I read that a concentration of bacteria in one place, the bag, is the concern. Machine washing after each use is too much hassle for me. I don't like doing laundry to begin with. I keep a long laundry basket in my trunk to unload the bags from the store. I take the basket in, unload, and return to fill it again. I do everything as simple and stress free as I can. I think it keeps me healthier. I know it frees up tons of time to be quilting!