Wash your reusable shopping bags

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© Rick Gomez/Corbis

A new study from researchers at the University of Arizona and Loma Linda University finds that reusable shopping bags can harbor potentially harmful bacteria if they aren’t cleaned — and 97% of people surveyed said they’d never washed theirs. Yet the study, funded by the American Chemistry Council, also found that washing the reusable bags, either by hand or machine, cut bacterial contamination by more than 99%.

The analysis of 84 bags collected from shoppers entering supermarkets in Los Angeles, Tucson, Arizona and the San Francisco Bay area, found that nearly all bags tested had some bacterial contamination. Roughly half had coliform bacteria, and 12% had Escherichia coli (E. coli). The researchers found that if the reusable bags — most of which are made of woven polypropylene — were exposed to juices from meat products, and then stored in the warm trunk of a car for two hours, bacteria grew 10-fold.

The study was released as debate continues over a California bill that would ban single-use plastic grocery bags. (The American Chemistry Council, which funded the study, represents some of the plastic bag manufacturers, NPR points out.)

Single-use plastic bags are unlikely breeding grounds for bacteria, but washing reusable bags is also a reliable way to limit contamination risk, the authors conclude. They write:

“These results indicate that reusable bags can play a significant role in the cross contamination of foods if not properly washed on a regular basis. It is recommended that the public needs to be educated about the proper care of reusable bags by printed instructions on the bags or through public service announcements.”

Read the full study here.