WHERE THE RUBBER MEETS THE ROAD—BUT THEN WHAT?

“WHY WERE SALMON Dying?” asked Science, December 4, 2020. This weekly magazine of the American Association for the Advancement of Science responded, “The Answer Washed Off the Road.” The culprit, it turns out, is tire residue.  

Ozone and Antiozonants. Tire rubber is a composite of a great many things beyond its basic elastomers. These include heavy metals such as zinc and cadmium, hydrocarbons, and sulfur-containing compounds. Antiozonants, for example, prevent ozone cracking, a deterioration of tires that used to be a common problem.

In one sense, ozone, aka trioxygen, is life-enhancing: The ozone layer, between 6 and 31 miles above the Earth’s surface, diminishes the sun’s ultraviolet radiation that would otherwise cause DNA damage to plants and animals.

On the other hand, ozone lower in the Earth’s atmosphere is produced by VOCs (volatile organic compounds) and sunlight. Its familiar name down here is photochemical smog. 

Ozone Cracks Solved, But…. Antioxidants mitigate tire cracking, but it has long been known that tire compound residue is a potential health hazard. In 2002, for instance, Alison J. Draper of Bucknell University published “What Happens to Rubber That Wears Off Auto Tires?” Draper began with clean samples of water inhabited by several kinds of aquatic organisms. To these she added finely ground tire particles, let them remain for ten days, then filtered them out. The resulting “leachate” contained substances that proved lethal to the organisms. 

Her research, though preliminary, showed that “tire wear particles may have negative impacts on small organisms in water habitats. Airborne tire particles may also aggravate respiratory problems in human beings (such as asthma or allergies).”

In 2013, Indiana Public Media’s Moment of Science website posted Amy Breau’s “When Rubber Wears Off Tires, Where Does It Go?” Breau described the problem: “… by the time the average passenger car tire ends up at the scrap yard, it weighs six pounds less than when it was new. Multiply six pounds by the number of tires scrapped each year in the U.S., and we’re talking millions of pounds of rubber that perform a disappearing act every year!”

Some of this residue remains as signs of burnouts and doughnuts, but Breau notes, “… the vast majority wears off as small particles that are rinsed off the road by rain, or blown off by wind, ending up in the soil, on plants, and in lakes, rivers and streams. We even breathe in rubber particles in the air.”

The Coho Salmon Kill. Erik Stokstad reports in Science, December 4, 2020, “For decades, something in urban streams has been killing coho salmon in the U.S. Pacific Northwest. Even after Seattle began to restore salmon habitat in the 1990s, up to 90% of the adults migrating up certain streams to spawn would suddenly die after rainstorms.”

Alas, note this was prior to their spawning.

A Detective Tale. “This was a serious mystery,” said Edward Kolodziej, an environmental engineer at the University of Washington’s Tacoma and Seattle campuses. Stokstad writes, “…researchers led by Kolodziej report the primary culprit comes from a chemical widely used to protect tires from ozone, a reactive atmospheric gas. The toxicant, called 6PPD-quinone, leaches out of the particles that tires shed onto pavement. Even small doses killed coho salmon in the lab.”

In sampling auto tires, the researchers found several thousand chemicals that are unidentified (tire-company formulations are highly proprietary). Two years of research narrowed down the list to the tire-compound antioxidant 6PPD, N-(1,3-dimethylbutyl)-N’-phenyl-1,4-benzenediamine, with ozone as a co-conspirator.

A Mystery Solved. The antioxidant 6PPD gets converted by ozone into a previously unknown organic compound 6PPD-quinone. Stokstad writes, “The team synthesized 6PPD-quinone and found it was highly lethal to coho salmon. Kolodziej and his colleagues say other species of fish should also be evaluated for sensitivity. Because you can’t buy the molecule, Kolodziej’s team is making it.”

Stokstad writes, “The researchers suspect the compound is present on busy roads everywhere. They’ve found it washes off pavement and into streams in Los Angeles and San Francisco, for example. The simplest solution might be for tire manufacturers to switch to an environmentally benign alternative.”

CNN reports, “In response to the findings, Sarah Amick, the US Tire Manufacturer’s Association vice president of environment, health safety and sustainability, called the study results ‘preliminary,’ but said the industry is committed to working to produce environmentally friendly products.”

I am reminded of science class demonstrations in my youth of asbestos insulation and of playing with globules of mercury (with nary a hand washing afterwards), back before their environmental tradeoffs were identified. ds 

© Dennis Simanaitis, SimanaitisSays.com, 2021