Most plastic bags don’t biodegrade, so reusable ones are a smart option. But when researchers tested reusable bags shoppers were using, almost all had large numbers of fecal bacteria, according to a 2011 study in Food Protection Trends.
“Some bags had more E. coli than underwear,” says Charles P. Gerba, Ph.D., study co-author and environmental microbiologist at the University of Arizona in Tucson. Even worse? Bacteria increased tenfold when used bags sat in warm car trunks for two hours.
The solution: Wash bags regularly—and after each use if they carried meat products. “Most of the bacteria came from meat,” Gerba says. Machine-washing in hot water is best, so Gerba recommends cloth bags.