Here is a list of foods often thought to be healthy in retail markets but often have been treated with chemicals in the growing or shipping process to make them look nice:
The Environmental Working Group (EWG) recommends going organic if you purchase these items from retail markets or direct from growers.
The EWG also reports that the following foods have the lowest risk of containing pesticide residue and are therefore the safest to eat when purchased from any source:
Sometimes the foods from the first list above can cost more when you buy organic but, if eating these helps your family avoid health risks such as developing cancers during their lifetime, it is probably worth the expenditure now to avoid often enormous costs later in life. Organic foods are those grown without the use of conventional pesticides, synthetic fertilizers, sewage sludge, ionizing radiation or genetic modification (GMO).
This information is available in more detail from the October issue of the Journal of the American Medical Association from a study of 70,000 subjects.