Even healthy-seeming oils like avocado oil can break down into harmful stuff when heated because they have too much of a fragile kind of fat.

Oils with polyunsaturated fatty acid content exceeding 10% are susceptible to thermal oxidation during cooking, generating cytotoxic aldehydes and lipid peroxides.

The kind of fat in an oil—like how much linolenic, linoleic, or oleic acid it has—decides what chemicals are made when you heat it up, and how much of them.

When you heat different cooking oils really hot for a long time, they break down and make harmful chemicals, and the type of oil you use determines which chemicals are made.

When you cook with certain plant oils like sunflower or soybean oil, even at normal cooking temps that aren’t hot enough to make them smoke, they can break down into harmful chemicals — and if you heat them for 30 minutes straight, those harmful chemicals become ten times more toxic.

When you fry food over and over again, some cooking oils break down faster than others—oils with more saturated fats (like coconut or palm oil) hold up better and make fewer harmful gunk particles than oils with lots of unsaturated fats (like sunflower or olive oil).