The Difference Between UVA and UVB Rays
The sun emits two types of ultraviolet (UV) radiation that reach our skin: ultraviolet A (UVA) and ultraviolet B (UVB). Both types damage unprotected skin, but how they go about doing that differs. Knowing how UVA and UVB rays and their ultraviolet light differs is essential to understanding the need for broad-spectrum sun protection.
UVA light and UVB light are not within the spectrum of visible light that the human eye can see. All light rays, including UVA and UVB rays, have different wavelengths, measured in nanometers, or “nm” for short. To give you some perspective, a nanometer is one billionth of a meter. Some of those rays—like blue light—are visible to us, some aren’t. Now let’s get to UVA vs. UVB and why you need to protect your skin from both!
All About UVA
UVA light, also known as long-wave light, accounts for about 95% of the UV light that reaches our skin. Although both UVA and UVB are bad for skin, UVA rays are more of a threat because a much larger percentage of them reach earth’s surface. They’re present all day long, year-round, even when it’s cloudy and the sun “isn’t out.” If you see daylight at any hour, UVA rays are present.
UVA light has a wavelength of 320 nm to 400 nm. There are two types of UVA rays: UVA1 and UVA2. UVA1 light is in the range 340–400 nm; UVA2 light is in the range 320–340 nm.
These differences in wavelength are important because specific sunscreen actives show peak performance in protecting from UVA light within these wavelengths—that’s one reason sunscreen products often contain several active ingredients. But some actives (like zinc oxide and, depending on how it’s formulated, titanium dioxide) protect across the entire UVA spectrum. Regardless of the combination of active ingredients, if the sunscreen is labeled “broad spectrum” it has been tested and proven to protect against the full range of UVA light.
UVA rays are considered the sun’s silent killers because, unlike UVB rays, you do not feel the effects of UVA rays damaging your skin. UVA rays are the cause of tanning, and unless you burn first, getting a tan isn’t painful—but, those unfelt UVA rays are reaching deep into skin, causing havoc in every layer.
UVA rays penetrate farther into skin than UVB rays, steadily destroying key substances in skin that give it its firmness and elasticity. UVA rays are a leading cause of wrinkles and a cause of, or major contributor to, every type of skin cancer.
One more difference: UVA rays penetrate glass, while UVB rays do not. Unless the window you sit by at work or the windows in your car are specially treated to filter UVA radiation, your skin is being exposed to UVA rays, making sunscreen an absolute necessity.
To UVB or Not to UVB
UVB light has a wavelength of 290 nm to 320 nm, a much smaller range than UVA light. Although not as skin-penetrating or ever-present as UVA rays, UVB light is powerful: It’s directly responsible for sunburn and other visible changes to skin’s surface, including discolorations. UVB radiation also plays a role in skin cancers.
Unlike UVA rays, the intensity of UVB rays varies to a much greater degree based on geographic location, time of day, and season. In the northern hemisphere, UVB rays are strongest between April and October, when there are more daylight hours, with peak intensity between 10:00 am and 4:00 pm.
Just like UVA light, UVB light is also present year-round, but UVB is more prevalent in sunny climates than in non-sunny climates. UVB light (and UVA light) is reflected from sand, water, and snow (80% of UVB rays reflect from snow!). UVB is also more damaging at higher altitudes than at lower altitudes, which is why skiers and mountain climbers need sunscreen. The same is true for UVA light, with the difference being the higher intensity of UVB when it’s at its most potent.
The SPF rating of sunscreens is related to protection from UVB rays. To ensure adequate protection from UVA as well as from UVB, look for sunscreens labeled “broad spectrum,” which indicates they have been tested and are permitted to make that claim.
Some sunscreens are sold with a different rating, known as the PA+ rating system.
References for this information:
International Journal of Molecular Sciences, January 2015, pages 68–90; and June 2013, pages 12,222–12,248
Experimental Dermatology, October 2014, pages 7–12
PLoS One, August 2014, ePublication
Molecules, May 2014, pages 6202–6219
Indian Journal of Dermatology, Venereology, and Leprology, June 2012 Supplement, pages S9–S14
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About the Experts
Paula Begoun is the best-selling author of 20 books about skincare and makeup. She is known worldwide as The Cosmetics Cop and creator of Paula’s Choice Skincare. Paula’s expertise has led to hundreds of appearances on national and international radio, print, and television including:
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