Claire, first peanut oil belongs to the group of monounsaturated fats, contains 48% oleic acid, 18% saturated fat and 34% omega-6 linoleic acid. Unlike polyunsaturated fats or oils (clearly avoid) it is relatively stable. Your body makes monounsaturated fatty acids from saturated fatty acids. The monounsaturated fatty acid most commonly found in our food is oleic acid, the main component of olive oil as well as the oils from almonds, pecans, cashews, peanuts and avocados. But the high percentage of omega-6 presents a potential danger, so use of peanut oil should be strictly limited. Researchers are just beginning to discover the dangers of excess omega-6 oils in diets.
Extraction: Oils naturally occurring in fruits, nuts and seeds must first be extracted. In the old days this extraction was achieved by slow-moving stone presses. But oils processed in large factories are obtained by crushing the oil-bearing seeds and heating them to 230 degrees. The oil is then squeezed out at pressures from 10 to 20 tons per inch, thereby generating more heat. During this process the oils are exposed to damaging light and oxygen. In order to extract the last 10% or so of the oil from crushed seeds, processors treat the pulp with one of a number of solvents—usually hexane. The solvent is then boiled off, although up to 100 parts per million may remain in the oil. Such solvents, themselves toxic, also retain the toxic pesticides adhering to seeds and grains before processing begins.
High-temperature processing causes the weak carbon bonds of unsaturated fatty acids to break apart, thereby creating dangerous free radicals. In addition, antioxidants, such as fat-soluble vitamin E, which protect the body from the ravages of free radicals, are neutralized or destroyed by high temperatures and pressures. BHT and BHA, both suspected of causing cancer and brain damage, are often added to these oils to replace vitamin E and other natural preservatives destroyed by heat. (from The Skinny on Fats and other sources).