Sunlight consists of five forms of radiation ranging from wavelengths of 100 nanometers to beyond one million nm (infinity). These radiation wavelengths trigger a variety of adverse photosensitive responses. Radiation emitted from artificial sources (mercury vapor lamps, welding arcs, and indoor tanning beds) also plays a role in damaging skin.
Skin is our largest organ and an integral part of the immune system. In many cases, skin is the primary line of defense against external trauma. It can be an effective barrier against many environmental insults, but its natural capacity to protect against the sun’s radiation is limited, skin type dependent, and generally ineffective. Protecting skin from all injury, especially damage from sunlight, is critical to overall health. The damaging effects of ultraviolet radiation (UVR) from the sun are well documented and include everything from sunburn to photocarcinogenesis, immunosuppression, and photoaging.
SUNLIGHT’S FIVE FORMS OF RADIATION
Ultraviolet C (UVC) – (100-280 nm)
UVC wavelengths are the shortest ultraviolet rays and are recognized as the most carcinogenic. UVC is severely photodamaging to the skin, resulting in skin burn with exposure. These wavelengths can very efficiently kill germs, giving rise to their common name, “germicidal waves.”
Fortunately, virtually all UVC generated by the sun is screened from reaching us by the protective ozone layer. Unfortunately, pollution damage to the ozone layer is undermining its protective ability, and UVC may become increasingly problematic for those living at high altitudes. If ozone depletion persists, the consequences will threaten life on a large-scale. Artificial sources such as some mercury arc-welding units and germicidal lamps also emit UVC.
Ultraviolet B (UVB) – (280-320 nm)
UVB is the intermediate wavelength of ultraviolet rays and causes the initial appearance of erythema, commonly called “sunburn”; usually perceived as redness. UVB primarily damages the epidermis (the outer most layer of the skin). The immediate result is skin redness and thickening of the skin’s outer stratum corneum, which is the body’s attempt to reduce UVB‘s impact on the epidermis. The superficial redness and painful irritation caused by UVB subsides after a relatively short period of time, but the underlying damage remains and accumulates, potentially resulting in basal cell and squamous cell cancers. Excessive exposure to UVB is the foremost promoter of premature aging of the skin. UVB also has significant immunosuppressive effects. Some indoor tanning devices also emit UVB.
In addition to staying out of the sun between 10 a.m. and 4 p.m. when UV rays are strongest, we can protect ourselves from UVB by applying sunscreen, wearing protective clothing and sunglasses, and avoiding tanning devices. The SPF rating assigned to a sun protection product is an indicator of its ability to protect against the UVB wavelength of light and is determined based on acute erythemal reaction, i.e., sunburn.
IMPORTANT NOTE: Choosing sun protection based solely on SPF is not enough. The common rule of thumb to “use SPF 30 or above” to protect your skin from sun damage is outdated, misleading and can be dangerous. SPF ratings pertain to UVB radiation only. UVB is not the only ultraviolet wavelength that damages the skin. SPF is not a sufficient or reliable indicator of the level of protection against any biological damage other than sunburn. As our understanding of light’s impact on skin has evolved, it has become clear that UVA adversely affects the deep dermis far more than the superficial “sunburn” caused by UVB rays, and mounting evidence suggests that UVA plays a relatively greater role in long-term photodamage.
Ultraviolet A (UVA) – (320-400 nm)
UVA are the longest of the ultraviolet wavelengths. While energy from the shorter UVB wavelength is absorbed in greater amounts in the epidermis and in keratinocyte DNA, energy from UVA penetrates more deeply into the dermis. Keratinocyte is the epidermal cell that synthesizes keratin, known in its successive stages in the layers of the skin as basal cell, prickle cell, and granular cell.
Once thought of as essentially harmless, UVA causes the pigmentation changes associated with what we misguidedly refer to as a “healthy tan.” Scientific evidence now indicates that UVA is anything but harmless. In addition to skin darkening (tanning), UVA induces cutaneous photodamage, usually seen as dryness, uneven pigmentation, inflammation, fine wrinkles, and skin cancer. Even a low dose of UVA can cause photodamage leading to wrinkles and sagging skin. UVA adversely affects the deep dermis resulting in a loss of the elastic quality of its supportive collagen, causing premature aging. UVA exposure has been linked to the development of basal and squamous cell cancers, as well as precancerous lesions. Recent reports note that depletion of vitamin A in the skin caused by UVA exposure may contribute to both photoaging and cancers of the skin. Tissue damage from UVA rays is cumulative, and generally, the effects will not appear until after years of exposure.i
Several studies have demonstrated UVA’s involvement in tumor development and suppression of immune function. Skin’s upper layers present a weak resistance to UVA penetration and are not equipped to counteract photons that impact immune functions.ii Epidermal pigment cells have been used in order to investigate the induction of DNA breaks due to UVA. What was determined is that with relatively low doses of UVA significant DNA breaks were found.(ii)
UVA represents 90-95% of terrestrial UV radiation. It is estimated that 10 to 12 times more UVA than UVB reaches the earth’s surface at sea level. Unlike the shorter UVB wavelengths, UVA easily penetrates window glass and retains essentially the same energy level all day long, every day of the year, presenting the same damaging effects at 9 a.m. in December as it does at the 4 p.m. in July. Indoor tanning devices also emit UVA.
As with UVB, we can protect ourselves against UVA by staying out of the sun, applying sunscreen, wearing protective clothing and sunglasses, and avoiding indoor tanning devices. Products that address this wavelength specify “UVA Protection” or “Broad-Spectrum Protection” on the label. However, ”Broad-Spectrum Protection” is not total spectrum protection, and safely maintaining unaltered essential biological functions can only be achieved by well-balanced, photostable sunscreens that protect against the entire spectrum of UV radiation. Unfortunately, there is presently no standardized indicator, numerical or otherwise, to signify the level of protection against UVA, but sunscreens containing titanium dioxide and zinc oxide will protect against UVA radiation.
Visible Light – (400-780 nm)
As the name describes, visible light is the wavelengths that humans can see (violet, indigo, blue, green, yellow, orange, and red.). Nearly 50% of the sun’s radiation that reaches us at sea level is within the visible range. The energy level of visible light is lower than that of the ultraviolet wavelength, but it is a misconception to think of visible light as harmless to human skin. This lower energy has the ability to penetrate deeper into the skin than UVA and reach-down within the dermis to create adverse skin reactions. Prestigious journals such as the Journal of Investigative Dermatology, Cancer Research, and the British Journal of Dermatology have published reports showing that visible light is capable of precipitating phototoxic reactions, promoting DNA cross-linking and enhancing tumor growth. Visible light’s significance as an active wavelength is evidenced by its current role in multiple Photo Dynamic Therapies (PDTs), some of which are used for the treatment of esophageal cancer, certain lung cancers and premalignant skin cancer.
Infrared- “IR” – (greater than 780-1,000,000 nm)
Infrared goes from above 780 nm to infinity, but most of the energy is in the 780 nm to about 1800 nm range. It comprises more than 40% of sunrays that reach us at sea level. These wavelengths warm us when we stand in the sun (perceived as deeply penetrating heat) and are emitted by stoves, furnaces, light bulbs, heat lamps, ovens, and space heaters. A number of studies have implicated infrared waves as photodamaging. Chronic exposure to infrared light leads to mottled pigmentation, loss of elastin (elastosis) and the typical characteristics seen in photoaged skin (wrinkling, sagging, leathery-feel). Infrared has also been known to cause cancer, such as Kang Cancer in China, Kangri in Kashmir, Kairo in Japan, and Peat Fire Cancer in Ireland.