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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,experts say.

"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, meshlike 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 effects 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.

Ninety percent of today's grocery bags are plastic. And fewer than 5 percent of plastic bags historically have been recycled, compared with 21 percent of paper bags, studies show.

Plastic bags are a particular problem in coastal regions, where they often end up in rivers and oceans, poisoning or strangling marine life, experts say.

Sixty percent to 80 percent of ocean debris is plastic, according to the Algalita Marine Research Foundation. And while plastic might gradually shred into smaller pieces, those fragments will persist and threaten sea life for up to 1,000 years, the foundation says.

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 Natural Resources Defense Council senior scientist 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, studies show: Reusable bags beat 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 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," Hershkowitz said. "That's a truism everyone can agree upon."