Ask the average person what a tire is made of, and he'll likely say rubber. In fact, only 17 percent of a passenger car tire comes from plant-derived rubber. The rest is synthetic rubber, i.e. petroleum. That’s why tires burn so well. With the rise of laws that forbid dumping tires in landfills, they have become an increasingly popular fuel for power plants, cement factories and other industrial furnaces. Tires have more energy density than coal and about the same energy density as heavy fuel oil.

Cutting down on our use of coal and oil sounds good. But are tires safe to burn? After all, Michelin designs tires to be safe on the road, not in the oven. I asked Didier Miraton, one of Michelin’s three managing partners, about his company’s stance on the tire life cycle and if Michelin cares what happens to old tires. He assured me that Michelin was one of the first tire companies to care about the issue and said that using tires for fuel is in line with Michelin’s environmental standards. Burning tires in cement plants, he said, is totally clean, with no secondary emissions.

Most modern factories and power plants have advanced pollution-abatement technology, but does that make them clean? Not everyone is convinced. Environmental groups have fought against burning tires in cement factories, saying they release toxins into the air. The release of dioxin, a powerful carcinogen, is of particular concern. Last year, a cement plant near Dallas withdrew a request to burn tires instead of coal after locals appealed to the Environmental Protection Agency. Given that Michelin tires are sold around the world, it’s probably a safe guess that a cement factory on the outskirts of Dhaka, Bangladesh, isn’t as clean as one in Dallas.

Another, increasingly common fate for used tires is in athletic surfaces and as ground cover for playgrounds. Even the Obamas have “rubber crumb” surrounding the White House jungle gym. Not everyone is pleased with this use, either: The nonprofit organization Environment and Human Health believes there is evidence to suggest negative health impacts from exposure to recycled tire chunks. The EHH points to laboratory tests that show rubber crumb releasing more than two dozen potentially harmful chemicals under ambient conditions, as well as leaching heavy metals including lead and cadmium. (At the moment, the EPA endorses the use of tire crumbs in playgrounds, but hasn’t yet done a comprehensive analysis of possible risks to kids’ health.)

Further cause for concern comes from the factories that make tires. The International Agency for Research on Cancer did a study of the rubber industry, finding strong evidence that workers were being exposed to cancer-causing chemicals, with more news surfacing seemingly daily about the connections between cancer and many petroleum-based products.

This is not to say that recycling tires is bad. Each year in the U.S., 220 million pounds of recycled tires are mixed into asphalt. This is probably a good use of an abundant waste material. Close to a quarter of all tires are used for retreading before they’re ready to be ground up, and more experimental projects are using tires to build flexible retaining walls in earthquake-prone areas.

But these all still look like ways of sweeping tires under the rug. The real holy grail would be a truly elegant and cyclical system for recycling tires. Industrial designers would call this a cradle-to-cradle approach. Ideally, tires and recycling technology should be designed so that an old tire can become a new one again (or at least something equally valuable), so that the cycle rolls on and on forever.