The green problem: Organic cotton can leave you chafed and chilled, thanks to the fact that it absorbs and holds sweat. Bamboo, soy, and hemp require serious processing (and chemicals) to turn into soft fabrics.
I was shocked to find out that polyester is a smart way to go. Companies such as Nau.com and Patagonia.com use recycled polyester. So if you spend enough time talking to people who know about these things, you find that while their petroleum origin makes them seem earth-unfriendly, they're actually eco-smart.
But there's an even better way to smarten up your workout clothes without buying new ones: Wash them in cold water with low-impact detergent and hang them up to dry. And next time, skip the antimicrobial fabrics: Compounds in them are now being regulated by the EPA as pesticides (and if you wouldn't carry a pest strip, why wear one?).