Reusing oil degrades its quality and poses health hazards. Each time you reuse an oil, it gets more and more destabilized until it decomposes. Sometimes, physical appearance may not be enough to determine if it is still safe for reuse.
During the frying process, the oil is exposed to atmospheric oxygen and moisture at high temperatures (over 160 °C) for a long time. As a result, many chemical reactions take place during the frying process. These reactions produce free fatty acids (FFA), polymers, and free radicals collectively known as total polar compounds (TPC), that can favor the development of several health problems such as stomach and lung cancer. With this, many countries have already made the monitoring of oil quality (TPC) a legal obligation among food producers, while others are on the way to do it.