According to a recent study published in Science, researchers at the University of Washington have identified vehicles as a primary culprit – and their tires as the worst offenders.The piece, published in December 2020, describes how the research team tested over 2000 chemicals known to be used in tires, and were eventually able to narrow down their results to just one toxin.

The data pinpoints a specific chemical used to preserve car tires that is a lethal toxin for salmon.

This tire preservative, also known as 6PPD, works to protect tires from degradation and maintains their structure. But it’s chemical structure changes when it is exposed to the pollutants emitted from cars, producing 6PPD-quinone, which is highly toxic for salmon.

As 6PPD-quinone makes its way into storm drains through rain runoff, and as it seeps into local streams and creeks it poisons the salmon that live there. When the toxin infiltrates these water systems, it is potent enough to cause immediate and even long-term damage to local ecosystems.

This process is accelerated during and after heavy rainfall, as the water quickly flushes more and more small tire particles off the street and into the surrounding drains.

Interestingly, not all salmon are equally vulnerable to the chemical. Coho salmon in particular appear to be much more affected by exposure to 6PPD-quinone, while chum salmon are believed to be less sensitive. However, researchers still have a lot of work to do to understand which other species and types of marine life are harmed by the chemical.

One proposed solution is to treat rain runoff before it makes its way into the storm drains, but it would be difficult to implement widely. Another possible solution could be to change the way tires are made by using more environmentally friendly and sustainable materials. While such alternatives have yet to be identified, researchers hope they can be discovered in the future.

Unfortunately, it’s not just the tires on our cars that are harming salmon. A 2017 study by Washington researchers has also linked salmon die-off to other contaminants being released by vehicles. Motor oil, brake linings, fuel, and anti-freeze are just some of the automobile toxins that also leech their way into our storm drains via water runoff; some of which are so toxic, they can kill off adult salmon in as little as 24 hours. Copper used in brake pads is also highly toxic to salmon, and young salmon are even more vulnerable to the metal. Brake dust, which is caused by the grinding on brake pads when a vehicle is braked becomes airborne, eventually settling onto the roads and into our storm drains after rainfall.

When salmon are exposed to brake dust, they become disoriented as they lose their sense of smell. It make them more vulnerable to predators and confuses their travel, affecting their ability to make their back to their home streams. Fortunately, copper is slowly being phased out of brake pad manufacturing, but it will still take years before the benefits will be realized.

For now, the best that vehicle owners can do is ensure that our tires are properly inflated, that our cars don’t leak, and that brakes and tires are in good working order –  as we share our urban space with other commuters essential to our ecosystem.


Thanks to Fernando Lessa of the urban salmon project  for the image of the Coho fry.  We have a story coming up about Fernando shortly1