You feel pretty good about yourself toting those green woven bags back and forth to the supermarket. But how much — and what — do they really save? And what's wrong with the old plastic and paper kinds?

"It's a conundrum," explains Kevin Ott from the Society of the Plastics Industry, based in Washington, D.C. "We don't live in a perfect world. There is a cost to the environment, regardless of which bag is used. To understand which bag is the greenest, we need to look at the life cycle analysis of each bag."

That "life-cycle analysis" follows each bag from raw material through production and distribution and to the consumer's hands. It also takes into account whether it's reused, recycled or thrown in the garbage.

Making Bags

Reusable fabric bags are most commonly made from cotton, but the cotton-farming process is extremely fossil-fuel-intensive because of the machinery involved.

According to the Pesticide Action Network of North America (PANNA) conventionally grown cotton uses more insecticides than any other single crop. Worldwide, cotton growers use more than 10 percent of the world's pesticides and nearly 25 percent of the world's insecticides.

Cotton is also responsible for 25 percent of all chemical pesticides — insecticides, fungicides and herbicides — used on American crops. Chemical fertilizers are used to enrich the soil.

Well, then, what about organic cotton? Those crops generally yield less usable fiber, which means an organic farmer needs more land to make a profit.

Most of the cotton grocery bags are woven outside the U.S. where labor is less costly, but that increases the use of fossil fuels in getting them from the factory to these shores.

So far, the life-cycle analysis of reusable cloth bags doesn't sound too green.

The process for making paper bags is also far from ideal. Huge machines log, haul and pulp trees. The entire paper-making process is heavily dependent on chemicals, electricity and fossil fuels.

To the paper industry's credit, it says it's been working to make its process cleaner and more efficient.

"Between 1975 and 2006, American Forest and Paper Association member pulp and paper facilities decreased the volume of discharged water per ton of production by 53 percent," says Scott Milburn, spokesman for the aforementioned industry group. "During the same period, members have also reduced their emissions of key air pollutants by an average of 62 percent."

Surprisingly, plastic may be greener than paper. The EPA reports that making paper bags generates 70 percent more air pollution and 50 times more water pollution than making plastic bags. The former also uses more energy and generates more solid waste.

"Our plastic bags are made from ethane gas pulled out of abundant natural gas," explains Sam Longstreth of Brentwood Plastics, Inc. in Brentwood, Mo. "Through a process of heat and pressure, the ethane is made into long chains of hydrogen and carbon which can be formed into polyethylene and then into bags. It's a much more efficient process than milling paper."

So far, plastic bags come out ahead. But what happens all three kinds of bag are thrown in the trash?

"Landfills are not designed for trash to biodegrade," Ott bluntly states. "Landfills are designed to be anaerobic."

In other words, there's no oxygen — and no light. There's no degradation of materials in a landfill. It only takes up space.

"People don't want the stink from things rotting in landfills," explains Longstreth. "They are designed so that nothing rots."

Arguing over which type of bag breaks down faster is thus pointless. For most purposes, it's all about space — and with the EPA stating that nearly 100 landfills close nationwide every year because they're full, every bit saved counts.

"Plastic bags take up a lot less space in a landfill than paper bags," says Longstreth. "To give you an example, you could put 10,000 plastic bags in the trunk of your car, but it would take a warehouse to store 10,000 paper bags."

Two thousand plastic bags weigh 30 pounds. Two thousand paper bags weigh 280 pounds. It's easy to see how plastic bags are cheaper and easier for local municipalities to cart away in garbage trucks.

Recycling Bags

With any sort of bag, the best solution is to not throw it away at all. Paper should have an advantage here, right?

"Paper is the material of choice for environmentally conscious consumers because it is renewable and recyclable — and recycled at far higher rates than plastic," argues the paper industry's Scott Milburn.

But, according to the American Chemistry Council, it takes 91 percent less energy to recycle a pound of plastic than it takes to recycle a pound of paper.

To many environmentalists, it seems counterintuitive that plastic bags could actually be a better choice than paper. After all, we see plastic bags flying through the air, stuck on trees, clogging our water ways and harming marine life.

"Plastic bags are more of a litter issue," explains Ott. "The focus of legislation and consumer behavior should be on recycling."

The Sierra Club says that when one ton of plastic bags is reused or recycled, 11 barrels of oil are saved — and plastic bags can be made into dozens of useful new products, such as building materials, low-maintenance fencing and decking and, of course, new bags.

What's Your Bag?

So between paper and plastic, plastic wins, right? Not exactly.

"There is no free lunch," says Ott. "Each has its merits and drawbacks. The issue is with all packaging and what the consumer does with it after its initial use."

All sides agree that the reusable cotton grocery bag is the right way to go — if it's reused over and over, and not left in the trunk of your car.

"Canvas will last forever," explains Bob Lilienfeld, editor of the Michigan-based Use Less Stuff Report. "You're lucky if you can use a paper bag two or three times, and plastic will maybe get three to five uses."

"The reusable cotton bag has become a symbol of consumption. We have an emotional connection to the products we buy," he adds. "In the grand scheme of things, it's what we put into the bags, how much of it, and how we transport it."

If your goal is to be a good steward of the environment, limit consumption and buy in bulk.

And when the kid at the checkout asks "Paper or plastic?", the greenest answer is, "Neither — I brought my own."