The distinction between a liquid being classified as flammable or combustible is a technical definition based on a specific physical property. This classification is guided by established regulatory standards, setting clear boundaries for safe handling, storage, and transport. The defining factor is the flash point, which dictates the minimum temperature at which a liquid produces enough vapor to ignite in the presence of an ignition source.
Defining Flammable and Combustible Materials
The terms “flammable” and “combustible” are defined by a liquid’s flash point. This is the lowest temperature at which the liquid gives off enough vapor to form an ignitable mixture with air. It is the vapors, not the liquid itself, that burn.
A liquid is classified as flammable if its flash point is below 100°F (37.8°C). These liquids ignite readily at normal room temperatures because they constantly release enough vapor to form a flammable mixture. Gasoline, for example, has a flash point far below freezing, making it highly volatile.
Combustible liquids, by contrast, have a flash point at or above 100°F (37.8°C). These liquids require external heating or a higher ambient temperature to produce enough ignitable vapor to sustain a flame. The primary regulatory distinction centers on this 100°F threshold.
While some regulatory schemes, such as those from the Occupational Safety and Health Administration (OSHA), have moved toward classifying nearly all liquids with a flash point under 200°F (93°C) as “flammable,” the practical distinction based on the 100°F flash point remains the clearest way to understand the difference in fire risk. This temperature divide dictates the specific safety measures required for storage and use. The higher the flash point, the lower the inherent fire hazard under typical operating conditions.
Kerosene’s Ignition Properties and Classification
Kerosene is classified as a combustible liquid. This classification is directly determined by its flash point, which typically falls in the range of 100°F to 150°F (38°C to 66°C), depending on the specific grade of the fuel. The US Energy Information Administration notes that kerosene has a minimum flash point of 100°F.
Because its flash point is at or above the 100°F regulatory threshold, kerosene does not readily ignite from a spark at standard room temperature. It must be heated, or its vapors must be concentrated, to reach its flash point for ignition to occur. This is why kerosene is commonly used in wick-fed lamps or heaters; the wick draws the liquid up and heats it sufficiently to vaporize and burn safely.
By the NFPA definition, kerosene is a Class II or Class III combustible liquid. This places it in a lower fire hazard category than highly volatile flammable liquids like gasoline. The specific physical property that distinguishes kerosene is its lower vapor pressure compared to flammable liquids, meaning it releases far less ignitable vapor into the air at the same temperature.
Safe Handling and Storage of Kerosene
The combustible classification of kerosene translates into specific safety requirements for storage and handling. Although less volatile than gasoline, it still presents a fire hazard and must be treated with caution. Kerosene should be stored in containers specifically approved for combustible liquids, such as those made of metal or high-density polyethylene.
Many jurisdictions require that kerosene be stored in blue containers to help distinguish it from other fuels like gasoline, which is often stored in red cans. These containers must be clearly labeled and kept tightly sealed to prevent the release of vapors and contamination.
The storage location requires a cool, dry, and well-ventilated area away from ignition sources, including heat, sparks, or direct sunlight. Kerosene vapors are heavier than air, meaning they can travel along the floor and collect in low areas, potentially finding a distant ignition source and flashing back. Proper ventilation helps disperse any released vapors, minimizing the risk of forming an ignitable concentration.