Even for fashion and beauty lovers who think sustainability is important, wading through all the sometimes-conflicting information about it can be tricky. To combat the confusion, we're asking scientists, researchers and other authorities to answer all your most pressing questions in our column Ask a Sustainability Expert.   Dear Fashionista: What's the most responsible way to dispose of shoes once they're worn past the point of no return? I don't want them to end up in a landfill, but I know I can't just put them out on the curb with my cardboard and glass for recycling.   Perhaps your soles look like Swiss cheese or your high heels are so chewed up you're telling people the dog got ahold of them. If it's time to retire your loved-to-death footwear and you're looking for responsible, sustainable ways to do so, there's good news and bad news: You have options, but none of them are perfect.   Of the 24.2 billion pairs of shoes manufactured globally each year, experts say that most of them end up in a landfill or incinerator, because there are simply too many shoes and not enough recycling solutions.   "When you are generating this much product, most of it is sent back to landfill," says Dr. Sahadat Hossain, the director of the Solid Waste Institute for Sustainability at the University of Texas at Arlington.   Still, you're right to resist the urge to toss shoes in the garbage. Once they're kicking back in the dump, shoes can leach plasticizers, heavy metals and other toxic chemicals, says Hossain. They also take a literal eternity to break down. While natural materials decompose quickly (cotton takes about six months while leather requires 20 to 40 years), most of our shoes contain plastic-based components that last much, much longer.   "PVC and EVA are around 35% of all shoe materials, globally," explains Hossain. "They can take as much as 1,000 years to decompose." Of course, this is all theoretical, he adds. In modern landfills, which are lined in plastic and then sealed shut, our shoes sit intact "as long as you can imagine."  

WHEN IN DOUBT, DONATE

  The simplest advice is to donate used shoes.   "Worn out to you doesn't necessarily mean worn out to someone else," says Tiffany Fuller, a deputy director of Reuse, Special Waste and Apartment Programs at the New York City Department of Sanitation.   In fact, according to the American Apparel and Footwear Association, we buy more than seven pairs of shoe per year, the equivalent of one new pair every seven weeks. Purchasing at this clip means that most of our shoes never truly get the chance to wear out. And reusing is better for the environment than recycling.   By donating, shoes will likely find their way to emerging nations (charities sell about 25% of what we donate locally and export the rest), some of which boast a thriving market for worn and even refurbished footwear.   "Some of them get washed and cleaned and, if need be, the heel is replaced," explains Steven Bethell, founder of Bank & Vogue, a global used clothing broker. In Guatemala, there's a sneaker cleaning plant, says Bethell, while in Pakistan men's dress shoes get resoled in large recycling facilities. If you're wondering if your shoes are fit to be reworn, Bethell's advice echoes Fuller's: "When in doubt, donate."   There's one major caveat with donating: Not all footwear will find a new home and developing countries are filled to the brim with our old stuff.   "There is greater supply than there is demand," says Liz Ricketts, co-founder of the OR Foundation, a non-profit that researches the secondhand industry in Ghana. In Ghana, she says, as much as 40% of clothing imported from the West is directly landfilled or burned. She suspects the percentage is even higher with footwear.   Still, Bethell says there are ways to boost the chances that your shoes will make the cut. Sneakers (both men's and women's), soccer shoes and men's dress shoes are most in demand in the international secondhand trade and are good options to donate. Fashionable women's shoes, on the other hand — especially high heels — are a "dime a dozen" and are a better option to repair and resell at home.      

