Shining Light on Ultraviolet Radiation

Ultraviolet radiation is composed of three wavelengths: UVA, UVB and UVC.  While UVC isn't a concern for skin cancer, UVA and UVB play different roles when it comes to tanning, burning, and photoaging.

Shining Light on Ultraviolet RadiationWith the ongoing debate about the best way to get Vitamin D and the controversy surrounding tanning beds, there is a huge amount of misinformation surrounding ultraviolet radiation (UV). However, one thing is clear: UV radiation is the main factor responsible for skin cancers, including basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and possibly melanoma. In fact, the National Institutes of Health and the World Health Organization have identified broad spectrum UV as a human carcinogen.

While the solar UV spectrum is continuous, it is a scientific convenience to describe the light within three specific wavebands - UVA, UVB and UVC - classified according to their wavelength. They differ in their biological activity and the depth to which they penetrate into the skin.

UVA is long wavelength (320-400 nm) UV and accounts for up to 95 percent of the solar UV radiation reaching the Earth's surface. It can penetrate into the deeper layers of the skin and has for years been thought to play a major part in skin aging and wrinkling. Importantly, recent studies strongly suggest that it may also initiate and exacerbate the development of skin cancers. UVA rays are present during all daylight hours and throughout the winter months. Although UVA rays are less intense than UVB rays, they are present all year round and depending upon the time of the year, can be 30 to 50 times more prevalent than UVB rays. Furthermore, UVA radiation can penetrate glass and clouds. Thus, we are exposed to large doses of UVA throughout our lifetime.

New research suggests that UVA exposure may be as damaging to the skin as UVB. Although scientists have known for several years that UVA penetrates more deeply into the skin than UVB, they believed that less of it was absorbed by DNA, causing fewer dangerous mutations. However an Australian-US study shows that UVA causes more genetic damage than UVB in skin cells where most skin cancers arise - the keratinocytes in the basal layer of the epidermis. UVB tends to cause damage in more superficial epidermal layers.

UVB is the middle-range of UV with wavelengths between 290-320 nm. It responsible for burning, tanning, acceleration of skin aging and plays a very key role in the development of skin cancer. The intensity of UVB varies by season, location and time of day. The most significant amount of UVB hits the U.S. between 10 AM and 4 PM between April and October. UVB rays do not penetrate glass.

UVC  is the shortest and highest energy UV with wavelengths less than 290 nm. However, since it is filtered by the ozone, these wavelengths do not reach the earth's surface and do not contribute to skin damage in humans.

While the differences between UVB and UVA need to be explored further, it's proven that exposure to the combination of UVB and UVA is a powerful attack on the skin. It can create irreversible damage that ranges from sunburn to premature aging to skin cancer. Protection from these rays is the only way to avoid these problems.

Sun Protection Factor (SPF)  and How it Relates to UV Radiation:

The most important issue to understand about an SPF rating is that it mainly indicates relative protection from erythema produced by UVB. In order to get adequate protection against both UVA and UVB, you should select a sunscreen that provides multispectrum protection, broad-spectrum protection or UVA/UVB protection - not just a sunscreen with a high SPF (UVB) rating. Additionally, the label should list a FDA-recognized long wavelength UVA sunscreen, such as avobenzone or zinc oxide.

Sunscreens are classified by the Food & Drug Administration (FDA) as an over-the-counter drug rather than a cosmetic. There are currently 17 active ingredients approved by the FDA for use in sunscreens in the United States. These active ingredients fall into two broad categories: organic ("absorbers") and inorganic ("blockers"). Most UV filters are organic and protect by absorbing UV. They form a thin, protective film on the surface of the skin and absorb the UV radiation before it penetrates the skin. The inorganic sunscreens, such as metal oxides or particulate UV filters, are often termed "physical" or "mineral". The metal oxides, namely titanium dioxide and zinc oxide, are insoluble particles which absorb and reflect UV away from the skin. Today, many sunscreens contain a mixture of organic and inorganic sunscreens and are offered with new base formulations and methods of application, making sunscreen use more appealing. In addition, new UVA filters such as Tinosorb currently being used in Europe are under FDA review.

Common FDA-Approved Active Ingredients in Sunscreen Include:

  • UVB absorbers/blockers: Padimate O (Octyldimethyl PABA), Homosalate, Octisalate (Octyl salicylate), Octinoxate (Octyl methoxycinnamate or OCM)
  • UVA absorbers/blockers: Avobenzone (Parsol 1789), Zinc Oxide, Ecamsule (Mexoryl)
  • UVA and UVB absorbers/blockers: Octocrylene, Titanium Dioxide, Zinc Oxide

Published on February 13, 2012. Updated on September 20, 2017.