Two recent studies might end the great grocery bag debate. The debates over which type of grocery bag is best break down into two issues: which are better for the environment, and which are better for your health.

The first study, in June 2010, looked at the question of cloth bag contamination. It’s well-known that though they appear clean, supermarkets offer many opportunities for people to pick up contaminated food: other shoppers contaminate shopping carts, stockroom clerks contaminate containers, and fruit and veggies come from farms pre-equipped with things like salmonella, e.coli, and other dangerous microbes. When shoppers put such things into cloth bags, they can contaminate the bag, and then wind up contaminating uncontaminated foods on their next shopping trip. In “Assessment of the Potential for Cross Contamination of Food Products by Reusable Shopping Bags,” Charles Gerba and his colleagues at the University of Arizona and Loma Linda University studied reusable bags and found that a goodly percentage were indeed contaminated:

Large numbers of bacteria were found in almost all bags and coliform bacteria in half. Escherichia coli were identified in 12% of the bags and a wide range of enteric bacteria, including several opportunistic pathogens. When meat juices were added to bags and stored in the trunks of cars for two hours, the number of bacteria increased 10-fold indicating the potential for bacterial growth in the bags.

Washing the bags might help, but Gerba et al surveyed people who use cloth bags and found that “reusable bags are seldom if ever washed and often used for multiple purposes.”

The second study that should end the “use reusable grocery bags” argument is by the Environment Agency of England and entitled “Evidence: Life Cycle Assessment of Supermarket Carrier Bags.” Published in February 2011, the study performs “cradle to grave” lifecycle assessment of seven different types of bag: the conventional lightweight bags made of high-density polyethylene (HDPE); an HDPE bag doped with a chemical to speed its degradation; a lightweight bag made from a biodegradable starch-polyester blend; a regular paper bag; a heavy-duty “bag for life” made from low-density polyethylene (LDPE); a heavier duty polypropylene bag; and a cotton bag, favorite accessory of the eco-chic. The table below, taken from the study, illustrates what the bags look like, what they can hold, and how much they weigh.

The British study evaluated the bags on a set of environmental criteria including global warming potential; abiotic depletion; acidification; eutrophication; human toxicity; fresh water aquatic ecotoxicity; marine aquatic ecotoxicity, and petrochemical oxidation. Their conclusions?

• The conventional HDPE bag had the lowest environmental impacts of the lightweight bags in eight out of nine impact categories;
• The biodegradable HDPE bag had larger environmental impacts than the regular kind;
• The starch-poly bag (similar to HDPE bags, but made of a mixture of starch and polyethylene) was worse yet, with the highest environmental impact rankings on seven of the nine categories examined;
• The heavy-duty LDPE bag must be used five times in order to get its global warming potential below that of a conventional HDPE bag;
• The non-woven polypropylene “bag for life” had to be used 14 times to get its global warming potential down to that of HDPE;
• Paper bags performed poorly on the environmental impact tests, and must be used four or more times to match the global warming potential of the HDPE bags; and, finally,
• Cotton bags were found to have greater environmental impacts that the conventional HDPE bag in seven of nine categories, even when used 173 times, which is needed for its global warming potential to drop down to that of HDPE.

The performance of alternatives to HDPE looks even worse when you consider that many people recycle HDPE bags as trash bags. The table below shows how many times that non-HDPE bags must be reused in order to bring their global warming potential down to that of an HDPE bag under a range of assumed reuse rates. The first column, for example, shows that you have to reuse a paper bag three times to reduce its global warming potential to that of the HDPE bag, while you have to use an LDPE bag four times, a non-woven polypropylene bag 11 times, and a cotton bag a whopping 131 times to achieve the same end.

The Environment Agency study also did not include the energy requirements of washing cloth bags in hot soapy water or bleach to sanitize them.

So the verdict seems clear: the traditional, thin plastic bag, though increasingly demonized and taxed, has better environmental performance and is likely to be considerably safer for human health. Time to wash and re-task those cloth bags. Maybe grow tomatoes in them or something.

Kenneth P. Green is a resident scholar at the American Enterprise Institute.

FURTHER READING: Green also wrote “Cap-and-Trade by Any Other Name,” “Moderate Republicans and the Ratchet Effect,” and “The National Academy of Blacklists.” He reveals “The Myth of Green Energy Jobs: The European Experience,” explains “Empowering the Free Energy Markets,” and says these are “Not Going Away: America's Energy Security, Jobs and Climate Challenges.”

Image by Rob Green/Bergman Group.