After heavy rains, healthy salmon in urban streams were dying before they could spawn. Scientists spent decades searching for the culprit.

The killer is 6PPD, a highly effective chemical preservative added to vehicle tires to prevent cracking and degrading from ozone exposure.

When 6PPD reacts with ozone in the air, it transforms into 6PPD-quinone. This secondary chemical washes off roads and straight into waterways.

6PPD-quinone is incredibly toxic. It can kill Coho salmon within mere hours at trace concentrations of just 1 part per billion.

In heavily trafficked watersheds, this chemical runoff causes up to 90% of returning salmon to die before reproducing, threatening the species.

This crisis highlights a broader systemic issue. Tire wear particles represent one of the largest, yet least discussed, sources of microplastic pollution globally.

Our modern transportation networks directly intersect with fragile aquatic ecosystems. Untreated runoff exposes systemic flaws in urban design.

There is hope. Bioretention systems and rain gardens can filter out up to 95% of 6PPD-quinone before the runoff ever reaches sensitive salmon habitats.

Regulators and global tire manufacturers are now racing to develop a non-toxic alternative to 6PPD to protect aquatic biodiversity long-term.