Essential oils (EOs) are concentrated aromatic extracts derived from plants, typically obtained through steam distillation or cold-pressing. These potent liquids are composed of numerous volatile organic compounds (VOCs) that give them their distinct scents. Because these compounds are volatile, essential oils are combustible and should generally be treated as flammable liquids under specific conditions. While no essential oil is absolutely “non-flammable,” some possess chemical properties that make them significantly less prone to ignition than others. The primary factor differentiating the fire risk among these oils is a measurable property called the flash point.
Understanding Essential Oil Volatility
Essential oils are categorized as Volatile Organic Compounds because they readily evaporate into the surrounding air at standard room temperatures. This rapid vaporization allows us to smell them, but it also makes them susceptible to ignition when exposed to sufficient heat or a spark. Unlike non-volatile carrier oils, such as olive or jojoba oil, essential oils do not burn as a liquid; instead, the concentrated vapors they release are what ignite.
This inherent ignition potential is quantified by the flash point. The flash point is the lowest temperature at which a liquid produces enough vapor to form an ignitable mixture with air near its surface. This measurement determines how easily an oil’s vapors can be set alight by an external flame. For safety and shipping regulations, a liquid with a flash point below 61°C (142°F) is often classified as a flammable liquid, a category that includes many common essential oils.
Chemical Factors Determining Ignition Risk
The wide variation in flash points among essential oils is directly linked to their chemical structure and molecular weight. Essential oils are complex mixtures primarily composed of terpenes, which are classified based on the number of carbon atoms they contain. Smaller molecules evaporate more easily, meaning they have a higher volatility and consequently a lower flash point.
The most common components are monoterpenes, which contain ten carbon atoms, such as limonene and pinene. Their small size and lower molecular weight make them highly volatile, contributing to the low flash points of citrus and pine oils. Conversely, sesquiterpenes are larger molecules containing fifteen carbon atoms, making them bulkier and less prone to vaporization. Oils rich in these heavier sesquiterpenes naturally exhibit higher flash points, resulting in a significantly reduced ignition risk.
Essential Oils Categorized by Flash Point
Essential oils can be grouped into categories based on their typical flash point range, providing a practical guide for handling and storage safety.
Highest Ignition Risk
Oils posing the highest ignition risk are dominated by light monoterpenes, generally having flash points below 50°C (122°F). This group includes all cold-pressed citrus oils, such as Lemon (around 48°C/118°F) and Orange oil. The vapors from these oils can reach ignitable concentrations at temperatures not far above a warm room, demanding caution near heat sources.
Moderate Ignition Risk
Oils in the moderate ignition risk group typically have flash points ranging from 60°C to 75°C (140°F to 167°F). Lavender oil (around 70°C) and Peppermint oil (near 66°C) fall into this range. While still classified as flammable liquids, their vapors require a higher temperature to reach the critical concentration for ignition compared to the citrus oils.
Lowest Ignition Risk
The oils that pose the lowest ignition risk have flash points well above 100°C (212°F). This is primarily because they are rich in heavy sesquiterpenes and other high-molecular-weight compounds. Vetiver oil is a prime example, with reported flash points often exceeding 119°C. Other oils in this safer category include Sandalwood and Cedarwood, where the bulky molecules suppress the release of ignitable vapor at common temperatures.
Minimizing Risk Through Safe Storage and Use
Managing the ignition risk of essential oils begins with proper storage, which focuses on suppressing the vaporization of the organic compounds. All essential oils, especially those with low flash points, must be stored in a cool, dark environment away from direct sunlight or any heat source. Heat accelerates the evaporation rate, increasing the concentration of flammable vapors inside the container and the surrounding area. Using tightly sealed glass containers prevents vapor escape and reduces the risk of a spill.
Handling spills requires immediate attention to prevent the formation of a flammable vapor cloud. Spilled oil should be cleaned up promptly using an absorbent material, such as a paper towel or rag. Contaminated material must be disposed of safely, as oil-soaked cloths can self-heat and spontaneously combust if left in a pile.
During use, avoid placing diffusers near open flames, stoves, or other high-heat appliances. For topical applications, dilution with a fatty carrier oil significantly raises the overall flash point of the blend. The presence of the non-volatile carrier oil greatly reduces the concentration of flammable vapors released, making the product safer for everyday use.