|Apples are #1 on the 2011 "Dirty Dozen"|
Regular farms often use synthetic pesticides that are terrible for the people applying them, contaminate the soil and water, and often end up on your food. By contrast, synthetic pesticides are banned or strictly limited on organic farms. Instead organic farmers often use natural pest control methods like predation.
It doesn't look like there are any studies that show the health effects of prolonged exposure to low doses of pesticide from food. (Seriously, who would sign up for that study anyway?) However, Wikipedia informed me that larger pesticide doses can cause everything from birth defects to cancer. The American Medical Association recommends you limit your exposure to pesticides.
In the US, the Environmental Protection Agency sets the 'tolerance' level for how much pesticide residue is allowable on food, however this legal tolerance level is not a safety level. The Environmental Working Group gives this great example of why these tolerances are not fit protection:
"We estimate that EPA's "safety" levels for 11 pesticides permitted on apples are too high. Theoretically, a 4-year-old boy who ate one large apple with the maximum permitted amount of just one of those 11 pesticides would exceed the daily safe intake of that pesticide. Kids who eat more than one apple a day and more fruits and vegetables will consume even greater amounts of pesticides. Those who live near farm fields or in homes where pesticides are used will pick up even more toxic chemicals. "Fittingly, apples were #1 for pesticide contamination on the EWG's 2011 "Dirty Dozen". Even if you can't afford to buy all organic, the Dirty Dozen are the foods with the most pesticide residue, which the EWG suggests you buy organic. (Or you could substitute a cleaner conventional alternative.) They also publish the "Clean Fifteen", which are the 15 foods with the least pesticides out of all the ones they test. You can click here to see both lists and here to see some commonly asked questions about pesticide residue.
If you want to buy organic, but find your grocery store doesn't really have a great or affordable selection you could try a farmer's market. Farmer's markets are great because you often get to meet the people growing the food and ask questions. In many cases small farms grow food organically, they just lack the USDA's certification. You can also haggle to some extent, especially if your buying in bulk.