When the rubber meets the road, where does it go?

R.D. Morgan of South Carolina asks a question that I've often wondered about myself: as automobile tires wear down, where does all the rubber go exactly? I had read, years ago ( pre-blog, so cannot find it) in a Smithsonian Magazine about a specialized rubber eating bacteria which inhabited the world's highways. Cecil doesn't mention that, but has other suggestions as to where the black cloud of rubber ends up.

The Straight Dope: When the rubber meets the road, where does it go? All over the place, bud--including maybe in your lungs. For a long time conventional wisdom had it that tire particles were too coarse to do much harm and simply wound up as one more component of urban grit. Now we know better. Asthma and latex allergies have been on the rise in recent years, and some think tire dust is why.

Tires are a mix of materials, mostly synthetic and natural rubbers but also including carbon black, oil, sulfur, steel, and chemicals added as antioxidants, strengtheners, and fillers. They also contain varying amounts of potentially hazardous metals such as zinc, nickel, chromium, cadmium, and copper. As you drive, and especially as you corner and brake, your tires continually abrade against the road surface, and to some extent wear away just due to flexing as they roll along. This tire wear takes the form of rubber left on the road, heavy particles that quickly settle on the road and shoulder, and lighter particles that become airborne.
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Tire dust that doesn't make it into the air can be problematic as well. Originally deposited on the pavement, it gets washed by rain into lakes and streams. Environmental scientist Alison Draper has shown that chemicals leaching out of tire dust can kill water organisms such as algae, plants, minnows, and snails. An Italian study found that the organic components of tire debris were toxic to frog embryos and to cultured human lung and liver cells.

How much rubber gets worn off of tires? Estimates vary widely--much depends on driving habits, vehicle weight, the type of road surface, and the type of tire. In the U.S., the amount is estimated to be on the order of 650,000 tons per year. A British study finds that about 10 to 20 percent of a tire's total weight is worn off during its lifetime, which works out to about 58,000 tons a year in the UK alone. Of that, the fraction consisting of those potentially toxic metals I mentioned is surprisingly high--36 tons of cadmium, more than 1,000 tons of copper, and nearly 3,300 tons of zinc. Pollution studies in the Los Angeles basin in the 1980s concluded that more than five tons of breathable tire dust were released into the atmosphere there each day, and there's no reason to think that figure's gone down since.

One of the items-you-can-do-to-save-the-planet mentioned in

An Inconvenient Truth

(or at least at its website ClimateCrisis.net) is to ensure your tires are fully inflated. Cecil Adams concurs.

... So just because tire dust isn't accumulating in huge black drifts along the roadside doesn't mean we can ignore it. What to do? One critical factor that determines how much rubber is lost from tires is inflation. Properly inflated tires create less rolling resistance than underinflated ones, build up less heat, flex less as they roll down the road, and release less latex into the environment. They also last longer and give you better gas mileage. So check your tire pressure next chance you get--not only will you be doing right by the environment, you might save a couple bucks.

Tags: Al_Gore, /automotive, /environment