Reusable shopping bags help cut down on plastic waste every time you hit the store. But without proper cleaning, they could also be harmful to your health. That's because your eco-friendly shopping bags could be carrying loads of bacteria. One study found harmful bacteria lurking in almost all tested reusable bags. Among them, half carried coliform bacteria and 12% contained E. coli. The bags also carried a wide range of enteric bacteria (a form of salmonella) that can cause food poisoning in humans. However, the same study found that only 3% of shoppers who use reusable bags regularly wash them. Luckily, cleaning your reusable grocery bags is simple and research shows it reduces bacteria by more than 99.9%. The best way to clean canvas or cloth bags is in the washing machine. First, shake out the bags to remove bits of food, dirt, and other debris. To help prevent the bags from fraying, toss them in with other items, such as towels or linens. To kill bacteria, be sure to use a hot water cycle and detergent. Avoid using bleach, as it could weaken or damage the bag's fibers. Plan to wash your reusable bags weekly or immediately after any spills or leaks. Bags made from recycled plastics, like polypropylene or PET, need to be washed by hand. After shaking them out, wipe them down with warm, soapy water. Be sure to clean around the inner and outer seams, flipping the bag inside-out if necessary. To help prevent mildew growth, thoroughly wipe down the bag with a dry towel after washing or let it air-dry completely before putting it away. With all bags, avoid cross-contamination by stashing meat, produce, and dry ingredients in separate sacks. Use an insulated bag for your raw chicken and ground beef and wipe down the interior with a disinfecting wipe after each use. You should also reserve each one for a single purpose (don't substitute the same grocery sack as a gym bag or diaper bag, for example). Avoid storing your bags in the car or another warm area, which can speed up bacteria growth, and keep them in a cool, dry place like the pantry instead.