UVA and UVB

Understanding UVA and UVB

UVA 

Most of us are exposed to large amounts of UVA throughout our lifetime. 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, which penetrates the skin more deeply than UVB, 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.

UVB 

UVB, the chief cause of skin reddening and sunburn, 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. However, 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.  

Sun Protection Factors (SPF) and UV radiation 

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 per se. Rather, 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 15 sunscreen screens 93 percent of the sun's UVB rays; SPF 30 protects against 97 percent; and SPF 50, 98 percent. The Skin Cancer Foundation maintains that SPFs of 15 or higher are necessary for adequate protection. 

FDA-Approved Sunscreens
Active Ingredient/UV Filter Name	Range Covered
	UVA1: 340-400 nm
	UVA2: 320-340 nm
	UVB: 290-320 nm
Chemical Absorbers:
Aminobenzoic acid (PABA)	UVB
Avobenzone	UVA1
Cinoxate	UVB
Dioxybenzone	UVB, UVA2
Ecamsule (Mexoryl SX)	UVA2
Ensulizole (Phenylbenzimiazole Sulfonic Acid)	UVB
Homosalate	UVB
Meradimate (Menthyl Anthranilate)	UVA2
Octocrylene	UVB
Octinoxate (Octyl Methoxycinnamate)	UVB
Octisalate ( Octyl Salicylate)	UVB
Oxybenzone	UVB, UVA2
Padimate O	UVB
Sulisobenzone	UVB, UVA2
Trolamine Salicylate	UVB
Physical Filters:
Titanium Dioxide	UVB, UVA2
Zinc Oxide	UVB,UVA2, UVA1

Physical:  zinc oxide,  titanium dioxide,  

Chemical:  avobenzone (Parsol, Eusolex, Escalol),  bemotrizinol (Tinosorb S), benzophenone-3 (Oxybenzone),  benzophenone-4 (Sulisobenzone),  ecamsule (Mexoryl SX),  phenylbenzimidazole sulfonic acid (ensulizole),  homosalate,  octyl methoxycinnamate (octinoxate),  octyl salicylate (octisalate),  octocrylene 

Hybrid:  bisoctrizole (Tinosorb M) 

UVB blockers:  phenylbenzimidazole sulfonic acid (ensulizole),  homosalate,  octyl methoxycinnamate (octinoxate),  octyl salicylate (octisalate),  benzophenone-3 (oxybenzone),  benzophenone-4 (sulisobenzone) 

UVA blockers:  avobenzone (Parsol, Eusolex, Escalol),  ecamsule (Mexoryl SX), 

UVA+UVB blockers:  zinc oxide,  titanium dioxide,  bisoctrizole (Tinosorb M), bemotrizinol (Tinosorb S),  octocrylene