Sunlight is composed of packets of energy called photons. The visible colors we can see by eye are relatively harmless to our skin; it’s the sun’s ultraviolet (UV) light photons that can cause skin damage. UVA and UVB are the sun's ultraviolet A (longwave) and ultraviolet B (shortwave) rays. UV radiation is part of the electromagnetic (light) spectrum that reaches the earth from the sun. It has wavelengths shorter than visible light making it invisible to the naked eye. These wavelengths are classified as UVA, UVB, or UVC. UVA is the longest of the three at 320-400 nanometers (nm or billionths of a meter). UVA is further divided into two wave ranges, UVA I, which measures 340-400 nanometers (nm or billionths of a meter), and UVA II which extends from 320-340 nanometers. UVB ranges from 290 to 320 nm. With even shorter rays most UVC is absorbed by the ozone layer and does not reach the earth.
UVA and UVB penetrate the atmosphere and play an important role in conditions such as premature skin aging, eye damage (including cataracts) and skin cancers. They also suppress the immune system reducing your ability to fight off these and other maladies.
By damaging the skin's cellular DNA, excessive UV radiation produces genetic mutations that can lead to skin cancer. Both the U.S. Department of Health and Human Services and the World Health Organization have identified UV as a proven human carcinogen. UV radiation is considered the main cause of nonmelanoma skin cancers (NMSC), including basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). These cancers strike more than a million and more than 250,000 Americans each year. Many experts believe that especially for fair-skinned people, UV radiation also frequently plays a key role in melanoma, the deadliest form of skin cancer which kills more than 8,000 Americans each year.
UVA rays account for up to 95 percent of the UV radiation reaching the Earth's surface. Although they are less intense than UVB, UVA rays are 30 to 50 times more prevalent. They are present with relatively equal intensity during all daylight hours throughout the year and can penetrate clouds and glass.
UVA penetrates the skin more deeply than UVB. It has long been known to play a major part in skin aging and wrinkling (photoaging), but until recently scientists believed it did not cause significant damage in areas of the epidermis (outermost skin layer) where most skin cancers occur. Studies over the past two decades, however, show that UVA damages skin cells called keratinocytes in the basal layer of the epidermis, where most skin cancers occur. (Basal and squamous cells are types of keratinocytes.) UVA contributes to and may even initiate the development of skin cancers.
UVA is the dominant tanning ray and we now know that tanning, whether outdoors or in a salon causes cumulative damage over time. A tan results from injury to the skin's DNA. The skin darkens in an imperfect attempt to prevent further DNA damage. These imperfections, or mutations, can lead to skin cancer.
Tanning booths primarily emit UVA. The high-pressure sunlamps used in tanning salons emit doses of UVA as much as 12 times that of the sun. People who use tanning salons are 2.5 times more likely to develop squamous cell carcinoma, and 1.5 times more likely to develop basal cell carcinoma. According to recent research first exposure to tanning beds in youth increases melanoma risk by 75 percent.
UVB is the chief cause of skin reddening and sunburn. It tends to damage the skin's more superficial epidermal layers. It plays a key role in the development of skin cancer and a contributory role in tanning and photoaging. Its intensity varies by season, location, and time of day. The most significant amount of UVB hits the U.S. between 10 AM and 4 PM from April to October. UVB rays can burn and damage your skin year-round, especially at high altitudes and on reflective surfaces such as snow or ice, which bounce back up to 80 percent of the rays so that they hit the skin twice. UVB rays do not significantly penetrate glass.
Since the advent of modern sunscreens a sunscreen's efficacy has been measured by its sun protection factor or SPF. SPF is not an amount of protection. It indicates how long it will take for UVB rays to redden skin when using a sunscreen compared to how long skin would take to redden without the product. For instance, someone using a sunscreen with an SPF of 15 will take 15 times longer to redden than without the sunscreen. An SPF 30 sunscreen protects against 97 percent of the sun's UVB rays and SPF 50 protects against 98 percent of the sun's UVB rays.
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