This story started in the Pacific Northwest of the United States. For years, coho salmon that migrate upstream for spawning were mysteriously dying from an unknown cause. Not only were the fish succumbing to an untimely demise, scientists could not understand why the fish massacre seemed to increase after rain events. An environmental whodunit was afoot, with very few plausible answers.

A typical river scene in the Pacific Northwest USA.

Step in researchers Professor Edward KolodziejAssistant Professor Jenifer McIntyre, and team from the University of Washington. Interested in solving the mystery, the team set about trying to figure out the cause of the sudden fish death. Was it habitat? Was it temperature? Was it genetics? Or something more sinister?

In December 2020, after extensive environmental detective work, the researchers had their eureka! moment. Published in the internationally renowned preeminent journal Science, the result was astounding. The cause? I hear you ask.

Tire rubber

A very particular part of tire rubber too. A chemical that is used in the manufacturing of tires known as 6PPD, or more specifically, its quinone transformation product 6PPD-q, or a N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine.

As 6PPD is an industry formulation, with a tightly guarded recipe, the exact ingredients of the chemical are relatively unknown. What is known is that this chemical is part of a much larger family of approximately ten similar chemicals also used in rubber manufacturing.

Tires, the culprit causing salmon death.

Used in tires, and other forms of rubber, as an antioxidant – that is, it helps prevent degradation of the rubber – the study authors found 6PPD-q to be “widespread” in toxic concentrations across the US West Coast.

Detected in runoff and road-wash, one of the primary contributors of 6PPD-q to the aquatic environment was tire wear particle ‘crumb’, with creeks near busy roads containing the largest amount.

Since that initial ground-breaking study, researchers from Europe, Asia, Japan and even Australia have turned their attention to 6PPD-q, and the results are cause for concern. Just like in the US, 6PPD-q has become widespread in freshwater ecosystems. It has all the hallmarks of a ubiquitous chemical of concern and one poised to become the next major environmental challenge.

Right under our noses, or more correctly, our wheels, an environmental toxicant has been lurking for decades. One might say that the chemical has been around for so long and nobody has been sick yet – yet – but this is a typical industry protecting tactic. The real truth is, it probably has been causing problems and scientist have not had the opportunity to detect the chemical so far.

Although discovered as a degradation product of tire wear, 6PPD is present in a wide variety of rubber use applications, including artificial turf and in piping. Toxicity testing is therefore critical for understanding the risk posed by this chemical of potential concern.

In the short few years since the ground breaking discovery, scientists have been working to figure out the answer to the question of toxicity. Various studies have determined toxicity in a range of organisms, including fish and mice.

To date, there is no clear definitive evidence of human toxicity. However, a study released in 2022 that examined urine samples of 150 people in China, including pregnant women, gave this concluding remark:

“Considering that 6PPD-Q was a lethal toxicant to multiple aquatic species, the potential human health risks posed by its long-term exposure require urgent attention.”

A stark warning for the future of this environmental hazard.