Below is the best article I have read on cooking with extra virgin olive oil. In essence, the phenolitic compounds found in high quality extra virgin olive oil protect the oil during the cooking process so it does not break down into harmful substances. Low quality olive oils do not have these compounds so it is actually more harmful, not less harmful, to cook with them. There are many other factors that make high quality extra virgin olive oil the best choice for cooking. I highly recommend reading the entire article.