An aerosol can is a specialized, high-pressure container designed to hold a product and a propellant, which is typically a liquefied gas that forces the contents out as a fine mist or spray. The safety of these common household items is dependent on maintaining the internal pressure within their engineered limits. Exposing these sealed metal vessels to heat presents a safety hazard because the temperature increase directly causes a significant and dangerous rise in internal pressure. Understanding the physical principles at play is essential to preventing a catastrophic failure, which often results in a violent rupture and potential fire.
The Critical Temperature Range
Most commercial aerosol cans carry a warning label advising against storage above 120 degrees Fahrenheit (49 degrees Celsius), as the risk of rupture begins to increase significantly at this temperature. Although there is no single, precise temperature point for an explosion, the range of 120°F to 140°F (49°C to 60°C) marks the threshold where the can’s internal pressure starts to exceed its intended safety margin. This range is the point at which the can’s structural integrity becomes compromised.
If the temperature continues to rise, deformation and seal failure may begin around 150°F (65°C). In extreme heat, such as 180°F (82°C) or higher, an explosion becomes highly likely, often resulting in a Boiling Liquid/Expanding Vapor Explosion (BLEVE). This variation in the precise failure point is due to factors like the can’s material thickness, the type and volume of propellant used, and the initial fill pressure.
The Science Behind Pressure Buildup
The rapid increase in internal pressure when an aerosol can is heated is a direct consequence of the physical properties of the gases and liquids inside. Aerosol cans contain a mixture of the product and a liquefied propellant, such as butane or propane, which is stored under pressure. When the can is exposed to heat, the kinetic energy of the molecules within the fixed volume increases dramatically. This molecular acceleration causes the internal pressure to rise sharply. Since the can’s volume cannot expand, the pressure is directly proportional to the absolute temperature.
The propellant, which is a liquid under normal pressure, begins to vaporize much more rapidly as the temperature climbs, quickly transitioning into a gas. This phase change from liquid to gas is the primary driver of the pressure spike, as the gas phase occupies a volume thousands of times greater than the liquid phase. The internal pressure can quickly move to a level that exceeds the can’s design limit.
The can’s weakest points, often the seams or the dome where the valve is crimped, are the first to fail under this excessive force, leading to a rupture. The sudden release of this highly pressurized, often flammable, vapor can then lead to a secondary hazard, such as a fireball or deflagration, if an ignition source is present.
Safe Storage and Disposal Practices
Preventing accidents involves translating the science of temperature and pressure into daily habits. A common scenario for reaching dangerous temperatures is leaving cans inside a closed vehicle, where the interior can easily exceed 130°F (54°C) even on a moderately warm day. Direct sunlight is also a significant hazard, as the dark metal of the can absorbs solar radiation, causing the internal temperature to rise far above the ambient air temperature.
To ensure safety, aerosol cans should always be stored in a cool, dry, and well-ventilated location, away from any potential sources of direct heat. This means keeping them away from radiators, stoves, ovens, furnaces, and any open flames or sparks. Storing cans in a cool pantry or a dedicated safety cabinet is preferable to areas like garages or sheds that can experience extreme temperature fluctuations.
Proper disposal is just as important as safe storage because even “empty” cans retain residual product and pressure. You must never attempt to pierce or puncture an aerosol can, as the rapid, forceful release of pressurized contents can cause injury. Similarly, never place a can in a fire or attempt to incinerate it, even if it seems completely empty. Cans that still contain product or propellant are classified as hazardous waste in many jurisdictions and should be taken to a local hazardous waste collection site for safe processing.