Isopropyl alcohol (IPA), often used as a disinfectant or solvent, is a clear, colorless organic compound recognized for its rapid drying time. This substance is highly flammable in its liquid state, prompting questions about the safety of surfaces after its application. Once isopropyl alcohol has completely disappeared from a surface, it poses no fire hazard. The chemical’s inherent physical properties dictate that the flammable component does not remain on the surface.
The Defining Factor: Extreme Volatility
The reason surfaces cleaned with IPA become non-flammable is due to high volatility. Volatility describes how quickly a substance transitions from a liquid state into a gaseous state, or vapor, at room temperature. Isopropyl alcohol possesses a low boiling point of approximately 180°F (82.5°C), meaning it readily evaporates even at standard ambient temperatures.
Unlike non-volatile flammable liquids such as oils or certain paint thinners that leave behind a combustible film, IPA vanishes entirely. The substance’s flash point—the minimum temperature at which it produces enough vapor to form an ignitable mixture in the air—is quite low, around 53°F (11.7°C). The fire risk exists only when the concentration of alcohol vapor falls within its flammable range of about 2% to 12.7%. Once the liquid has evaporated, the localized vapor concentration drops to zero, eliminating the possibility of ignition.
The Immediate Safety Risk (While Wet)
While the dried surface is safe, the period when the isopropyl alcohol is still liquid or actively evaporating presents a significant fire hazard. Since the liquid constantly releases flammable vapors, keep all ignition sources away from the area until the surface is visibly dry. IPA vapor is approximately twice as heavy as air, meaning it does not dissipate upward easily.
The vapor tends to sink and travel along the floor or ground, accumulating in low-lying or enclosed spaces. This traveling vapor can be ignited by a distant spark or flame, causing a flash fire that races back to the liquid source. Proper ventilation is necessary when using IPA, especially in smaller rooms or confined areas, to prevent the buildup of a dangerous vapor concentration. Common ignition sources to avoid include pilot lights, lit cigarettes, electrical switches that can produce a spark, and static electricity.
What Happens to the Residue? (Purity Matters)
Commercial isopropyl alcohol is typically diluted with water to concentrations like 70% or 91%. Water is non-flammable and is the primary non-alcohol component in these products. When the solution is applied, the volatile alcohol evaporates quickly, leaving the non-flammable water behind.
In solutions of 70% IPA, the remaining 30% is mostly water, which evaporates more slowly than the alcohol component. Any remaining trace residue is either water or non-volatile additives present in lower-grade solutions, neither of which contributes to a fire hazard. The component responsible for the fire risk, the alcohol, is completely gone through evaporation.