What Oil Should You Not Cook With? The Dangers of Unstable Oils

November 25, 2025 •

4 min read

According to numerous studies, heating certain unstable oils beyond their smoke point can release toxic compounds like aldehydes and free radicals, posing health risks. Understanding what oil should you not cook with is essential for ensuring both food safety and nutritional integrity in your kitchen.

Quick Summary

Heating oils high in polyunsaturated fats, such as common vegetable and seed oils, can degrade them, producing harmful substances and trans fats. It is vital to consider an oil's thermal stability and smoke point, choosing wisely to avoid toxicity and inflammation and protect long-term health.

Key Points

Avoid Unstable Seed Oils: Common vegetable oils like corn, canola, and soy are high in polyunsaturated fats that degrade under high heat, releasing toxic compounds.
Respect Smoke Points: Oils with low smoke points, such as flaxseed or unrefined olive oil, should only be used for low-heat cooking or cold preparations like dressings.
Prioritize Thermal Stability: Look beyond the smoke point. Oils with stable fatty acids, like monounsaturated fats in avocado oil or saturated fats in ghee, resist breaking down during heating.
Don't Reuse Oil: Reheating oil, especially unstable types, drastically increases the concentration of harmful trans fats and toxic byproducts.
Opt for Safer Alternatives: For high-heat cooking, choose oils with high thermal stability, such as refined avocado oil, refined olive oil (pure or light), or ghee.
Choose Minimally Processed Oils for Cold Use: For dressings or finishing, select minimally processed, nutrient-rich oils that haven't been exposed to high heat or chemicals.

The Science Behind Unsafe Cooking Oils

Choosing the right cooking oil goes far beyond just flavor. The stability of an oil when heated is crucial for your health. An oil's 'smoke point' is the temperature at which it starts to smoke and break down, releasing harmful free radicals and toxic compounds. However, a high smoke point alone isn't enough; 'thermal stability'—how well an oil resists breaking down—is an even more important factor.

The Problem with Polyunsaturated Fats

Many common, inexpensive vegetable oils—often called 'seed oils'—are high in polyunsaturated fatty acids (PUFAs). While our bodies need some PUFAs, these fats are chemically less stable than monounsaturated or saturated fats, especially when exposed to high heat and oxygen. The industrial refining process used for these oils, which often involves chemical solvents and high heat, can strip away beneficial nutrients and antioxidants. The result is a cheaper, but far more unstable, oil that is prone to oxidation and the formation of toxic byproducts like aldehydes and trans fats during cooking.

What Oil Should You Not Cook With? The Key Offenders

For high-heat cooking methods like searing, deep-frying, and stir-frying, certain oils should be avoided entirely. These include:

Corn Oil: Commonly used for deep-frying, but high in omega-6s and thermally unstable.
Canola Oil (Rapeseed Oil): Despite a moderately high smoke point when refined, it is high in PUFAs that break down under heat.
Soybean Oil: Another unstable seed oil, often genetically modified and high in pro-inflammatory omega-6s.
Sunflower Oil (Standard Refined): Regular refined sunflower oil, with its high omega-6 content, oxidizes easily and becomes toxic when overheated. High-oleic versions are more stable.
Safflower Oil (Standard Refined): Similar to sunflower oil, standard refined safflower oil is unstable for high-heat cooking.
Vegetable Oil Blends: Often a mix of unstable seed oils like corn, soy, and sunflower, these highly processed blends should be avoided for high-heat applications.
Walnut Oil and Flaxseed Oil: These have very low smoke points and are extremely sensitive to heat. They should be reserved for cold uses like dressings and dips to preserve their delicate nutrients.

The Dangers of Overheating Unstable Oils

When unstable oils are heated past their smoke point or reused repeatedly, the health consequences are significant. The oil's fatty acids break down, leading to the formation of toxic compounds that can have systemic effects on the body.

