With the arrival of reusable grocery bags, there appeared to be, at long last, a definitive eco-friendly answer to that question posed daily to consumers: Paper or plastic? Alas, it was not to be. Researchers recently discovered that the supposedly green alternative actually harbors bacteria, mold and other unappetizing organisms, albeit natural.

That we still lack a neat resolution to the checkout line dilemma exposes a fundamental truth about all environmental issues. For every choice related to resource use, trade-offs are inevitable. In some cases, the upsides clearly offset the downsides, or vice versa. But in many matters, as with the grocery bag quandary, the calculations can get dizzyingly complex.

Some governments certainly have not considered the issue in any depth before issuing regulatory edicts. In June, Achim Steiner, executive director of the United Nations Environment Program, called for a global ban on plastic bags. Even China's State Council has weighed in on the issue by forbidding shops from offering free plastic bags and encouraging the masses to tote their groceries in baskets instead.

Retailers are instituting bag policies, as well. On Earth Day last year, Whole Foods announced plans to discontinue the use of conventional plastic bags, although the store will sell upscale ones for 99 cents apiece (or canvas ones for $6.99 to $35).

The effective endorsement of paper over plastic has offended some green groups who (justifiably) argue that such moves lack an objective scientific basis.

Myriad factors must be considered when calculating the pros and cons, benefits and costs, of each bag type, including all the environmental and economic impacts of various energy and chemical inputs and outputs involved in production, distribution, and reclamation (or recycling).

Conventional wisdom holds that plastic bags -- being synthetic -- are environmentally destructive, while paper bags -- the spawn of trees -- are the greener option. But life cycle analyses of both products indicate that bags made from paper require more energy to produce, create more pollutants and take up considerably more landfill space than plastic.

According to the Progressive Bag Affiliates, a division of the American Chemistry Council, paper bags generate 70 percent more emissions and 50 percent more water pollutants than plastic, which requires 40 percent less energy to produce and generates 80 percent less solid waste.

Using plastic bags involves trade-offs, too. Earth Day Canada (2009) reports that production of plastic bags involves five of the top six chemicals responsible for the greatest proportion of hazardous waste generation. Moreover, most plastic bags are made from fossil fuels.

In their favor, paper bags are recycled at a higher rate than plastic ones and require less energy to be recycled. They are also biodegradable, although neither type of bag breaks down all that well in the dark, dry and oxygen-deprived confines of modern landfills.

When exposed to ultraviolet light from the sun, the plastics will, over time, degrade into smaller particles. But they will not reduce to organic matter, as is the case with paper.

Not all reusable bags are created equal, either. For example, those made from polypropylene (plastic) are cheap to manufacture but wear out quickly, limiting their reusability. Canvas types are more durable, but cotton production is water intensive and typically involves large quantities of pesticides. Bags made from jute are strong, but most of the fiber used to create the bags is imported and requires considerable fuel consumption to get to market.

Perhaps most troubling for consumers is the recent discovery that reusables pose a potential health risk, according to testing by two independent laboratories (and an evaluation of the results by a third). Researchers obtained the bags for testing from shoppers leaving major grocery stores. Each shopper was offered a new reusable bag as replacement for their existing bag, and the participants were asked a series of questions about their bag, including its age, how often it was used and whether it was ever washed. Four new bags also were tested as controls.

More than 30 percent of the used bags had unsafe levels of bacterial contamination, 40 percent had yeast or mold, and there was fecal bacteria embedded in the surface of some.

In contrast, conventional plastic bags showed no evidence of bacteria, mold, yeast, or coliforms. According to the researchers, The moist, dark, warm interior of a folded reusable bag that has acquired a small amount of water and a trace of food contamination is an ideal incubator for bacteria.

Each type of bag has merits and drawbacks. That's all the more reason for governments to avoid dictating the choice for consumers and retailers, particularly when the elected officials doing the choosing are more intent on scoring green political points than actually improving the environment.