by Ilana DeBare

Paper or plastic grocery bags - which are better for the environment?

You probably think you know the answer. And you're probably wrong.

Paper bags are not necessarily better for the environment than plastic - despite many consumers' long-standing assumption that paper beats out plastic hands down when it comes to eco-friendliness.

"There definitely was a period of time when the message was, 'Choose paper over plastic,' " said Jenny Powers, a spokeswoman for the Natural Resources Defense Council. "That's not the way to view it."

Powers and other environmental experts now say the best choice is neither paper nor plastic - it's reusable shopping bags made of substances like cotton, hemp, nylon or durable mesh-like plastic.

"The ideal option is bring your own bag," Powers said. "Second choice is to ask for the type of bag that you know will be reused - plastic if you'll use it for holding trash, or paper if you will recycle it."

The question of the relative merits of various kinds of grocery bags sounds simple.

But in fact, scientists spend large amounts of time trying to nail down the environmental impacts of creating, transporting and disposing of products such as grocery bags - a process known as life cycle analysis.

The final answer depends on numerous details, including:

-- Whether the bags are made from recycled or virgin materials.

-- How far the raw ingredients and finished bags must travel before reaching consumers.

-- How much energy and water are used in the manufacturing process.

-- Whether bags that are labeled "recyclable" or "compostable" actually end up being recycled and composted, or just get dumped in the trash.

The stakes are high. Ninety percent of today's grocery bags are plastic. Californians alone use 19 billion plastic bags each year - 600 bags every second - according to the California Integrated Waste Management Board.

And fewer than 5 percent of plastic bags historically have been recycled, compared with 21 percent of paper bags.

Plastic bags are a particular problem in coastal regions like the Bay Area, where they often end up in rivers and oceans - poisoning or strangling marine life. Sixty to 80 percent of ocean debris is plastic, according to the Algalita Marine Research Foundation. And while plastic may gradually shred into smaller pieces, those fragments will persist and threaten sea life for up to 1,000 years.

But paper bags have other negative effects on the environment.

"If you're comparing a paper bag made from virgin timber with a plastic bag made with natural gas, the paper bag causes more global warming pollution, more biodiversity impacts and more water impacts," said Allen Hershkowitz, a senior scientist with NRDC who has worked on life cycle analyses for two decades. "If the paper bag is not recycled, it will generate greater carbon emissions during incineration than plastic would, or greater methane emissions if it is landfilled."

One thing is clear in every study that has been done: Reusable bags beat both paper and plastic on virtually all environmental criteria.

For instance, a 2002 Australian study concluded that someone using plastic grocery bags for a year would go through 520 bags and generate 6.08 kilograms of greenhouse gases, which contribute to global warming. Someone using paper bags would also go through 520 bags to generate 11.8 kilograms of greenhouse gases.

But a year's worth of reusable polypropylene bags - estimated at four bags, used twice a week - would generate less than 2 kilograms of greenhouse gases.

"The best thing is for people to be encouraged to take reusable bags," said Hershkowitz. "That's a truism everyone can agree upon."

Learn more about the environmental impacts

-- A Web site called Use Less Stuff, use-less-stuff.com, offers an easy-to-read summary of several European analyses of grocery bags.

A 2007 report by Los Angeles County summarizes some of the research on paper-versus-plastic at links.sfgate.com/ZBWC.

-- San Francisco's Department of the Environment offers another summary of bag analyses from Sweden and the United States at links.sfgate.com/ZBWB.

-- The 2002 Australian report can be found at links.sfgate.com/ZBWD.

E-mail Ilana DeBare at idebare@sfchronicle.com.
© 2007 The San Francisco Chronicle