There are a number of studies that show how various pesticides, even if applied correctly, end up in the bodies of farm workers. Organophosphate pesticides have been linked to health problems including abdominal pain, dizziness, headaches, nausea, vomiting, and skin & eye problems. Pesticide exposure has also been linked to respiratory problems, memory disorders, dermatologic conditions, cancer, depression, neurologic deficits, miscarriages, and birth defects.
Keep in mind that these pesticides work in very small quantities to efficiently kill very small animals and insects, but need to be applied in large amounts to ensure that all animals harmful to a crop (or substantially all) are killed. Pesticides are ingested by humans largely by eating the foods made from crops where pesticides have been applied. Food residue limits are established by law to limit a child's lifetime ingestion of each pesticide to a level presumed to be safe.
Imported fruits and vegetables from South America are more likely to contain high level of pesticides, including some pesticides banned in North America.
A 2002 study of organic foods, showed a one-third reduction in residues compared to conventionally grown foods. A US study of pesticide residues by the USDA (US Department of Agriculture) has been testing over 60 food items for over 400 different types of pesticides found in 2005 that 30% of foods had not detectible residues, 30% had one residue, and 40% had more than one pesticide residue.
A 2006 study measured levels of organophosphorus pesticide exposure in 23 schoolchildren, and found dramatic and immediate drops in pesticide exposure after replacing their diet with organic food.
The United States Environmental Protection Agency and state agencies periodically review the licensing of suspect pesticides, but the process of de-listing is slow.