The worst cooking oil you can use is any oil containing partially hydrogenated fats, which are the primary dietary source of industrial trans fats. These engineered fats are linked to more than 278,000 deaths globally each year, and the World Health Organization has called for their complete elimination from the food supply. Beyond that single worst category, the answer gets more nuanced: an oil’s harm depends less on the type sitting in the bottle and more on how it’s used, how many times it’s reheated, and whether it’s pushed past its limits.
Partially Hydrogenated Oils Are the Clear Worst
Partially hydrogenated oils are vegetable oils that have been chemically altered to become more solid and shelf-stable. The process creates industrial trans fats, which raise LDL (“bad”) cholesterol, lower HDL (“good”) cholesterol, and increase the risk of heart disease, stroke, and type 2 diabetes. The WHO recommends limiting trans fat to less than 1% of total daily calories, which works out to less than 2.2 grams on a 2,000-calorie diet. Their stronger recommendation: avoid industrially produced trans fats entirely.
Many countries have now banned or restricted partially hydrogenated oils, and almost half the world’s population lives under policies aimed at eliminating them. But they haven’t disappeared. An analysis by the Environmental Working Group found that 87% of more than 7,500 foods containing partially hydrogenated oils didn’t clearly disclose that fact on their labels. In the U.S., manufacturers can label a product as “0 grams trans fat” if it contains less than 0.5 grams per serving, a loophole that allows small amounts to accumulate across a day’s eating.
Other ingredients can also carry trace amounts of trans fats. Fully hydrogenated oils, monoglycerides and diglycerides, and even some flavoring compounds may contain small quantities. Refined oils like soybean, canola, cottonseed, and corn oil naturally contain minor amounts of trans fat from processing, though at levels far below those found in partially hydrogenated products.
What Makes Any Oil Harmful When Heated
Even oils that are perfectly fine at room temperature can become problematic when heated beyond their stability. When cooking oil breaks down under high temperatures, it generates a family of toxic compounds called lipid oxidation products. These include reactive aldehydes that, with long-term exposure, can bind to proteins and DNA in the body and raise LDL cholesterol levels.
The key factor is the oil’s fatty acid profile. Oils high in polyunsaturated fats (those with multiple fragile chemical bonds) are more vulnerable to oxidation than oils rich in monounsaturated or saturated fats. When researchers heated soybean oil, palm oil, olive oil, and lard to the same temperatures, the oils with more polyunsaturated fatty acids consistently produced higher levels of harmful aldehydes. At standard frying temperatures around 180°C (356°F), these toxic byproducts increase with both temperature and time, reaching equilibrium after roughly six hours of continuous heating.
This doesn’t mean polyunsaturated oils are dangerous in a salad dressing or a quick sauté. The concern is specifically about sustained high-heat cooking and, especially, reusing oil multiple times.
Oils That Perform Worst Under Heat
The oils most prone to breaking down during cooking are those with the highest polyunsaturated fat content and the lowest smoke points. Unrefined flaxseed oil, for example, is extremely rich in polyunsaturated fats and has a smoke point so low it should never be used for cooking at all. Unrefined walnut oil and wheat germ oil fall into a similar category.
Among commonly available cooking oils, these tend to be the least stable for high-heat methods like frying and deep-frying:
- Unrefined sunflower oil is high in polyunsaturated fats and degrades quickly at frying temperatures.
- Soybean oil is the most widely consumed oil in many countries but is relatively high in polyunsaturated fat, making it more reactive when heated repeatedly.
- Corn oil has a similar polyunsaturated fat profile and produces meaningful levels of oxidation products under sustained heat.
- Cottonseed oil carries an additional concern: it naturally contains gossypol, a compound toxic to humans. Refining reduces gossypol levels, but the process also increases aldehydes and hydrocarbons in the finished oil.
Refined versions of these oils have higher smoke points (often 400°F or above) and will tolerate more heat before visibly smoking. But smoke point alone doesn’t tell the full story. An oil can begin generating harmful compounds well before it starts smoking, particularly if it’s polyunsaturated.
Oils That Hold Up Better
Oils with more monounsaturated or saturated fat resist breakdown more effectively. Refined avocado oil has the highest smoke point of any common cooking oil at around 520°F, and its fat profile is predominantly monounsaturated. Extra virgin olive oil, despite its moderate smoke point of 375 to 410°F, contains natural antioxidants that help resist oxidation, making it surprisingly stable for everyday cooking temperatures. Refined coconut oil has a smoke point of 400 to 450°F, though its high saturated fat content is worth considering for overall dietary balance.
Reused Oil Is Worse Than Any Fresh Oil
One of the most overlooked risks in cooking oil safety isn’t which oil you pick but how many times you use it. Each frying cycle accelerates degradation. The oil darkens, its antioxidant capacity drops, and the concentration of polar compounds (a standard measure of oil deterioration) climbs. Restaurants and food manufacturers typically discard frying oil once polar compounds exceed 25% of the total, a threshold set by food safety regulators in many countries.
At home, you can spot degraded oil without a lab test. Oil that has turned noticeably darker, developed a sticky or thick consistency, foams excessively when food is added, or smells stale or “off” has broken down past the point of safe use. If your frying oil hits any of those markers, it’s time to replace it, regardless of the type.
The Omega-6 Debate
You may have seen claims that common seed oils like soybean, sunflower, and corn oil are harmful because they’re high in omega-6 fatty acids, specifically linoleic acid, which supposedly drives chronic inflammation. This idea has gained traction online, but the clinical evidence doesn’t support it. A systematic review of 15 randomized, placebo-controlled trials in healthy humans found that varying linoleic acid intake had no significant effect on C-reactive protein (the most common blood marker of systemic inflammation) or on a long list of other inflammatory markers, including several involved in blood clotting and immune signaling.
That doesn’t mean these oils are ideal for every purpose. Their vulnerability to heat-related breakdown is a real, well-documented concern. But the specific claim that the omega-6 fats in seed oils cause inflammation in healthy people isn’t backed by the available trial data.
A Practical Ranking
If you’re looking for a simple hierarchy of what to avoid, here it is, from worst to least concerning:
- Any partially hydrogenated oil: The only category experts recommend eliminating completely.
- Any oil reused multiple times past visible degradation: More dangerous than the wrong fresh oil.
- Unrefined, high-polyunsaturated oils used for high-heat cooking: Flaxseed, walnut, and unrefined sunflower oil should stay off the frying pan.
- Refined high-polyunsaturated oils used for deep frying: Soybean, corn, and cottonseed oil are common and inexpensive but degrade faster than alternatives under sustained heat.
For most home cooking, the practical move is straightforward: use an oil that matches your cooking temperature, don’t reuse it until it breaks down, and save delicate unrefined oils for dressings and finishing. The single worst choice isn’t a specific bottle on the shelf. It’s partially hydrogenated fat, and the second worst is any oil you’ve pushed well past its limits.