Cotton has been around for thousands of years, but it's drawing new interest these days with talk of sustainable clothing. That soft and comfy T-shirt you bought the other day may have begun in cotton fields in Tennessee or California, then traveled around the world and back from factories in China, India or Bangladesh. The shirt itself took only 8 ounces of cotton to make, but weighing its environmental impact is more complicated.

Cotton accounts for 40 percent of all world fiber production, according to PBS. China, the United States and India produce over half the world's cotton. Cotton is used in 40 percent of the world's clothing -- from a man's dress shirt that requires 10 ounces of cotton, to a pair of jeans (24 ounces) or even a diaper that takes just 2.5 ounces. One 500-pound bale can provide 800 men's shirts.

"Even if a T-shirt is made from an animal-free, all-natural material like cotton, there are still environmental consequences," the Huffington Post says -- including the 2,700 liters of water needed to make one shirt, according to a video made by the World Wildlife Fund and National Geographic. The website Cotton Today, however, argues that cotton is naturally drought tolerant and uses less water than most crops. Cotton growers also use less pesticides than 20 or 30 years ago -- about 0.38 ounces of pesticides to grow enough cotton for that one T-shirt, the site says. That's partly because technology has made it possible to grow more cotton on less land.

Low-cost imports, many from China, have led to a fashion trend many call "fast fashion" -- purchase of clothing as almost disposables. Americans buy about 1 billion garments from China annually, and throw away in excess of 68 pounds of textiles per person per year, according to "Waste Couture: Environmental Impact of the Clothing Industry." Beginning about 1990, meanwhile, fashion designers, including Armani, Stella McCartney, Rogan Gregory and Katharine Hamnett, have been attuned to sustainability using repurposed, organic, vegan, low-impact and other green fabrics and ideas. In 2008, the New York runway show FashionFuture enlisted McCartney, Versace, Givenchy, Calvin Klein and Saint Laurent to showcase green fashion.

So what to do with all those old unwanted shirts, and how much of it ends up in landfills anyway? According to EPA figures cited by "Secondary Materials and Recycled Textiles," about 3.8 billion pounds is discarded annually. Some is sold online as vintage. About 15 percent of discarded clothing is donated or consigned, then sold to other consumers, according to "Waste Couture." Some is exported to other countries. A study by a charity group and a university looked at whether recycling pays off, after considering transportation and other energy costs. It found it definitely does: Reuse of a ton of cotton clothing only used 2.6 percent of the energy needed to manufacture brand-new clothing from new materials.