both types of RT with or without feed, suggesting that goldfish recognized RTs as feed. The results of this study suggested that shoes released abrasion debris and particles into the environment as MPs. This study is the first to demonstrate the process of MP release from shoe outsoles and the ingestion of MPs from shoe outsoles by fish.