Measuring oxidative damage to human hair from solar radiation and pollution: A critical review

Human scalp hair evolved to protect the brain and scalp from environmental factors such as solar radiation and to provide thermoregulation. Whilst the evolutionary drivers for this protective function have diminished, ongoing global industrialisation has caused concerns about the deleterious effects of solar radiation and environmental pollution on hair. This critical review evaluates published data on the effects of solar radiation on human hair, alone and in combination with pollution (air- and waterborne). The majority of data were generated via in vitro experiments in which hair samples were subjected to defined artificial solar radiation and pollutant exposure. Protein and lipid oxidation were the main synergistic oxidative reactions taking place in hair fibres due to solar radiation exposure and in the presence of pollutants and trace metals. UVB radiation mostly impacted the hair cuticle, whilst combined UVA and visible spectra impacted the cortex. Light coloured and bleached hair incurred the greatest damage, whilst dark hair types are less affected. In addition to chemical analytical techniques, oxidative damage to the cuticle was evidenced by scanning and transmission electron microscopy, whilst mechanical fibre measurements were used to infer loss of cortex integrity. In conclusion, whilst solar radiation is the most significant factor, trace metals, air-borne pollution, and the application of products and styling techniques also play a role in hair degradation. These factors could also be referred to as “hair exposome”, and hair protection strategies considered according to the combination of hair characteristics and specific exposome exposure.