Scientists have suspected for several years that chemicals from tire wear particles are to blame for the deaths of thousands of coho salmon that have returned to spawn in Puget Sound’s urban streams. Sometimes referred to as “pre-spawn mortality” or “urban runoff mortality syndrome,” these deaths typically occur in streams near roads, and scientists have been analyzing a wide variety of automobile-derived chemicals to see if they produced similar toxic effects.

Now, thanks to some painstaking detective work by our partners at the University of Washington Center for Urban Waters (our groups are affiliated and share lab space) and many other collaborators, researchers can point to 6-PPD-quinone, a derivative from a preservative in tires called 6-PPD. The finding is as unexpected as it is fraught with implications. The chemical comes about only when the tire preservative is exposed to naturally occurring ground-level ozone in the environment creating a “transformation product” not previously identified. A paper outlining the discovery is published today (Dec. 3) in the journal Science and you can read more details from Christopher Dunagan in our magazine Salish Sea Currents.

In some ways the research on 6-PPD-quinone is just beginning. What started out as a local mystery could now catalyze studies around the world. Scientists wonder if the newly identified chemical is harming more than just coho.

“This is the first thing I’ve worked on in my career where I have no idea where the story ends,” says the paper’s co-senior author Ed Kolodziej. “It’s kind of what keeps you up at night. You’re wondering, ‘How wide is it?'”

Tires and similar rubber products are found everywhere in the world, he points out, and while 6-PPD-quinone has not been shown to kill some other species of salmon (it doesn’t appear to harm chum, for example) there is speculation that the impacts could be more widespread.

“We just have no idea,” he says. “All these questions are just totally wide open because there’s just no information out there.”

Given the potential ramifications, scientists may now begin to search for similar impacts among often vulnerable species such as stream invertebrates and amphibians, but it is also clear that humans are sometimes exposed to similar 6-PPD compounds. “We know the 6-PPD parent compound [has been documented] in house dust,” Kolodziej offers as an example. It also occurs in recycled tires that are used for crumb rubber playing fields and gym mats. “We’re generating a billion tires a year globally that need to be disposed of,” says Kolodziej. “All these things and all those recycled products likely contain some level of 6-PPD and the 6-PPD quinone as well. So, humans, I think, have a variety of exposure pathways.”

Could that endanger human health? “Again, we just have no idea,” Kolodziej says. 

What is known, however, are the implications for coho salmon. In the short term, Kolodziej hopes that the revelations in the Science paper will at least lead to more “salmon safe” tires.

“Tires need these preservative chemicals to make them last,” Kolodziej told UW News. “It’s just a question of which chemicals are a good fit for that and then carefully evaluating their safety for humans, aquatic organisms,” and other species, he says. “We’re not sure what alternative chemical we would recommend, but we do know that chemists are really smart and have many tools in their toolboxes to figure out a safer chemical alternative.”

View a video about the discovery below.