Herman Miller's Aeron chair was launched 15 years ago, millions of copies the chair have been sold, and the Aeron even holds a spot in the New York Museum of Modern Art's permanent collection.

Monday brings another milestone for the Aeron, when the first Cradle to Cradle (C2C) certified versions of the chair start shipping out.

After three years of work, the Aeron has achieved C2C Silver certification from McDonough Braungart Design Chemistry, adding to Herman Miller's roster of C2C products and pushing it further down the path to eventually selling only products designed with the environment in mind.

"We've gone back over the last year and a half to re-engineer the arm pads as well as re-engineer a number of other components," said Scott Charon, program manager for Herman Miller's Design for the Environment team.

The chair's arm pads used to be covered in PVC stapled to foam; they're now made out of a PVC-free, self-skinning urethane foam that creates its own hard, outer layer. Herman Miller also had to take a flame retardant out of one of the plastic components and eliminate lead from some of the steel parts.

For many years, Herman Miller has been applying C2C principles to its new products, and four of its chair lines and various storage, filing and office setups meet C2C standards for material choice, recyclability and disassembly.

When Brian Walker took over as Herman Miller's president and CEO in the early 2000s, he pushed to apply C2C principles to past products as well.

By 2010, the company aims for 50 percent of its sales to be from products that meet its internal Design for Environment protocol, which includes C2C evaluation. By 2020, Herman Miller wants that figure to reach 100 percent.

"It's much easier to apply the protocol for new products because you can make the tools and make the materials selection," Charon said. Existing products are trickier to deal with for a number of reasons, including finding new suppliers or swapping out materials without changing how an item looks or performs.

Even with all of the changes Herman Miller was able to make to the Aeron, it wasn't able to meet a higher C2C standard. One thing that kept it from earning Gold certification is the fact that the chair contains polyester (though it's recycled polyester), which hurts a product's C2C certification since it contains antimony.

The Aeron chair is also 94 percent recyclable, but Charon said the chairs aren't meant to be just used for a few years and then recycled. They're meant to last. Aeron chairs come with a 12-year warranty, and all the new materials used to meet C2C certification were put though the company's regular testing. "An Aeron chair can last for a long time," Charon said. "It can be refurbished and fixed up."