Polyunsaturated vegetable oils are highly unstable at high heats and easily damaged and subject to rancidity producing huge amounts of free radicals. The refining process also strips them of their natural antioxidants including vitamin E which in turn depletes your body of valuble antioxidants when you eat them.
The only oils stable at high heats include, ghee and coconut oil, rice bran, peanut for cooking, olive oil, sunflower, avocado and mustard seed to name a few for salad dressings and to turn steamed vegetables from a boring side dish to tasty.