Hairspray is an aerosolized cosmetic product designed to hold styled hair in place. Its flammability is a direct consequence of the chemical ingredients needed to dissolve styling agents, dispense the product as a fine mist, and ensure quick drying. The potential for ignition arises from the combination of highly combustible liquid solvents and pressurized, volatile gas propellants used in the typical aerosol formula. Understanding the role of these components explains the physics behind hairspray’s rapid flammability.
Flammable Liquid Solvents
The core of hairspray’s flammability rests with the liquid chemicals used to dissolve the film-forming polymers that hold the hairstyle. These solvents must evaporate almost instantly to prevent wetting the hair. They are overwhelmingly short-chain alcohols, primarily ethanol (SD alcohol 40) and occasionally isopropanol. Ethanol readily transitions from a liquid to a vapor at low temperatures, making it an effective solvent.
Ethanol’s hydrocarbon structure allows it to combust easily when its vapor is mixed with oxygen and exposed to an ignition source. These solvents possess a very low flashpoint—the minimum temperature required for a liquid to produce enough vapor to ignite. Since ethanol’s flashpoint is far below room temperature, vapors are present in the air even before the product is sprayed.
Volatile Gas Propellants
The second element involves the highly volatile gases necessary to propel the liquid contents out of the can and atomize them. Modern hairsprays primarily use liquefied petroleum gas propellants, such as propane, butane, and isobutane, which are stored as liquids under high pressure. When the nozzle is pressed, the sudden drop in pressure causes the propellant to flash-boil, instantly converting into gas that forces the liquid product out. This rapid expansion releases a cloud of highly combustible hydrocarbon gas, significantly enhancing the fire risk by acting as an accelerant to the flammable liquid solvent.
The Physical Mechanism of Ignition
Spraying hairspray creates a temporary cloud of fuel vapor highly susceptible to ignition. This cloud is a dense mixture of rapidly vaporizing alcohol solvent and expanding hydrocarbon propellant gas, both of which are highly combustible. For any gas or vapor to burn, its concentration in the air must fall within the flammable or explosive range. If the fuel vapor concentration is too low (too lean) or too high (too rich), ignition will not occur.
The aerosol device is engineered to create a temporary mixture near the nozzle that often falls within this flammable range. The fine mist produced by the propellant dramatically increases the surface area of the liquid solvent, allowing it to vaporize almost instantaneously. This rapid vaporization results in a high concentration of fuel vapor that quickly finds its flammable limit when mixing with surrounding oxygen. When an external heat source, such as a lighter or static spark, is introduced to this volatile cloud, the combustion reaction begins immediately and propagates rapidly.