IF THEY CAN'T BE SAVED, SEE IF THEY CAN GET NEW LIFE IN ANOTHER FORM

  If your shoes are truly beat and you can't fathom another person loving them — a good rule of thumb is that if the uppers are still in good condition, the shoe can have a second life — toss them into a recycling bin instead of the trash. Footwear recycling is not nearly as common as clothes recycling (apparel can be shredded and turned into new fibers or insulation, for example), because shoes are more complex. However, there are a handful of programs attempting it. And the options are expected to grow in the next few years.   Nike's Reuse a Shoe program, running since 1993, recycles worn out sneakers by any brand. They've processed 33 million pairs of shoes to date. After separating the shoes into leather, foam, plastic and rubber, the pieces are ground down and reused as surfacing for playgrounds, track tops, carpet padding and even new Nike gear, like maybe the soles of your Air Jordans. You can get in on the action by dropping your shoes at participating Nike and Converse Factory stores (here's a list of them).   You can also take beat-up shoes by any brand to Columbia stores and participating Asics stores (here's a list). These in-store take back programs are run by I:CO, a global waste handler whose parent company opened the world's first industrial-scale shoe recycling facility in 2018, which is working to find solutions for any kind of shoe waste, according to a company rep via email. Recently I:CO partnered with Adidas to turn bits of rubber from running shoes into rugs, for example.   Another option is to organize a shoe drive and send what you collect to Terracycle, a New Jersey-based recycler that finds buyers for leather shoe bits that can to be turned into flooring and furniture, while plastic becomes containers and soundproofing materials, among other uses, says Ernel Simpson, VP of research and development. But you'll have to pay for it. Collection boxes start at $109, making it a better option for offices, apartment buildings or schools.   Fortunately, footwear recycling options should expand in the near future.   "We are looking at shoe to shoe to recycling. Conceptually it's a new way of looking at things," explains Dharan Kirupanathan, technology lead behind Adidas's Futurecraft.Loop, an allegedly "infinitely recyclable" running shoe made of a single material that's currently in its pilot phase. And I:CO is planning to expand its footwear recycling options to more stores and brands in 2020.  

THE BEST SOLUTION OF ALL? KEEP THEM KICKING

  Before you act on any of the above, take a second look at the shoes in question. Chances are that your shoes aren't actually on death's door and could be brought back to life. In fact, there's almost no such thing as a worn-out shoe in the hands of a good cobbler.   "We can fix everything," says Jair Antonio Hernandez, owner of J&C Shoe Repair in Brooklyn, who has 48 years of experience and happens to be my personal cobbler. Many cobblers will take on unusual repairs, like replacing the cork on Birkenstocks, shampooing and reconditioning Uggs and rejuvenating your sneakers, too.   "We can do full resoles on sneakers, new heels and change the color," says Hernandez. Repairing truly worn-out shoes is more expensive, he adds, so next time, don't wait so long.   If you don't have shoe repair nearby, try an online shoe repair service, like My Shoe Hospital, NuShoe or Cobbler Concierge. Jersey-based SoleFresh has a mail-in option for sneakers. If you've bought a pair of high-end shoes (like Red Wing), check if the company provides in-house repairs.   You can also try your hand at some DIY shoe recovery at home. Shoe adhesives like Shoe-Fix, Barge and Shoe Goo are cheap and effective and can be used to plug holes in those paper-thin soles, reattach a flopping sole and — my favorite trick — build up a worn-down heel so you're not walking at a slant.  

THE FUTURE OF FOOTWEAR

  Luckily, the shoe industry is working on new ways to help curb this cycle of waste — and make it simpler to part with shoes without all the guilt. From upstarts like Veja and Everlane to industry stalwarts like Adidas, Nike, New Balance, Converse and Saucony, more footwear companies are moving towards non-toxic, biodegradable and recycled components. It's also easier than ever to give a pair of shoes a second life by listing them on resale platforms like TheRealReal, Depop and ThredUp or on sneaker trading platforms like Sole Supremacy, StockX and Fight Club.   At the end of the day, there's no magic bullet solution to getting rid of old shoes. Like everything else to do with sustainability, it's complicated and requires tradeoffs. And even recycling innovations can't replace the time-tested advice to buy the best quality shoes you can afford (leather still lasts the longest and is easiest to repair, says Hernandez) and to care for each pair as long as possible. Once you're past that point? It's worth remembering that old shoes also make great flower pots.