The green giveaway of the moment - the reusable shopping bag - is a case study in how tricky it is to make products environmentally friendly.

    It's manufactured in China, shipped thousands of miles overseas, made with plastic and could take years to decompose. It's also the hot "green" giveaway of the moment: the reusable shopping bag.

    The bags usually are printed with environmental slogans as well as corporate logos and pitched as earth-friendly substitutes for the billions of disposable plastic bags that wind up in landfills every year. Home Depot distributed 500,000 free reusable shopping bags last April on Earth Day, and Wal-Mart gave away one million. One line of bags features tags that read, "Saving the World One Bag at a Time."

    But well-meaning companies and consumers are finding that shopping bags, like biofuels, are another area where it's complicated to go green. "If you don't reuse them, you're actually worse off by taking one of them," says Bob Lilienfeld, author of the Use Less Stuff Report, an online newsletter about waste prevention. And because many of the bags are made from heavier material, they're also likely to sit longer in landfills than their thinner, disposable cousins, according to Ned Thomas, who heads the department of material science and engineering at Massachusetts Institute of Technology.

    Used as they were intended, the totes can be an environmental boon, vastly reducing the number of disposable bags that do wind up in landfills. If each bag is used multiple times - at least once a week - four or five reusable bags can replace 520 plastic bags a year, says Nick Sterling, research director at Natural Capitalism Solutions, a nonprofit focused on corporate sustainability issues.

    Just as digital music downloads were the giveaway of choice last year, reusable shopping bags are the new "it" freebie. Earlier this month, Google handed out 525 nylon bags bearing the company's logo at its "Zeitgeist" conference, a meeting of business and political leaders held at its campus in Mountain View, Calif. The Sundance Institute gave out 12,000 fabric bags at its annual film festival earlier this year. Elisa Camahort Page, cofounder of BlogHer, an online community for women bloggers, says she even gave away 150 reusable bags to guests at her wedding last year.

    Fueling the reusable-bag boom is the growing unpopularity of the ubiquitous throwaways known as T-shirt bags, so-called because the handles look like the top of a sleeveless T-shirt. An estimated 100 billion plastic bags are thrown away in the U.S. every year, according to the Worldwatch Institute.

    Last year, San Francisco became the first U.S. city to ban the bags from supermarkets and chain drug stores, and this month, the city of Westport, Conn., banned most kinds of plastic bags at retail checkout counters. Boston, Baltimore and Portland, Ore., are also considering bans.

    Earlier this year, Whole Foods Market grocery stores stopped using the T-shirt bags, and now offer paper bags or sell reusable totes priced at 99 cents to $29.99. Next month, Ikea will also discontinue their use, forcing customers to carry their purchases to their cars, bring bags from home or buy the chain's 59-cent reusable blue plastic substitute.

    Such efforts are helping make reusable totes the nation's fastest-growing fashion accessory, with sales this year up 76% to date over last year, according to Marshal Cohen, chief industry analyst at the market researcher NPD Group. At Bags on the Run, an online-based Phoenix company that sells nonwoven polypropylene bags, sales this year are up 1,000% to date over last year, according to Aerin Jacob, senior vice president of business development. Eco-Bags Products, which sells bags made of fabric, recycled materials and plastic, had $2.2 million in sales in 2007, a 300% increase over 2006, says Sharon Rowe, who heads the Ossining, N.Y.-based company. ChicoBag, in Chico, Calif., has tripled sales of its $5 reusable polyester tote this year, says president Andy Keller.

    Starting Monday, Target will move displays of its own 99-cent totes to the checkout lanes, to boost the bags' sales. On Wednesday, Rite Aid, which currently sells its branded bags in selected markets, will start stocking them in all of its 4,930 stores. CVS expects to have three million of its own bags in the marketplace within the next year.

    Finding a truly green bag is challenging. Plastic totes may be more eco-friendly to manufacture than ones made from cotton or canvas, which can require large amounts of water and energy to produce and may contain harsh chemical dyes. Paper bags, meanwhile, require the destruction of millions of trees and are made in factories that contribute to air and water pollution.

    Many of the cheap, reusable bags that retailers favor are produced in Chinese factories and made from nonwoven polypropylene, a form of plastic that requires about 28 times as much energy to produce as the plastic used in standard disposable bags and eight times as much as a paper sack, according to Mr. Sterling, of Natural Capitalism Solutions.

    Some, such as the ones sold in Gristedes stores in New York that are printed with the slogan "I used to be a plastic bag," are misleading. Those bags are also made in China from nonwoven polypropylene and have no recycled content. Stanley Joffe, president of Earthwise Bag Co., the Commerce, Calif., company that designed the bags, says the slogan is meant to point out that the bag itself is reusable, taking the place of a disposable plastic bag.

    Some plastic bags are, in fact, made with recycled materials. The polypropylene bags at Staples are made from 30% recycled content, according to company spokesman Mike Black. Target sells six types of bags, including a $5.99 variety made from recycled plastic bags, says spokesman Steve Linders.

    And yesterday, at the Clinton Global Initiative, a public-policy gathering in New York of business and political leaders, Wal-Mart pledged to reduce plastic bag waste by about 33% in every store world-wide in the next five years. Starting next month, the company will sell a new blue reusable plastic bag with a small amount of recycled material for 50 cents, half the price of its current black bag, which is 85% recycled plastic, says spokeswoman Shannon Frederick.

    Getting people to actually use the bags is another matter. Maximizing their benefits requires changing deeply ingrained behavior, like getting used to taking 30-second showers to lower one's energy and water use. At present, many of the bags go unused - remaining stashed instead in consumers' closets or in the trunks of their cars. Earlier this year, KPIX in San Francisco polled 500 of its television viewers and found that more than half - 58% - said they almost never take reusable cloth shopping bags to the grocery store.

    Phil Rozenski, director of environmental strategies at the plastic bag maker Hilex Poly Co., believes even fewer people remember to use them. Based on consumer surveys conducted by the company, he says roughly the same number of people reuse their bags as bring disposable bags back to the grocery store for recycling - a figure he puts at about 10% of consumers, according to industry data.

    This month at the Stanford Graduate School of Business, marketing professor Baba Shiv dedicated the first day of a weeklong seminar on green marketing to the "road blocks" facing reusable bags. He says it can take "years and decades" for consumers to change their shopping habits, and only when there's a personal reward or an obvious taboo associated with the change: "Is it taboo yet to be carrying plastic bags? I don't think so." Mr. Shiv also says that according to surveys done by his graduate students, many shoppers say they are less likely to carry a retailer's branded reusable bag into a competing store. "What these bags are doing is increasing loyalty to the store," he says.

    Dan Fosse, president of Cambridge, Minn.-based Innovative Packaging, produces a line of bags called SmarTote. Each one comes with a bar code that allows stores to track whether it is being reused. The idea, says Mr. Fosse, whose bags carry the slogan "Saving the World One Bag at a Time," is that companies can offer prizes or other incentives to customers who can prove their bag isn't just collecting dust at home.

    Grocery stores are starting to report incremental results, says Mr. Fosse, who added the bar codes last spring. "It's really hard to change customer behavior."

    Sarah De Belen, a 35-year-old mother of two from Hoboken, N.J., says she uses about 30 or 40 plastic bags at the grocery store every week. Late last year, she saw a woman at the supermarket with a popular canvas tote by London designer Anya Hindmarch and promptly purchased one online for about $45.

    But Ms. De Belen says she soon realized she'd need 12 of them to accommodate an average grocery run. "It can hold, like, a head of lettuce," she says. Besides, she adds, it's too nice to load up with diapers or dripping chicken breasts.