Where the rubber hits the road

Annually, tire manufacturers construct 3.1 billion tires worldwide. And 3.1 billion tires are discarded annually. Some are recycled, some are not. Regardless of their ultimate fate, it is a lot of tires.

But it also means 3.1 billion tires worth of tread is left on roads around the world. Annually, this amounts to 5.6 billion kilograms of rubber-based road dust and particulate matter.

Of course, we don’t see this dust as it tends to be in the form of very small particles – typically of in the PM2.5 to PM25.0 (2.5 to 25 micrometer) range.

But it is there.

And when a vehicle hits the brakes hard, the rubber becomes apparent as skid marks left on the road – although most of a skid mark results from the asphalt in the road heating up.

All of this rubber being released by normal tire wear leads to the question of where does it go? In Prince George, where people have been driving cars for almost a century, you would think there would be great big piles of rubber left on the sides of the roads. Or our roads would be rubber-covered like a sports track but they are not.

The answer to where tire rubber ends up is complex. The first step is to consider what exactly goes into making a tire. Generally, production techniques and rubber composition are proprietary with manufacturers employing slightly different blends of compounds.

However, using various sources, an approximate composition can be obtained. Natural rubber (poly-isoprenes) and synthetic rubber (poly-styrene-butadiene) typically make up 40 per cent, fillers such as carbon black, silica, and chalk about 25 per cent, vulcanizing agents such as sulphur and zinc oxide are three per cent, anti-oxidants and softeners around 15 per cent, minerals about five per cent, and various additives such as plasticizers around 12 per cent.

Of these components, natural rubber can be broken down by bacteria found in the environment.

This decomposition can take a long time but ultimately leads to a degradation in the structure of the tire particles releasing the fillers, minerals, and other components into the soil.

Some are innocuous as they are naturally occurring components of soil.

For example, carbon black is released into the environment during a forest fire and is incorporated into soil layers throughout the north.

But before decomposing particles are lifted by wind or stirred up by passing traffic leading to suspension in air where they can be transported long distances.

It is estimated 28 per cent of the plastics found in our oceans are from tire rubber. This amounts to about 2.8 billion kilograms per year or about half of the rubber left on the roads.

This flow of plastic particles into the ocean is particularly exacerbated by the distribution of population worldwide. Approximately 2 billion people live within 50 km of the ocean with many major cities serving as ports (i.e. Vancouver, Los Angeles, New York, London). Transport to the ocean is likely much higher from these areas.

But it is not just plastic particles that are an issue. The various additives, such as plasticizers, are not generally consumed by bacteria nor destroyed by combustion as the tire hits the road.

For decades, fishery scientists have noted an alarming trend in the streams and rivers where coho salmon collect to spawn. Up and down the west coast, fish have been turning up dead in huge numbers after heavy rain events – a phenomenon labeled urban runoff mortality syndrome.

A recent report in Science, entitled ‘A ubiquitous tire rubber-derived chemical induces acute mortality in coho salmon’ suggests the culprit may be the compound N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine) or 6PPD.

More accurately, the 6PPD released as tires break down over time reacts with ozone generating a by-product called 6PPD-quinone which is the actual culprit. It is 100 per cent toxic to coho and researchers have found it in roadway runoff samples collected from urban areas spanning the west coast.

In a typical healthy stream, less than one per cent of adult coho die before spawning. But in rainfall related mass die-off events, anywhere from 40 per cent to 90 per cent

For decades, fishery scientists have noted an alarming trend in the streams and rivers where coho salmon collect to spawn. Up and down the west coast, fish have been turning up dead in huge numbers after heavy rain events.

of fish can perish. As streams located closest to high-traffic urban areas and highways fare the worse, the researcher suspected for years the runoff from roadways played a role.

Further, the streams most affected often had a chemical profile matching roadway run-off. After years of careful research, Ed Kolodziej, the lead researcher, concluded: “We believe 6PPD-quinone is the primary causal toxicant… It’s exciting to start to understand what is happening because that starts to allow us to manage these problems more effectively.”

Eliminating the use of 6PPD from tire manufacture may simply be a matter of regulation as cleaner alternatives may be available. But it will take three to five years before all of the tires containing the compound are removed from the road. And in the meantime, billions of kilograms of micro-particles will find their way into our waterways and oceans.