For two decades, researchers worked to solve a mystery in West Coast streams. Why, when it rained, were large numbers of spawning coho salmon dying? As part of an effort to find out, scientists placed fish in water that contained particles of new and old tires. The salmon died, and the researchers then began testing the hundreds of chemicals that had leached into the water.

A 2020 paper revealed the cause of mortality: a chemical called 6PPD that is added to tires to prevent their cracking and degradation. When 6PPD, which occurs in tire dust, is exposed to ground-level ozone, it’s transformed into multiple other chemicals, including 6PPD-quinone, or 6PPD-q. The compound is acutely toxic to four of 11 tested fish species, including coho salmon.

Mystery solved, but not the problem, for the chemical continues to be used by all major tire manufacturers and is found on roads and in waterways around the world. Though no one has studied the impact of 6PPD-q on human health, it’s also been detected in the urine of children, adults, and pregnant women in South China. The pathways and significance of that contamination are, so far, unknown.

Seventy-eight percent of ocean microplastics are synthetic tire rubber, according to one estimate.

Still, there are now calls for regulatory action. Last month, the legal nonprofit Earthjustice, on behalf of the fishing industry, filed a notice of intent to sue tire manufacturers for violating the Endangered Species Act by using 6PPD. And a coalition of Indian tribes recently called on the EPA to ban use of the chemical. “We have witnessed firsthand the devastation to the salmon species we have always relied upon to nourish our people,” the Puyallup Tribal Council said in a statement. “We have watched as the species have declined to the point of almost certain extinction if nothing is done to protect them.”

Read the full article about pollution from tires by Jim Robbins at Yale Environment 360.