1. The smoke point drops with every use, and that's when acrolein forms
Fresh mustard oil has a smoke point around 250°C. Refined sunflower oil sits close to 230°C. Reuse either of them twice and that threshold falls, sometimes by 30 to 40 degrees, because the free fatty acids released during the first fry lower the oil's thermal stability. Once oil crosses its degraded smoke point, glycerol breaks down into acrolein, a volatile aldehyde the WHO classifies as a probable human carcinogen. You can see this happening: the oil starts smoking at a lower temperature than it used to, and the smoke has a sharper, more acrid edge. Most Indian households fry at 180°C to 200°C, which is well inside the danger zone for oil that has already been used twice.
