UVA rays (320 - 400 nm) account for approximately 95% of the UV energy reaching the Earth's surface, while UVB rays (290 - 320 nm) make up the remaining 5%. Although they are not as energetic as UVB rays, UVA rays penetrate deeper into the skin, thus having a maximum effect at the dermis level. Studies associate them with immediate short-term tanning and other long-term damage, whereas they were once considered relatively harmless.
UVA rays are known to generate an excess of reactive oxygen species, which risk indirectly damaging the DNA of skin cells, if they are not quickly neutralised. Although skin cells have antioxidants, DNA repair enzymes and stress signals to minimise this damage, excessive exposure to UVA can lead to mutations and genetic instability. UVA rays are also responsible for premature skin ageing (photoaging). Regular exposure to UVA rays can lead to the appearance of deep wrinkles, age spots and a dull complexion.
Furthermore, prolonged exposure to UVA rays can increase the risk of melanoma. They are thought to facilitate the carcinogenesis induced by UVB rays. It has also been demonstrated that exposure to UVA weakens the skin's immune function (photoimmunosuppression), which can make the skin more vulnerable to infections, diseases, and allergic reactions. Indeed, UVA rays are believed to reduce the activity of human epidermal antigen-presenting cells and the number of Langerhans cells. Additionally, this UV-induced immunosuppression likely plays an indirect role in photocarcinogenesis.
Photoprotective measures are therefore widely recommended for everyone, regardless of their skin type. These include wide-brimmed hats and protective clothing, limited exposure to the sun during peak intensity hours (between 11am and 3pm), and the regular use of sun protection products with a broad spectrum (UVA/UVB) and a sun protection factor (SPF) of at least 30.