Aerosol cans are common in homes and workplaces, delivering everything from paint to air freshener in a convenient spray. Despite their utility, these sealed containers carry a specific safety warning: they must be protected from heat. When exposed to elevated temperatures, a dangerous runaway reaction transforms the metal cylinder into a potential projectile. Understanding this explosive hazard requires looking inside the can to see how its contents and structure react to thermal energy.
The Anatomy of an Aerosol Can
A typical aerosol can is a self-contained pressure vessel, usually constructed from tin-plated steel or aluminum, designed to withstand a specific internal force. It holds two primary components: the product concentrate and the propellant. The propellant is the invisible engine that pushes the product out when the valve is activated.
Many modern aerosol cans use liquefied gases like propane or butane as propellants, existing as both a liquid and a gas inside the can under normal pressure. This combination maintains a constant pressure throughout the product’s lifespan. The can’s sturdy metal body and the valve assembly are engineered to contain this pressure safely under standard conditions.
How Internal Pressure Skyrockets
The danger begins when the aerosol can is exposed to excessive heat, such as being left in a hot car or near an open flame. The rise in temperature immediately affects the gas molecules within the fixed volume of the can. An increase in temperature causes gas molecules to gain kinetic energy, moving faster and striking the container walls with greater frequency and force.
This elevated molecular activity directly translates into a rapid increase in the internal pressure. Even more significant is the behavior of the liquefied propellant inside the can. Under normal conditions, the liquid propellant slowly vaporizes to maintain the required internal pressure.
When the temperature soars, the liquid propellant begins to boil much more quickly, undergoing a phase change. This rapid vaporization converts a small volume of liquid into a significantly larger volume of high-pressure gas. The combination of faster-moving gas molecules and the dramatic increase in the total amount of gas causes the internal pressure to skyrocket beyond its intended design limit. This process quickly overwhelms the can’s ability to contain the expanding vapor.
The Critical Point of Structural Failure
The can’s metal body, typically aluminum or steel, has a defined tensile strength, which is the maximum stress it can endure before tearing apart. While manufacturers design cans to tolerate internal pressures higher than their operating level, the force exerted by the expanding gas eventually becomes too great. Most consumer aerosol cans are rated to safely withstand temperatures only up to about 50°C (120°F).
When the internal pressure exceeds the structural limits, the can will fail abruptly and violently. The weakest points are often the seams, such as where the bottom is joined to the body, or the seal around the valve assembly at the top. The failure is a catastrophic rupture, causing the metal container to tear apart and fragment. This event is often classified as a Boiling Liquid Expanding Vapor Explosion (BLEVE), where the sudden release of high-pressure contents, which are often flammable, can also create a large fireball.
Safe Handling and Storage
Preventing an aerosol can explosion depends on controlling the thermal environment around the container. The most important preventative action is to store all aerosol products in a cool, dry location where the temperature will not exceed the recommended maximum of 50°C (120°F). This means avoiding storage areas like garages, sheds, or attics that can become superheated during warm weather.
Never leave aerosol cans inside a vehicle, as the interior temperature can quickly climb past the safety threshold even on a moderately sunny day. Cans should also be kept away from direct heat sources, including stoves, radiators, fireplaces, and open flames. When disposing of aerosol cans, always ensure they are completely empty of product and pressure, and follow local guidelines. Never attempt to incinerate or puncture a can that still contains propellant.