If they look clean, why do fruits and vegetables need to be washed?

By: Susan Brewer, Ph.D., University of Illinois,
Department of Food Science and Human Nutrition

Because fruits and vegetables grow outdoors, they are exposed to a variety of things most of us don’t want (and shouldn’t) eat: droppings from birds and other animals, mold and bacterial spores, soil, air quality contaminants, and agricultural chemicals.
 
Bird and animal droppings can contain salmonella and E. coli, both of which can cause food-borne infections in man. A few cells can grow and cause havoc in the intestinal tract. Mold spores can provide the “seeds” for mold growth later if the food is stored. If they grow on the food, some can produce toxic compounds (mycotoxins) that can cause cancer and liver damage.
 
Soil can contain a variety of unhealthy things including bird and animal fecal material, heavy metals, spores, and agricultural chemicals. The spores of Clostridium botulinum can often be found in soil. Root crops (such as potatoes and carrots) that grow in the soil and fruits and vegetables that grow low on the ground (such as lettuce, tomatoes and strawberries) are likely to be contaminated with soil, and therefore, with Clostridium botulinum spores. The spores themselves are not generally harmful (to adults), but if they germinate and grow in a low-acid food (such as a vegetable), they produce a neurotoxin which interferes with nerve function (botulism). These spores aren’t inactivated by normal cooking temperatures (boiling).
 
The air can contain substances exhausted from factories as well as chemicals sprayed to kill mosquitoes that can settle on produce. A variety of agricultural chemicals are used in the production of most food crops including fertilizers, weed killers, and insecticides. The “withdrawal” period between the last application and harvest is designed to be sufficient for the chemicals to break down. However, under some circumstances, residues can remain.
 
Finally, water used to irrigate the crop can become a carrier of all of these elements, contaminating the fruit or vegetable. Washing under running water will remove most of these things.
 
Source(s): Susan Brewer, Ph.D., University of Illinois, Department of Food Science and Human Nutrition