UVB radiation reaches the earth in relatively low amounts (about 0.5 percent of the solar spectral irradiance at ground level, integrated in the range 290-2500 nm) and is highly energetic.

Seventy percent of UVB radiation reaching the skin is absorbed by the stratum corneum, 20 percent reaches the viable epidermis, and only 10 percent penetrates the upper dermis.

UVA radiation has lower energy but is at least 20 times more abundant. UVA radiation is partly absorbed by the epidermis, but 20-30% reaches the deep dermis.

Ninety-five percent of the UV rays that reach the ground level are

UVA 1.

These are unstable and highly reactive oxygen radicals that tend to donate oxygen atoms to other substances or compounds. 

When the oxygen atom (or free radical) is given up, the substance receiving it is oxidized, its structure is changed, and, from then on, a series of chemical reactions ensue, resulting in its damage and altered functionality.

At the skin level, the structures that can be affected by ROS are lipids and cell membrane enzymes, DNA and RNA, but this oxidation process also underlies several systemic diseases.

The harmful effects of the sun on the skin, at whatever wavelength it irradiates us, if not well protected by the appropriate creams sun products and physical screens (e.g., clothing) are the damage to cellular structures both at the superficial level (epidermis) and in the deep dermis.

This damage is done by the generation of ROS, oxygen radical species, which affect cellular structures in various ways altering their structures and causing premature skin aging and skin changes given by the damaged underlying dermal structure.