Smoking points and healthy vs. unhealthy oils
Every oil will eventually be damaged if heated above a certain temperature. Each oil has its own smoke point, the temperature at which the oil begins to be damaged by heat. The word smoke point is a bit deceiving, as you may not see a billow of smoke appear when an oil hits exceeds its smoke point temperature, but the oil will be damaged and begin to form harmful compounds, such as trans fats and other harmful compounds. If you keep heating it, eventually you will see smoke, but by then it was way too late.