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.

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.

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.

As you heat any oil, the number of smelly, toxic aldehyde chemicals goes up steadily — and soybean oil spikes the most right at frying temperatures.