Besides the lower levels of heavy metals, there are the chemical residues themselves. The big question is whether or not the accumulation of pesticide residues in non-organic foods is a real health concern or not. Studies have never been able to conclusively show a direct correlation between residues in food and a decline of human health, but there are numerous problems in doing any such study. The first is that you would need a population of people who are free of chemical residues to compare to, and no one has been able to find such a group. According to an ongoing EPA study of fat samples taken from surgeries and autopsies across the country, we are all loaded with chemical residues.
Similar studies done on other countries all show the same results.
The clearest studies that we have about pesticide residues and disease are those looking at breast cancer. In the last few years there have been a series of studies, each building upon the other, looking at the level of DDT, DDE, and PCB in women, They have very clearly shown that chemical residues in the serum and fat cells of women greatly increase the risk of breast cancer. I am not waiting for them to prove it before I change my eating habits. As a clinician who sees numerous environmentally poisoned people with health problems, I am convinced of an association between chemicals and disease. The biggest source of exposure for many people is their workplace, then their homes, followed by air, food and water.