Formation of Toxic Chemicals: Overheating produces toxic aldehydes and free radicals, which have been linked to inflammation, DNA damage, and chronic illnesses. One particularly harmful compound is HNE (4-hydroxy-trans-2-nonenal), which is linked to vascular, neurological, and diabetic diseases.
Increased Risk of Heart Disease: The process creates harmful trans fats and oxidizes cholesterol, increasing levels of 'bad' LDL cholesterol and raising the risk of cardiovascular disease.
Inflammation: Excessive intake of omega-6 fatty acids from these oils can disrupt the body's omega-3 to omega-6 balance, promoting chronic inflammation.
Oxidative Stress: Free radicals cause oxidative stress, which accelerates the aging process and can harm the liver and other organs.
Reusing Oil: Restaurants often reuse oil multiple times, and this repeated heating massively increases the concentration of toxic byproducts. Home cooks should also avoid reheating oil.

Cooking Oil Comparison: Unstable vs. Safer Alternatives for High Heat


Oil Type	Ideal Use	Stability for High Heat	Key Issues
Standard Seed Oils (Corn, Soy, Sunflower)	Frying, Sautéing (lower temp)	Low. Prone to oxidation and degradation.	High in unstable polyunsaturated fats (PUFAs); releases toxic compounds when overheated.
Flaxseed & Walnut Oil	Cold uses (dressings, finishing)	Very Low. Should not be heated.	Low smoke points; delicate PUFAs oxidize easily and create bitter flavors.
Extra Virgin Olive Oil	Low-medium heat sautéing, dressings	Medium-high (more stable than smoke point suggests)	Low smoke point of ~350-406°F. Best for lower temperatures to preserve flavor and antioxidants.
Avocado Oil (Refined)	High-heat cooking (searing, roasting)	High. Very heat stable due to monounsaturated fats.	High smoke point of ~520°F; neutral flavor and antioxidant-rich.
Ghee (Clarified Butter)	High-heat cooking, roasting	High. Heat stable due to milk solids being removed.	High smoke point of ~450-485°F; nutty flavor.
Algae Oil	All-purpose high-heat cooking	High. Very high smoke point and excellent stability.	Up to 535°F smoke point; neutral flavor and high monounsaturated fat content.

Conclusion: Choosing Wisely for Health

For the sake of your health, it is essential to be selective about your cooking oils. The common, inexpensive seed oils, while convenient, are not built to withstand high temperatures and can compromise your food's nutritional value and introduce harmful compounds. Instead, opt for oils with high thermal stability and appropriate smoke points for your cooking method. Use delicate, unrefined oils like flaxseed or extra virgin olive oil for cold applications, and switch to stable, refined options like avocado oil or ghee for high-heat cooking. Your body will thank you for making the healthier, more informed choice.

More information on healthy dietary fats can be found via the World Health Organization.

Frequently Asked Questions

Some oils are bad for high-heat cooking because they contain unstable polyunsaturated fatty acids (PUFAs) that oxidize and break down when heated. This process releases toxic free radicals and aldehydes, which are harmful to health.

Smoke point is the temperature at which an oil begins to visibly smoke. Thermal stability, however, is a more crucial measure of how well an oil resists chemical breakdown when heated. An oil can have a high smoke point but low thermal stability, making it a poor choice for cooking.

Many commercial 'vegetable oils' are highly refined and high in unstable omega-6 fatty acids, which can promote inflammation when consumed in excess. For this reason, many health experts advise against using them for high-heat cooking.

No, it is generally unsafe to reuse cooking oil. Each time oil is heated, its fat molecules break down further, and reheating drastically increases the amount of toxic compounds, like trans fats, that are released into the food and air.

Cooking with an oil past its smoke point causes it to burn, which not only gives food an unpleasant, burnt flavor but also releases harmful free radicals and toxic compounds into the food and the air.

The best oils for high-heat cooking are those with high thermal stability, such as refined avocado oil, light or refined olive oil, and ghee (clarified butter). These oils have a more stable fatty acid profile that resists oxidation.

Extra virgin olive oil has a relatively low smoke point and is best for low-to-medium heat cooking, sautéing, or dressings. While it is more heat-stable than its smoke point suggests, it's not ideal for deep-frying or very high temperatures, which can degrade its delicate antioxidants and flavor.