The short version
For years, coho salmon returning to spawn in urban creeks in the Puget Sound area were dying after rainstorms, and nobody knew why. A team led by the University of Washington Tacoma and Washington State University traced the cause to a chemical that comes from car tires. A preservative added to tire rubber, called 6PPD, reacts with ozone in the air to form 6PPD-quinone, which proved highly toxic to coho. The team then found it in road runoff and creeks along the US West Coast, often at levels high enough to kill the fish.
What the researchers did
Scientists had linked the die-offs to stormwater from roads but could not pin down which of the many chemicals in road runoff was responsible. This team set out to find it.
- Working from the “urban runoff mortality syndrome” seen in Pacific Northwest coho, they narrowed down a chemical mixture made from tire wear particles, splitting it into fractions and testing each on salmon until one highly toxic compound remained, according to the universities’ announcements of the study.
- They identified that compound as 6PPD-quinone, a product formed when the tire antioxidant 6PPD reacts with ozone.
- They exposed juvenile coho to the chemical to estimate the concentration that kills half the fish in a test (the “lethal concentration”).
- They went back to stored samples of roadway runoff and stormwater-affected creeks on the US West Coast and measured how much 6PPD-quinone they contained.
What they found
- The killer comes from tires. The toxic chemical is not the tire preservative itself but what it turns into once it reacts with ozone. Because 6PPD is used in tires around the world, its breakdown product can be expected wherever cars drive.
- A tiny dose is deadly to coho. In the team’s tests, about 0.8 micrograms per liter (0.8 ± 0.16), less than one part per billion, was enough to kill half of the exposed coho.
- It is already in the water. Runoff and creek samples from the West Coast contained from under 0.3 to 19 micrograms per liter, often at toxic levels.
- An unexpected risk. The authors say their results point to
unanticipated risks of 6PPD antioxidants to an aquatic species
(Tian et al., 2021), and suggest that worn-off tire rubber deserves attention as a pollutant. - A later correction made it look worse. Once a commercial standard for 6PPD-quinone became available, the same team re-measured it in 2022. They found their original measurements had been biased high, so both the lethal level and the concentrations in runoff were revised downward. The fish turned out to be even more sensitive than this paper reported, and runoff still commonly carried more than enough to kill coho (Tian et al., 2022).
What it means for Waquoit Bay today
This study was done on the West Coast, where coho live. The chemical’s source is the same everywhere: tire wear on roads. Rain washes road surfaces in Falmouth and Mashpee too, and some of that runoff drains toward the rivers and ponds that feed the bay. No one has measured 6PPD-quinone in Waquoit Bay’s tributaries, so we don’t know how much reaches them. Sensitivity also varies widely by species. Lab studies elsewhere have since found brook trout, the species of the Quashnet River’s sea-run trout, to be sensitive as well, though less so than coho (Brinkmann et al., 2022). We have not found any tests on river herring.
The practical takeaway is that stormwater carries more than nitrogen and sediment. Catching and filtering road runoff before it reaches streams helps on several fronts at once (see what stormwater runoff does to oxygen downstream). Sampling a few local outfalls after storms would tell us whether this tire chemical is a concern here. Our summary is based on the paper’s published abstract and on the authors’ 2022 follow-up, because the full article is behind a paywall.
Read the paper
Tian, Z., Zhao, H., Peter, K. T., Gonzalez, M., Wetzel, J., Wu, C., Hu, X., Prat, J., Mudrock, E., Hettinger, R., Cortina, A. E., Biswas, R. G., Kock, F. V. C., Soong, R., Jenne, A., Du, B., Hou, F., He, H., Lundeen, R., … Kolodziej, E. P. (2021). A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon. Science, 371(6525), 185–189. https://doi.org/10.1126/science.abd6951 (first published online December 3, 2020; see also the publisher’s 2022 erratum)
License: © 2021 American Association for the Advancement of Science. The article is behind a paywall and is not openly licensed, so we link to the publisher’s page rather than hosting a copy. The card image is a title card we made from the article’s citation details.
Drafted with AI assistance from the linked source.
