Each type of cooking oil makes a unique set of chemicals when heated, and scientists can tell them apart just by looking at what chemicals are made — because their fats are different.

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.

Oils with polyunsaturated fatty acid content exceeding 10% are susceptible to thermal oxidation during cooking, generating harmful aldehydes and polar compounds even below smoke point.

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