I recently received this question via the FutureDerm, Inc. Facebook page. Blocking both is important, but we are exposed to more UVA. Most people believe we are exposed to more UVA if we are indoors, and more UVB if we are outdoors. However, the truth of the matter is, we are exposed to just about all UVA if we are indoors, and 20 times more UVA than UVB if we are outdoors (Methods in Enzymology, 2000). Despite this, SPF largely measures protection from UVB and not UVA exposure!  The full answer just gets more and more interesting:
UVA = UV-Aging, UVB = UV-Burning
A simple way to remember the difference is above: UVA = UV-Aging, whereas UVB = UV-Burning. In truth, both contribute to aging and burning, but the burning effects of UVB rays are seen much sooner than UVA rays (Cosmetic Dermatology, 2009), so this acronym works.
UVA rays are longer and have been found to play a role in skin cancer, solar elastosis, and premature signs of skin aging (Journal of the American Academy of Dermatology, 1995; Journal of Investigative Dermatology, 1995).
On the other hand, UVB rays cause immediate skin erythema (skin burning). Given that we are sticklers for immediate gratification, it is no wonder than SPF ratings were developed to measure UVB protection and not UVA, signs of which take much longer to appear. SPF is specifically the level of sun exposure needed to produce a minimal skin erythema (skin reddening) divided by the amount of energy required to produce the same erythema on unprotected skin. In theory, a subject that applies SPF 10 could stay in the sun 10 times longer without incurring skin redness (Cosmetic Dermatology, 2009).
62.8% of UVA Rays Get in Through Windows
UVA rays are longer (320-400 nm) than UVB rays (280-315 nm). While window glass protects you from short UVB rays, 62.8% of long UVA rays get through, according to the American Cancer Society and the Skin Care Foundation. (So maybe you don’t want that nice corner office with all the windows!)
You’re also not safe with standard window tint. Ordinary window tint only blocks 3.8% of UV rays (sunaware.org). However, gray tinting will block 99.1% of UV rays. Just make sure to get your car professionally done to comply with state laws about window tinting (sunaware.org). For instance, where I live (Pennsylvania), window tinting is illegal.
Fluorescent Lighting and Old Monitors Emit UVA
Fluorescent lighting also produces small amounts of UV rays that can be damaging over time (GELighting), while old-school, big-box cathode ray tube (CRT) monitors similarly emit very low levels of UVA radiation. According to the Robbins Eye Center of Bridgeport, CT, these radiation levels are so low that a lifetime of exposure does not hurt the eyes. Still, the exposure is significant enough that those with xeroderma pigmentosum (XP), a rare disorder in which patients are ultra-sensitive to sunlight, need to take caution around these monitors with monitor anti-glare filters.
High SPF Sunscreens Do NOT Necessarily Block More UVA
Here’s where it gets tricky:Â In a study of six sunscreens with an SPF of 20 or higher, 2/3 sunscreens with a high SPF rating (45 and 50) were found to have a low UVA rating.
The same study concluded that PABA is the best UVA filter (Journal of the American Academy of Dermatology, 2000), although zinc oxide, benzophenones, cinnamates, salicyclates, and phenylbenzimidazole sulfonic acid have all been found to be excellent UVA blockers as well (Cosmetic Dermatology, 2009).
Avoid sunscreens that only block UVA or UVB
Thanks to new government regulations in the United States, sunscreens must block both UVA and UVB rays starting in 2013. However, in the meantime, keep in mind that some sunscreens only block UVA or UVB (Cosmetic Dermatology, 2009),:
UVA-Only Sunscreens
- Benzophenones
- Menthyl Anthranilate
- Parsol 1789
UVB-Only Sunscreens
- Titanium oxide (blocks only short-range UVA up to 340 nm, not long-range UVA)
- Para-Aminobenzoic acid (PABA)
- Cinnamates
- Salicylates
- Phenylbenzimidazole Sulfonic Acid
UVA Doses Tend to Be Much Higher than UVB
Unlike exposure to chemicals like parabens, which only stay in the system for 36 hours (Experimental Dermatology, 2007), exposure to sunlight is cumulative. Each dose of sunlight causes DNA damage, which accumulates over time (FASEB, 2002). While DNA repair enzymes in the cell help to mediate the damage caused after UV exposure, excessive sunlight is still a well-established carcinogen (PNAS, 2002; Journal of Investigative Dermatology, 1996; Nature, 1971). Most sunlight is 20 times more UVA than UVB (Methods in Enzymology, 2000).
Bottom Line:Â When In Doubt, Wear a Hat
That is actually meant to be humorous, though hats do help! The bottom line really is to remember SPF rating is not a measure of how much anti-aging protection you’re getting; rather, it’s a measure of how much burning protection you’re getting.  For the best of both worlds, look for a high SPF rating with additional UVA blockers like zinc oxide, benzophenones, cinnamates, salicyclates, and phenylbenzimidazole sulfonic acid.
Hope this helps,
Nicki
