A carcinogen is any substance or agent that can cause cancer in humans, typically by creating mutations in a cell’s DNA and disrupting normal growth regulation. The term “oil” encompasses a vast array of chemicals with fundamentally different compositions. Mineral oils derived from petroleum are complex mixtures of hydrocarbons, while cooking oils are composed primarily of triglycerides. This chemical difference means the carcinogenic risk associated with oil depends entirely on its source, refinement level, and specific exposure pathway.
Occupational Exposure: Mineral and Petroleum-Based Oils
The most definitive link between oil and carcinogenicity exists in the industrial and occupational sphere, specifically with unrefined petroleum products. Untreated and mildly treated mineral oils, which are liquid by-products of crude oil refining, have been classified by the International Agency for Research in Cancer (IARC) as Group 1 carcinogens, meaning they are carcinogenic to humans. This classification is based on sufficient evidence from human studies involving long-term skin exposure.
The primary cancer-causing agents in these minimally refined mineral oils are Polycyclic Aromatic Hydrocarbons (PAHs). PAHs are genotoxic compounds that directly damage DNA and initiate carcinogenesis. Historically, workers in occupations like mule-spinning and metal machining experienced an increased incidence of skin cancers, especially of the scrotum, due to repeated skin contact with these oils.
Modern industrial practices have largely mitigated this risk through extensive refinement processes. Techniques like hydro-treatment and solvent extraction effectively remove toxic PAHs. Highly-refined mineral oils, which have had their PAH content drastically reduced, are classified by IARC as Group 3, or “not classifiable as to its carcinogenicity to humans.”
However, the carcinogenic risk reappears when petroleum-based oils are used and degraded. Used engine oil (used crankcase oil) contains mutagenic compounds, regardless of whether it originated from gasoline or diesel engines. This degradation occurs as the oil breaks down and accumulates combustion byproducts and contaminants, turning a relatively safe product into a potential hazard upon prolonged skin contact.
Dietary Concerns: Cooking Oils and Thermal Degradation
The risk profile for common household cooking oils, such as olive, canola, or sunflower oil, differs significantly from industrial oils. In their unheated state, these oils are composed of triglycerides and are not classified as carcinogens. Potential harm emerges only when these oils are heated, particularly to very high temperatures or when reused multiple times for frying.
High-temperature cooking causes the oil to undergo thermal oxidation, hydrolysis, and polymerization, breaking down the fatty acids. This degradation generates a variety of toxic compounds, including aldehydes and other reactive compounds. Specific products of concern include malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), which interact with DNA and cellular structures, increasing oxidative stress.
The chemical stability of a cooking oil is determined by its fatty acid profile. Oils rich in polyunsaturated fatty acids (e.g., certain seed oils) are significantly less stable than those high in monounsaturated fatty acids (e.g., olive oil). The multiple double bonds in polyunsaturated fats make them more susceptible to oxidation and the formation of toxic aldehydes when heated.
Repeatedly heating oil, a common practice in commercial and domestic settings, accelerates this degradation, causing the oil to darken, increase in viscosity, and lose its smoke point. This accumulation of degradation products, including some PAHs and polymeric compounds, raises a health concern, as both the consumption of food fried in this oil and the inhalation of cooking fumes contribute to overall exposure.
How Carcinogens Are Classified by Global Health Agencies
The formal assessment of a substance’s carcinogenic potential is conducted by independent scientific bodies, most notably the International Agency for Research on Cancer (IARC), the specialized cancer agency of the World Health Organization. IARC evaluates scientific evidence, including human and animal studies, to determine the strength of the link between an agent and cancer risk.
IARC uses a five-group classification system to categorize agents based on the strength of the evidence, not the level of risk. Group 1 is reserved for agents that are “Carcinogenic to humans,” where there is sufficient evidence from human studies.
The subsequent categories are Group 2A, or “Probably carcinogenic to humans,” and Group 2B, or “Possibly carcinogenic to humans.” Agents are placed in these groups when the evidence is strong but not definitive in humans, often relying on convincing animal studies or mechanistic evidence. The final categories, Group 3 and Group 4, are for agents that are “Not classifiable” or “Probably not carcinogenic,” respectively.
An agent’s classification depends entirely on its specific preparation or state of exposure. For example, the difference between an IARC Group 1 mineral oil and an IARC Group 3 highly-refined mineral oil is the degree of refinement, which removes the PAH contaminants. This framework provides a scientific standard for differentiating risk across the diverse array of substances referred to as “oils.”