Hair is a highly resilient structure and has been designed to withstand what can amount to a huge number of physical processes over its lifetime. The cuticle, with multiple layers, imparts significant resistance, and the f-layer helps to decrease surface frictional forces. The cuticle has great internal strength, with a surface that has low friction. If a cuticle cell is removed, the cuticle cell below is merely left exposed with a fresh surface.
A person may brush their hair up to 1000 strokes per month with the consequence that the cuticle structure is degraded, and eventually all the cuticles may be removed to reveal the less resilient cortex structure. The degradation due to these physical processes can take several forms. The first is abrasion of the cuticle edges, eventually leading to complete removal. Combing and brushing wet hair is more likely to lead to cuticle abrasion than combing and brushing dry hair due to higher friction forces.
Breakage may occur during combing and brushing. Under normal circumstances, the force required to pull a hair fibre out of the follicle is less than the force required to break a hair fibre. It might therefore be predicted that broken fibres would be rare. However, several factors can lead to broken fibres. Excess chemical, UV light, and heat exposure can eventually weaken hair sufficiently to lower the force to break of hair to less than that required to pull hair out of the scalp. If the fibre is caught up in a knot during combing, the local forces in the knot can be much higher, causing the fibre to break. Breakage is particularly an issue for curly hair since it is more likely to form tangles and knots leading to breakage, especially when wet.
Physical processes causing cuticle damage and breakage will occur for almost all people but the severity of damage will vary according to hair morphology and habits and practices. Fine hair is more likely to break than thick hair simply because it has a lower tensile strength. Curly hair is more likely to break than straight hair due to higher abrasive forces between fibres and the likelihood of forming knots and tangles. This is especially the case for persons of African descent, in whom hair can be extremely fragile, especially if it has been regularly braided.
Often cuticle removal is seen preferentially on the outside edges of the fibre, as this is the part of hair where frictional forces are highest. Products that include conditioning actives such as quaternary ammonium compounds and silicones are hugely impactful to reduce this physical damage and work by creating lower frictional forces on the hair surface, making combing easier, with lower knot and tangle formation.
Environmental Processes
All hair is exposed to the “environment.” Unless the hair is covered for cultural or religious reasons, there will be some impact from the source of all life-the sun. UV exposure and its impact on hair have been widely studied.
There are changes to the underlying hair structure by breaking down the protein structure. There are also more visible signs of high levels of UV exposure. Extended exposure to sunlight can bleach melanin, giving hair a highlighted look, especially in lighter shades where melanin levels are lower and the bleaching effect is more obvious (i.e., medium blond and lighter). Those with medium or dark brown hair may never observe melanin bleaching.
People may also notice permanent hair dyes being bleached by UV, but this may be less easy to distinguish versus washing processes that can also rapidly fade dyes. Hair is exposed to light made up of a combination of different wavelengths, from UV to visible to infrared light. The atmosphere and ozone layer will filter out some of UVC light, and window glass will filter out UVB, so lifestyle and time spent outdoors will influence the level of light exposure. In addition, the irradiance (i.e., the radiant power per unit area) will vary according to time of year and location. As an example, irradiance in summer sun is ~0.55 W/m2 compared to 0.35 W/m2 in winter sun.
The mechanisms of UV damage are complex. It has been well established that all the components of hair structure are affected, including proteins, lipids, and melanin. In the protein structure the major amino acid residues that are impacted are the sulphur-containing residues cystine and methionine and the aromatic-containing residues tryptophan, tyrosine, and phenylalanine. The aromatic amino acids strongly absorb in the UV, forming excited state species which eventually form reactive oxygen species such as singlet oxygen, hydrogen peroxide, and hydroperoxides. shows basic pathways that can occur where proteins and lipids are slowly broken down via reaction with formed reactive oxygen species-in this case, singlet oxygen.
Contaminants in hair such as copper ions have been shown to accelerate these damage mechanisms by creating additional reactive oxygen species. Several oxidation products in the protein structure has been identified including kynurenines, which are yellow in colour and are thought to be responsible for the photo yellowing of hair sometimes observed in Grey hair.
Since all the hair structural components are susceptible to UV damage, many different hair properties are impacted. These include hair strength, both dry and wet, and also increased combing forces. Thus, hair exposed to UV may look lighter at the tips but it may also look less healthy, as structural damage manifests as broken fibres and split ends. All hair types will be impacted by UV damage, although it is documented that darker hair will experience less damage due to the protective effect of melanin, which will also strongly absorb light. In addition, it is documented that artificial dyes will offer a protective effect by acting as UV absorbers, and it is also known that these dyes are oxidized and destroyed during this process.
Water is an important part of the hair care process and can have beneficial effects by removing sebum lipids and dirt/dust from hair during the washing process.
