Hair is a complex biological fiber primarily composed of keratin, a fibrous protein that forms the building blocks of the strand. This protein is organized into the cortex, which provides the bulk, strength, and color of the hair. Surrounding the cortex is the cuticle, a transparent, protective layer of overlapping cells that function like roof shingles. Hair breakdown occurs when the degradation or erosion of this fundamental structure—the protective cuticle and the protein-rich cortex—compromises the hair’s integrity.
Chemical Assaults on Keratin
Chemical processing is one of the most aggressive forms of hair breakdown because it directly targets the molecular bonds holding the keratin structure together. The most durable bonds are the disulfide bonds, which are covalent cross-links responsible for the hair’s shape and resilience. Processes like permanent waving and chemical relaxing use reducing agents, such as ammonium thioglycolate, to cleave these bonds. The hair is then reshaped, and an oxidizing agent, often hydrogen peroxide, rejoins the bonds in their new configuration.
Bleaching and high-lift coloring inflict damage through an oxidation reaction, typically using hydrogen peroxide and ammonia in a highly alkaline environment. The chemical reaction oxidizes the sulfur atoms in the disulfide bonds, converting the strong cross-links into new chemical groups like cysteic acid. This process permanently weakens the internal support structure of the hair fiber, leading to a loss of elasticity and tensile strength. High alkalinity also causes the cuticle scales to swell and lift, allowing the chemicals to penetrate the cortex.
Extreme or repeated chemical exposure can lead to hydrolysis, a severe form of damage that breaks down the peptide bonds forming the long keratin chains. Peptide bonds form the backbone of the protein structure, and their cleavage results in the permanent fragmentation of the keratin chains. This molecular disintegration causes a substantial loss of hair mass and irreversible deterioration of the fiber. This leaves the hair highly porous, weak, and susceptible to breakage.
Mechanical Stress and Thermal Damage
Physical forces and high heat cause significant structural failure by eroding the hair’s outer defense system. Mechanical stress occurs during routine grooming, such as aggressive brushing, combing, or friction from rough towel drying, which chips away at the delicate cuticle layer. As the cuticle scales are lifted and removed, the hair fiber loses its smooth surface, resulting in increased friction, tangling, and a dull appearance. This erosion exposes the vulnerable cortex beneath, leading to split ends and mid-shaft breakage.
Tension from styling, particularly tight hairstyles like ponytails and braids, acts as a mechanical culprit. This constant pulling stretches the hair fiber, placing significant stress on the internal cortex proteins. While hair can stretch up to 30% when wet, overstretching can cause micro-fractures in the cuticle and cortex. This continuous tensile strain permanently compromises the hair’s elasticity and reduces its ability to resist breakage.
Thermal damage from heat styling tools, such as flat irons, compounds damage through rapid and intense moisture loss. Temperatures above 95°C cause the water inside the hair shaft to flash-evaporate, creating internal voids known as “bubble hair” and lifting the cuticle edges. This steam expansion severely reduces the fiber’s strength and leaves it permanently brittle. High heat also weakens the internal protein bonds within the cortex, making the hair less resilient to subsequent mechanical stress.
Environmental Factors and Structural Weakness
External environmental elements, independent of styling habits, contribute to structural weakness. Sunlight, specifically Ultraviolet (UV) radiation, causes photo-degradation of the hair fiber. This process leads to the oxidative breakdown of keratin proteins and the fading of hair color pigments. The resulting protein loss reduces the hair’s overall strength and makes it more vulnerable to damage.
Hard water contains high concentrations of dissolved minerals like calcium and magnesium, which create a damaging coating on the hair shaft. This mineral buildup interferes with the cuticle’s ability to lie flat, leading to a dull, rough texture and reduced manageability. Over time, these deposits weaken the protein structure, making the hair brittle and prone to breakage.
Saltwater and chlorine exposure cause damage through dehydration and chemical reaction. Saltwater is highly hygroscopic, meaning its high salinity draws moisture out of the hair strands, causing severe dryness, frizz, and breakage. Chlorine, particularly in swimming pools, erodes the hair’s protective lipid layer and chemically degrades the disulfide bonds. This leaves the hair highly porous and rough.
The integrity of a hair shaft can be compromised from within before it emerges from the scalp. Systemic issues, such as malnutrition or deficiencies in key micronutrients, lead to the production of inherently weaker hair fibers. Since hair is composed mainly of keratin, insufficient intake of protein, iron, zinc, or biotin disrupts the normal hair growth cycle. This compromised internal structure results in brittle, fragile strands less capable of withstanding external assaults.