Acetone, a common solvent found in products like nail polish remover, is a highly volatile liquid. The question of whether it is flammable when “dry” is misleading because the primary fire hazard is the invisible gas it rapidly produces, not the liquid residue. Acetone evaporates quickly, even at room temperature, releasing flammable vapors into the surrounding air. If the liquid is completely gone, the surface is no longer a direct risk, but the rapid evaporation means a significant fire hazard remains in the atmosphere shortly after use.
Understanding Flammability: Volatility and Flash Point
The flammability of acetone stems from its inherent physical properties: high volatility and a low flash point. Volatility refers to how readily a substance turns into a gas. Acetone is extremely volatile, boiling at a relatively low temperature of about 56°C (133°F). This means it constantly releases a large volume of vapor into the air, even when its container is open briefly.
The flash point is the minimum temperature at which a liquid gives off enough vapor to form an ignitable mixture with the air above its surface. Acetone has a very low flash point, typically around -20°C (-4°F). Since this is far below normal room temperature, acetone always produces enough vapor to be flammable under standard conditions. For a fire to start without a spark or flame, the substance must reach its autoignition temperature, which for acetone is approximately 465°C (869°F).
The Real Danger: Invisible Vapor Concentration
The most significant hazard associated with acetone is its invisible vapor, which can create an explosive atmosphere far from the liquid source. Acetone vapor is heavier than air, with a vapor density of about 2.0. Because of this density, the vapor tends to sink and flow along the floor, collecting in low-lying areas or drains. This heavier vapor can travel considerable distances to an ignition source, such as a pilot light or a sparking electrical device.
Once ignited, the flame can travel rapidly back along the trail of vapor to the liquid source, a phenomenon known as “flashback.” This significantly expands the fire hazard beyond the immediate work area. The risk of fire or explosion is determined by the concentration of this vapor in the air, which must fall within the flammable limits.
The Lower Explosive Limit (LEL) for acetone is between 2.5% and 2.6% by volume in air; concentrations below this are too lean to burn. Conversely, the Upper Explosive Limit (UEL) is 12.8% by volume, meaning concentrations above this are too rich in fuel to ignite. Even if a surface appears “dry,” the surrounding air may be saturated with a vapor concentration within this explosive range. This dangerous atmosphere can persist for a long time, especially in poorly ventilated spaces where the heavy vapor remains trapped near the floor.
Practical Safety Guidelines for Acetone Use
Safe handling of acetone requires strict adherence to procedures that manage its volatile and flammable nature. Adequate ventilation is paramount to prevent flammable vapor from accumulating in the workspace. Working in a well-ventilated area, preferably with local exhaust ventilation like a fume hood, helps dilute and remove the heavy vapor before it can reach the LEL.
All sources of ignition must be eliminated from the area where acetone is being used or stored. This includes open flames, hot surfaces, and equipment that could generate a spark, such as ungrounded electronics. Containers must be kept tightly sealed when not in use to minimize vapor release and should be stored in a cool location away from heat and direct sunlight.
Spill and Transfer Procedures
When transferring acetone, use grounded and bonded equipment to prevent the buildup and discharge of static electricity, a common ignition source. In the event of a spill, the area should be immediately evacuated and isolated, and all ignition sources must be turned off. Spills should be absorbed with an inert, non-combustible material like sand, and cleanup must be performed using non-sparking tools.