In some cases, water can also contain harmful metals such as copper and iron, which are absorbed by hair. As mentioned elsewhere in this chapter, it has been demonstrated that copper absorbed by hair can accelerate damage from oxidative processes such as hair colouring and UV exposure. Water hardness, i.e. calcium and magnesium ions, can also impact hair health and product performance. For example, in hard water, deposition of conditioning actives such as silicones is reduced and lather amount is also reduced.
Heat Processes
There are a range of different structural changes caused by heat but the factors crucial to determine extent of these changes are firstly, the temperature that the hair gets to, and secondly the cumulative time of exposure to this temperature. Many people can use heat implements without noticeable damage. If they take care with frequency of use and moderate the temperature of the device. However, the benefits sought with heat implements, e.g. curl formation or straightening, are correlated with the level of heat used, so it can often be a trade-off between level of damage and desired style achievement.
For some people, heat implements are used simply to dry the hair but the majority use heat implements as a tool to style hair. Heat will evaporate water from between hair fibres (capillary water) and from inside hair (internal water). It is internal water that is crucial for style achievement, as removing this water will create hydrogen bonds between neighbouring protein chains, effectively temporarily locking the desired style in place.
The more water that is evaporated (i.e., the higher the temperature or the longer hair is dried for), the stronger these hydrogen bond links will be and the longer the style will last. Of course, during water evaporation hair has to be styled in the final shape, which is what curling tongs or flat irons do effectively by either creating a curl or straightening while heating. Blow dryers are the most common form of heat used, and in the US ~50% of women use a blow dryer regularly.
The air flows temperature is typically up to 100°C but hair will only get close to this temperature once capillary and internal water has evaporated. While hair is wet, heat will be absorbed by water while it is evaporating. No significant changes to the hair structure occur at these temperatures, but physical cracking of cuticles can occur at these temperatures if hair is dried very rapidly. These cracks occur as the rapidly drying cuticle contracts around a still swollen cuticle. Once cracks are formed the cuticle is more easily removed by grooming.
Chemical degradation of hair due to heat will only occur if hair is heated above 150°C. These temperatures can readily occur if heated straightening or curling irons are regularly used. With these implements on the highest setting, hair can reach temperatures of above 220°C, especially if sections of hair are repeatedly straightened or hair is slowly pulled through the flat iron. In addition, a critical degradation of protein takes place above 190°C which will involve cysteine disulphide bond breakage.
At very high levels of exposure to this temperature, melting of keratin can even occur; severe keratin degradation and hair breakage occur when hair is flat-ironed for 3 minutes at 210°C. These exposures are extreme and are unlikely to occur, but excessive heat damage can be seen in increased incidence of broken hair for some women who use high heat implements on a regular basis. These broken fibres will then lead to poor shine and smoothness, directly indicating poor hair health. However, used with care, heated implements can also be used to increase perception of healthy hair. Blow dryers and flat irons can be used to straighten and align hair, giving it shine and making it easier to comb. In addition, creating a good “wet-set” by removing internal water will increase resistance of style to humidity, thus reducing frizz.
Chemical Treatments
Chemical treatments are the leading cause of hair structure changes and ultimately hair damage, but they also drive dramatic changes to either colour (e.g., permanent colouring) or style (e.g., perming, straightening).
Some people accept the damage trade-offs for the desired benefits. Damage from these treatments is not just of a higher category but since all chemical treatments result in a breakdown of hair’s protein and lipid structures, they render hair more susceptible to subsequent damage from physical and heat processes. Some people often will moderate their habits and practices once they start employing chemical treatments such as colouring by reducing washing frequencies and moderating heat implement temperatures.
Hair Colouring and Bleaching
Hair colouring is the most frequent chemical treatment worldwide. Forty percent of women colour their hair in the US, 46% in the UK and 49% in Japan. Hair colouring is increasing in popularity in large population countries such as China, Brazil, and India. The colouring process involves aggressive chemistry designed to lighten the hair by bleaching the natural pigments melanin and complex chemistry designed to form artificial colour within the hair. It is the combination of lightening and colour formation that gives the final shades, but it is the oxidant chemistry that is the main source of hair structure changes and ultimately hair damage.
There are two reactive species formed during the colouring process that have a significant impact on hair health via several mechanisms. The first species is the per hydroxyl anion (HOO-) which is formed from hydrogen peroxide and ammonia at the high pH used in colorants. This species is crucial for bleaching melanin, so there is always a trade-off between colouring performance and damage caused.
The per hydroxyl anion is responsible for two major chemistries that directly impact hair health. The first is removal via per hydrolysis of the surface lipid layer (f-layer or 18-methyleicosanoic acid) that is present on each cuticle cell. This lipid provides a protective hydrophobic coating which reduces friction forces, especially when hair is wet, and gives hair its smooth and soft feel. Once removed after colouring, the surface properties of hair dramatically change, with wet friction increasing significantly in addition to loss of soft hair feel and shine.
