Tire wear particles (TWP) have been
identified as one of the major
sources of microplastics (MPs), and few studies have focused on their
environmental behaviors and impacts. However, a thorough characteristic
and toxicity assessment associated with environmentally persistent
free radicals (EPFRs) on the photoaged TWP is missing. In this study,
we investigated EPFRs in the process of TWP photoaging and evaluated
their toxicity using in vitro bioassays. Our results showed that a
total of around 1.0 × 1017 spins/g EPFRs (g-factors ranging 2.00308–2.00318) was formed on
TWP with 60 days of light irradiation, which contained more than 29%
of reactive EPFRs (r-EPFRs). Using macrophages as model cells for
bioassays, TWP-associated EPFRs trigged endpoints, including the decrease
of cell viability (27 to 45%) and the increase of oxidative stress
response (46–93%) and inflammatory factor secretion. The enhancement
of TWP toxicity with photoaging was confirmed to be attributed to
the generated EPFRs combined with other TWP’s chemical compositions
(e.g., various metals and organics). Most importantly, the toxicity
of photoaged TWP was closely correlated with the generated r-EPFRs,
which induced reactive oxidant species (ROS) generation. This study
provides direct evidence of toxicity on the photoaged TWP particles,
revealing the potential contributions of EPFRs to the adverse effect
on human health and highlighting the need for an improved understanding
of the impacts of EPFRs on the risk assessment of TWP released into
the environment.