The obvious purpose of washing produce is to get rid of dirt and other physical intruders. Run cool water over foods with smooth surfaces, such as apples and melons. But those with nooks and crannies, including lettuce, broccoli, artichokes, herbs, and even mushrooms, should be immersed in plenty of water; give them a good swish to dislodge stubborn boarders. (Caution: Mushrooms are like sponges; lift them out quickly.)
However, when the produce has been treated with pesticides, cleaner is better. To penetrate and remove pesticides effectively, water needs a surfactant - a substance that lowers water's surface tension so it can break through films on foods and rinse them away. Make your own surfactant using a nondetergent water softener such as Calgon (2 teaspoons per gallon of water), or you can buy a surfactant cleaner, such as Organiclean. Apply it to foods, then rinse them again with cool water.
But what about microbes like E. coli and salmonella? Commercial packers use a chlorine rinse on produce. You can make your own solution with 1 part household chlorine bleach to 10 parts water, plus 1 teaspoon vinegar for each quart of water. Wash foods in it, then rinse them with plain water to get rid of any chlorine odor.