The science of aging

Sunlight that reaches the ground consists of visible light, infrared radiation and ultraviolet (UV) radiation. UV radiation has wavelengths between 100 and 400 nanometres and is divided into three bands by wavelength. The shortest, UVC, is absorbed by the atmosphere and does not reach the ground. The other two do.

The two bands behave differently in the atmosphere. UVB is strongly absorbed by ozone, and when the sun is low its light crosses more of the atmosphere, so UVB falls sharply in the morning, in the late afternoon and in winter. UVA is absorbed far less, so it is present for more of the day and more of the year, and it changes less with the time of day and the season.

Premature aging of the skin caused by repeated exposure to UV radiation is called photoaging. It is distinct from intrinsic aging, the slower changes that come with age alone, and it accounts for most of the visible difference between skin that has been exposed to the sun and skin that has not.

UVA penetrates through the epidermis into the dermis, the layer that gives the skin its strength and elasticity. There, UV exposure generates reactive oxygen species and activates enzymes called matrix metalloproteinases, which break down collagen and elastic fibres. At the same time, the production of new collagen falls. Over years, damaged elastic material accumulates in the dermis, a change known as solar elastosis, and pigment is distributed unevenly.

The visible results are fine lines and wrinkles, loss of firmness, rough texture, and dark spots and uneven tone. UVB contributes to photoaging as well, mainly in the epidermis, but UVA is the larger share of the radiation that reaches the dermis.

The UV index is the international standard for reporting the strength of the sun’s UV radiation, defined by the World Health Organization together with the World Meteorological Organization, the United Nations Environment Programme and the International Commission on Non-Ionizing Radiation Protection.

The index is calculated by weighting the UV radiation at each wavelength by how effectively that wavelength reddens skin, known as the erythema action spectrum, and adding up the result. The weighting is far higher for UVB than for UVA, so the index is driven mostly by UVB, even though UVB is only about 5% of the UV radiation at ground level.

The UV index therefore describes the risk of sunburn well, but it is not a measure of aging. When the sun is low, in winter, under cloud and behind glass, the index can be low while the UVA reaching the skin remains substantial.

The Aeon Index measures the aging light, UVA, reaching the skin, on a scale from 0 to 10. What it is calculated from, how, and its limits are on its own page.