Human hair is flammable, but its unique biological makeup provides resistance that differs from materials like paper or fabric. Hair is an organic, fibrous material with a high surface-area-to-volume ratio, meaning it has plenty of surface exposed to oxygen to support combustion. While it ignites when exposed to an open flame, the way it burns is distinct and dictated by its complex internal structure. Understanding the difference between immediate ignition and sustained burning is key to grasping hair flammability.
How Hair Reacts to Direct Flame
When human hair encounters an open flame, it ignites almost instantly. Instead of sustaining a large, bright flame, hair undergoes pyrolysis, the thermal decomposition of organic material at high temperatures. This decomposition causes the hair to char, curl, and shrink away from the heat source. Because individual strands are thin, limiting the available fuel, fire generally does not spread rapidly unless the hair is in a thick mass. Burning hair emits a distinct, foul odor, often described as sulfurous, resulting from the chemical breakdown of its protein components. Once the direct flame source is removed, the hair often self-extinguishes because the small amount of fuel is quickly consumed.
The Chemical Structure That Resists Fire
Hair exhibits self-extinguishing behavior due to its unique chemical composition, centered around the fibrous structural protein, keratin. Keratin is extremely tough and insoluble, forming the core component of hair. Hair is classified as a hard keratin, highly resistant to heat and chemical degradation. Keratin naturally contains a significant percentage of sulfur, typically 3% to 5% by weight, derived from the amino acid cysteine. When hair burns, this sulfur and high nitrogen content are released as non-flammable gases, such as sulfur dioxide. These gases displace oxygen near the flame, inhibiting sustained combustion and acting as a built-in flame retardant, causing the hair to smolder and char.
External Factors That Alter Flammability
Several external factors can override hair’s natural resistance and increase the risk of rapid, sustained burning. Moisture content is a significant variable; dry hair ignites much faster than damp or wet hair. The physical arrangement also matters, as voluminous styles expose a larger surface area to oxygen, making ignition more likely. The greatest increase in flammability comes from certain hair products that act as accelerants. Styling products like hairsprays, gels, and mousses often contain highly flammable alcohol or volatile organic solvents. These chemicals provide a readily available fuel source that burns quickly, overriding the hair’s tendency to self-extinguish. Oils and oil-based serums also increase combustibility by coating the strands with a flammable substance.