This is an excerpt from Wikipedia on PTFE which is the key ingredient to make Teflon cookware.
While PTFE is stable and nontoxic, it begins to deteriorate after the temperature of cookware reaches about 533 K (260 °C; 500 °F), and decomposes above 623 K (350 °C; 662 °F). These degradation by-products can be lethal to birds, and can cause flu-like symptoms in humans. In May, 2003, the environmental research and advocacy organization Environmental Working Group filed a 14-page brief with the U.S. Consumer Product Safety Commission petitioning for a rule requiring that cookware and heated appliances bearing non-stick coatings carry a label warning of hazards to people and to birds.
Meat is usually fried between 400 and 450 °F (204 and 232 °C), and most oils will start to smoke before a temperature of 500 °F(260 °C) is reached, but there are at least two cooking oils (refined safflower oil and avocado oil) that have a higher smoke point than500 °F (260 °C). Empty cookware can also exceed this temperature upon heating.
As of October 29, 2012, environmental exposure (i.e., via water and air) to perfluorooctanoic acid (PFOA or C8) has been linked to high cholesterol, ulcerative colitis, thyroid disease, testicular cancer, kidney cancer, and pregnancy-induced hypertension"
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