Every Thanksgiving, millions of American drivers wear down their tires on long-distance road trips. Where does all that tire rubber go? You'll find out in this Science Update.

Transcript

Where the rubber leaves the road. I'm Bob Hirshon and this is Science Update.



Every day, the treads on millions of tires gradually wear down. That thought got listener R.D. Willis of Puyallup, Washington, to wondering:



Willis:

Everyone's tires are wearing out. Where does that rubber go?



Well, R.D., we asked Alison Draper, a toxicologist at Trinity College in Hartford, Connecticut. Her research focuses on that very question.



Draper:

Basically, there are two avenues by which the rubber disappears off the tire. The smaller particles are small enough that they're airborne, and then the larger particles are large enough that they're too heavy to be airborne and fall to the side of the road. And then the large particles are washed off the roadway with rainwater and end up in our lakes and streams and rivers and such.



Tire rubber contains a variety of different chemicals, some of which are pollutants. But Draper says no one has a good estimate of exactly how much rubber enters the environment and what it might be doing.



Draper:

So part of the thrust of our research is really to try to figure out what chemicals are leaching out and causing those problems.



If a science question's wearing you out, call us at 1-800-WHY-ISIT. If we use your question on the show, you'll get a free Science Update mug. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

One of the basic rules of physics is that matter is never really created or destroyed—it just changes to another form. And yet, we often forget this when it comes to the technologies that we use. It's hard enough to remember that throwing garbage away doesn't make it disappear from the planet. But tire rubber? It just vanishes, right?



Not so fast, says Draper. All that rubber that gets worn off your tires has to go somewhere. And according to her research, a lot of it winds up in lakes, streams, and rivers.



Is that bad? Well, to figure that out, you have to look at what's in the tires and how it gets into the environment. The black color of tires comes from a chemical called carbon black, which is basically soot. And soot contains some chemical by-products that are known environmental pollutants. Tire rubber also contains hazardous sulfur and zinc compounds that are used to speed up the rubber-making process.



The main ingredient in tire rubber is a synthetic polymer called styrene-butadiene, which is a big, stable chemical that doesn't break down easily. It's often assumed that chemicals like these are not as toxic because they don't leach easily into the environment. But Draper and her colleagues have studied chunks of rubber that are left in water, and they've noticed that some of the chemicals do, in fact, leach into the water. And if anything, that same amount of rubber would probably release more chemicals into the water if it were in the form of small particles, because smaller particles have much more surface area in direct contact with the water.



The rest of the rubber—the really tiny particles—ends up in the air. How much? Well, in Los Angeles, tire rubber is the 13th most significant source of air pollution. That's pretty striking when you consider how small the particles of rubber are from any one car. And the fact that the rubber's in the air means that sooner or later some of it will end up in your lungs. Could tire rubber contribute to diseases like asthma and lung cancer? It's hard to say for sure, since cars produce so many other pollutants besides rubber. But I wouldn't want to inhale a jar full of powdered rubber, would you?



Tire rubber pollution is just one of many environmental problems in which the research is lagging far behind the damage we may have done. Tire rubber has been used continuously for a whole century, but Draper's research is only just beginning. The longer we wait to study problems like these, the more effort we may have to expend in order to correct them in the future.



Now try and answer these questions:

What are the two main environmental destinations for particles of rubber that wear from car tires? What are some of the potentially hazardous chemicals in tire rubber? What possible steps might be taken to reduce tire rubber pollution? Can you think of other kinds of environmental waste that people might not realize they're creating?

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