The sunlight spectrum is made up of UV , VIS and IR light (10, 40 and 50% respectively).1 UV light (100-400 nm) has the highest energy and is mostly absorbed by oxygen and the tropospheric ozone.
Thus, the Earth’s surface is protected from UVC light (λ100-290 nm) but is reached by the more energetic and seasonal UVB radiation (λ290-320 nm, which is concentrated in the middle of the day and is more intense in summer) and UVA radiation (λ320-400nm, more penetrating due to its greater wavelength, constant throughout the day, all year round). Depending on its wavelength, UV radiation reaches various layers of the skin (epidermis most superficial-, dermis, or hypodermisdeepest): the UVB radiation acts mainly on the epidermis, causing erythema (sunburn), and partially penetrating the dermis; while the UVA radiation passes through the dermis and is responsible for photoageing. Both types of radiation, through direct or indirect DNA damage, are associated with cellular damage and can have carcinogenic effects.
However, it would be wrong to ignore the fact that the UV light that reaches us has beneficial health effects such as promoting vitamin D (UVB) synthesis, boosting the immune response and the haemoglobin in the blood. It also boosts cardiovascular health and brings about wellbeing and good humour. In addition, heliotherapy is used to treat psoriasis, vitiligo or atopic dermatitis.
Balance, therefore, should be sought through moderate exposure, according to medical recommendations, and the use of suitable photo-protection for each skin type. For that purpose, the classical solutions of the cosmetics market are the incorporation of chemical or physical filters in their formulas. The chemical filters contain aromatic groups and carbonyl groups that absorb UV radiation (mainly UVB) and disperse it through conformational changes in the molecules, emission of radiation with larger wavelength and energy in the form of heat.2 The physical filters are of titanium oxide TiO2 and zinc oxide ZnO, and act through the dispersion, reflection and absorption of the UV radiation (they form a physical barrier on the skin). They are primarily known as TiO2 nanoparticles for UVB protection and ZnO for UVA protection. They are often used in combination with chemical filters to facilitate the preparation of formulas.
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