Pesticides are chemicals used to eliminate or control insects, plants, fungi, animals, or microorganisms in order to protect food crops and other plants. Some pesticides have been classified as carcinogens.
According to the National Cancer Institute, general studies of people with high exposures to pesticides, such as farmers, pesticide applicators, manufacturers, and crop dusters, have found high rates of blood and lymphatic system cancers; cancers of the lip, stomach, lung, brain, and prostate; as well as melanoma and other skin cancers. The general population may be exposed to low doses of pesticides from fruits and vegetables bought from the supermarket or from contaminated surface or ground water.
On the other hand, organically grown foods are not subjected to harmful substances used in conventional farming. They are also richer in nutrients, more flavorful, and promote a stronger immune system.
Read this guide to pesticides presented by the Environmental Working Group: Shopper's Guide to Pesticides