Deep frying, shallow frying, sautéing, or roasting changes the molecular structure of the oil as it breaks down through oxidation.  

Fat molecules react with oxygen in the air to form lipid oxidation products (LOPs) including aldehydes. The higher the cooking temperature or the more frequently it is reheated, the more of these potentially toxic products are produced in the oil, especially if it is close to 'smoke point'.

• Sunflower and seed oils are high in polyunsaturated fats and degrade the easiest.
• Olive oil and rapeseed oil are high in monounsaturated fats and are better for heating as they produce fewer aldehydes. The highly processed ones have a higher smoke point and are safer to cook with than unrefined ones.

To reap health benefits, use extra virgin olive oil cold in things like salad dressings or mayonnaise.

Rapeseed oil is extracted from the black seeds of the yellow-flowering rape plant. Rapeseed oil is unsaturated fat and high in monounsaturated fat, making it a healthier type of oil. It is rich in Vitamin E and Omega 3,6, and 9 fats.

It is available as refined and unrefined. In the UK, oil labelled 'vegetable oil' is usually refined rapeseed oil. Unrefined rapeseed oils are labelled 'cold-pressed', 'premium', 'virgin' or extra virgin' and have a delicate nutty flavour.

Rapeseed oil is great for frying, but it is important to use as little oil as possible and cook at lower temperatures.

Saturated fats are normally solid at room temperature, for example, red palm oil, lard or beef dripping. They generate fewer aldehydes when heated but might carry other risks.

Coconut oil is extremely high in saturated fat. The refined version is the best of all to cook with.

An oil's sustainability depends on the way it is produced.

Sunflower and rapeseed oils made outside Europe and the UK are sprayed with neonicotinoids and insecticides implicated in killing bees, butterflies, moths and other insects. As a result, they are now banned in Europe and the UK. 

Oils made from crops known to cause environmental damage, such as avocado and soy (deforestation), and almonds (water requirements, insecticide use), might not be a green choice either.