When people ask at what temperature alcohol burns, they are usually seeking the temperature required to start the fire, not the temperature of the flame itself. Alcohol, such as ethanol, methanol, and isopropyl alcohol, are volatile compounds whose flammability is dictated by how easily they release ignitable vapors. Understanding the fire risk of these liquids requires knowing a set of temperatures that determine when and how combustion begins. This focus on ignition temperatures informs safety standards for the storage and use of alcohol-based products.
Defining Critical Ignition Temperatures
Flammable liquids like alcohol have three distinct temperature thresholds that define their fire hazard. The Flash Point is the lowest temperature at which a liquid emits enough vapor to form a flammable mixture with air that will ignite momentarily when an external ignition source, like a spark or flame, is applied. This ignition is not sustained.
The Fire Point is a slightly higher temperature, typically only a few degrees above the flash point, where the liquid produces enough vapor to sustain combustion for at least five seconds after the external ignition source is removed. A liquid at its fire point will continue to burn, whereas at its flash point, the fire extinguishes almost immediately.
The third and highest temperature is the Autoignition Temperature (AIT), which is the minimum temperature required for the substance to spontaneously ignite without any external spark or flame. At this temperature, the substance’s own heat energy is sufficient to initiate combustion, making it a measure of the risk of spontaneous combustion from contact with hot surfaces.
Ignition Points of Common Alcohols
The specific temperatures required to initiate combustion vary significantly between different types of concentrated alcohol.
Methanol (wood alcohol), often used in industrial applications and as a fuel, has a flash point typically ranging from 11°C to 13°C (52°F to 55°F). Its autoignition temperature falls between approximately 433°C and 470°C (811°F and 878°F).
Ethanol (the alcohol found in beverages and bio-fuel), in its pure form, has a flash point of about 13°C to 14°C (55°F to 57°F), meaning it can ignite at or below average room temperature. The fire point for ethanol is close to 16°C (61°F). The autoignition temperature for ethanol is around 365°C to 369°C (689°F to 696°F).
Isopropyl Alcohol (rubbing alcohol), commonly found in household products at high concentrations, has a flash point of approximately 12°C to 13°C (54°F to 55°F). This low flash point means its vapors are easily ignitable at room temperature. Its autoignition temperature is typically listed around 399°C (750°F) in air.
How Alcohol Concentration Affects Burning
The figures for ignition points apply primarily to concentrated alcohols, but the presence of water dramatically changes the flammability profile. Water is an inert component that does not contribute to the flammable vapor mixture; instead, it acts as a heat sink. This means significant heat energy is required to vaporize the water before the alcohol can release enough flammable vapor to ignite.
The presence of water raises the flash point, a principle demonstrated by the proof system used for spirits. While pure ethanol has a flash point of 13°C, a 70% alcohol solution, such as many hand sanitizers, sees its flash point increase slightly to around 16°C to 17°C (61°F to 63°F). This small increase is still low enough to pose a fire risk at typical room temperatures.
Conversely, common alcoholic beverages like wine or beer are not flammable because their high water content elevates the flash point well above room temperature. Spirits that are 80-proof (40% alcohol by volume) have a flash point of approximately 26°C (79°F). The water content effectively prevents the formation of an ignitable vapor-air mixture at normal temperatures.
Practical Safety Implications of Alcohol Fires
The low flash points of concentrated alcohols mean that storing them in containers that are not tightly sealed is hazardous, as the liquid constantly releases flammable vapors at ambient temperature. These vapors are heavier than air and can travel along floors or low surfaces to a distant ignition source and flash back to the main container. Proper ventilation and cool storage are necessary for high-proof alcohols.
A serious hazard associated with alcohol combustion, particularly with methanol and high-purity ethanol, is the near-invisibility of the flame in bright light. Alcohol flames burn cleanly, producing very little soot or color, making them difficult to detect in daylight or well-lit environments. This characteristic has led to severe burn injuries when people have unknowingly approached or attempted to refill a container that was still actively burning.
Another risk is the phenomenon known as “flame jetting,” which occurs when refilling a container that is still warm or contains a persistent, invisible flame. The sudden addition of liquid fuel causes rapid vaporization and ignition of the vapor inside the container, leading to a violent expulsion of burning liquid.