If you’re one of the many Philly shoppers that stuff their reusable, green-friendly shopping bags into their bags and backpacks so they’re ready at a moment’s notice to tote home any and all grocery goods, it’s time to ask yourself one very important question: When was the last time you tossed ‘em in the wash?

According to a recent joint food-safety research report issued by the University of Arizona and Loma Linda University in California, reusable grocery bags can be a breeding ground for dangerous food-borne bacteria, as they create an opportunity for cross contamination of foods. The study randomly tested 84 reusable bags carried by shoppers and found bacteria levels, including those of E. coli, that were significant enough to cause a wide range of serious health problems and even death. (Blame leaky milk cartons, meat products, or that time you tossed your books or sneaks in there in between shops.) However, with regular hand- or machine-washing of these bags, the bacteria levels were reduced by 99.9%. So stay green, but be clean. — Kelly O’Shea