And while the C.D.C. recommends washing all produce with water, including heads of lettuce, it does not recommend washing other forms of bagged lettuce, which has already been washed before bagging. “Your chances of contaminating it in your kitchen” — with contaminants that may already be on your kitchen countertop, hands or elsewhere — “are actually higher than if you didn’t wash the salad greens,” notes Dr. Gieraltowski.

source: https://www.nytimes.com/2018/04/19/well/eat/romaine-lettuce-...