This is a very understandable concern that many people share. Conventional agriculture does use a host of chemicals as fertilizers and pesticides, many of which remain on produce as residues that are still present when the consumers buys them in the supermarket.

First of all, unless you are buying 100% organic (and even then), you are probably going to have some exposure to pesticides. This is true whether you are eating plant or animal foods, but actually the concentration of these chemicals is higher in animal foods because of bioconcentration. Chemical compounds, particularly fat-soluble ones, become concentrated in the animal's tissue over time, and are still present when the animal is slaughtered. This means that when you eat it, you are also eating residues of many things the animal consumed during its lifetime that stayed in the tissues.

As far as processed food goes, a majority of processed food contains either wheat, corn, or soy, all three of which are grown conventionally using petroleum-based chemicals.

Of course, it's best to eat foods as unprocessed as possible, with as much plant content as possible. One important point to keep in mind about the research on chemical carcinogens is that the levels at which they are tested (usually tested means using animals like mice) are much, much higher than what most people encounter in their daily life. The rational for this level of exposure is that in a research setting, you need to deliver a large dose of the chemical and compare that with no or low exposure in order to see whether or not there really is an effect. The laws regulating the exposure of carcinogens for use in industry prevent certain chemicals from being used if they demonstrate a carcinogenic effect at a certain level of exposure; this level of exposure is always much higher than what we typically encounter.

In contrast, the animal models used by Dr. Campell in his work demostrated a carcinogenic effect by adjusting the levels of dietary protein within the range of human consumption (rats need a fairly similar level of protein compared to humans). So the effect of 20% vs. 5% protein for those animals produced a clear dose-response relationship even at very small increases in consumption.

That being said, common sense dictates that a more conservative approach for health would be to avoid all chemical pesticides or other additives, if possible. The question of whether or not to buy, or how much to buy of organic food is one that many people grapple with because of cost. Because of this, the Environmental Working Group maintains an updated list of what they call "The Dirty Dozen" and "The Clean Fifteen". These lists include the produce with the highest and the lowest levels of chemical pesticides, so that consumers can make more informed choices about shopping. Their message is - if your budget can accommodate some organic but not all, it's better to avoid the foods on the Dirty Dozen and to not worry about buying organic versions of the foods on the Clean Fifteen.

Another way of making organic produce more affordable is to buy a farmshare - a share of the harvest directly from a farm. You can look for one in your zip code at LocalHarvest.org.