What Are UVA and UVB Rays?
Some of the sun's rays can be seen, like visible light, and some cannot be seen, like ultraviolet light. When it comes to skin health and acne, it is the unseen, ultraviolet (UV) rays that are of concern. Ultraviolet rays are further broken down into UVA, UVB, and UVC rays, but only UVA and UVB rays reach the earth's surface.1–4
Differences Between UVA and UVB Rays
The sun's rays come to us in the form of waves. You can think of these waves in the same way as ocean waves, except that these are energy waves. Waves comprise crests (high points) and troughs (low points). The distance between the crests of two adjacent waves is called the wavelength. Wavelength is what gives each type of ray its unique characteristics. For example, human eyes can see wavelengths only of visible light.
The main difference between UVA and UVB rays is that they consist of different wavelengths. UVA rays have a longer wavelength than UVB rays, and therefore carry less energy, but they can penetrate farther into the skin. UVB rays have a shorter wavelength and more energy than UVA rays, but they don’t penetrate as deeply into the skin.4,5
Amount of radiation that reaches the earth
Before reaching the earth’s surface, the sun’s UV rays have to pass through the ozone layer, which is a natural layer of ozone gas that is part of the earth’s atmosphere. The ozone layer absorbs some of the energy from these rays and protects the earth from the full force of the sun’s radiation. UVA and UVB rays reach the earth in different amounts. The ozone layer absorbs most of UVB, while almost all of UVA reaches the earth’s surface.4
The amount of UVA that reaches the earth’s surface remains relatively constant year-round. The amount of UVB that reaches the earth’s surface varies according to season, time of day, amount of cloud cover, latitude (distance from the equator), and altitude. Generally speaking, UV exposure is greatest when the sun is highest in the sky. Therefore, it is strongest during the summer months between 10:00am and 2:00pm near the equator. Exposure is also greater at higher altitudes. While strongest on sunny days, UV rays do pass through light cloud cover.4
How Do UVA and UVB Rays Affect the Skin?
- less powerful
- penetrate the skin more deeply
- primarily responsible for skin tanning
According to the dermatology department on the University of California’s website: “UVA radiation…is less powerful than UVB, but it penetrates deeper into the skin. Small daily doses of UVA cause long-term skin injury, even without signs of sunburn. UVA light is used in tanning booths. Tanning booths not only cause the same type of skin and eye damage as natural sunlight, [but] may also be as much as 20 times stronger.”6
- more powerful
- penetrate the skin less deeply
- primarily responsible for sunburn
According to the dermatology department on the University of California’s website: “UVB radiation…has the most energy and causes the most damage. UVB can do more damage more quickly than UVA rays. Because of its damaging [e]ffect to the DNA of skin cells, UVB radiation is the main cause of sunburn and skin cancer.”6
Historically, scientists considered UVB to be the primary culprit in causing skin cancer, and they didn’t think that UVA was harmful. However, more recent research indicates that both kinds of UV rays cause both skin damage and skin cancer.2,6
UVA and UVB Rays and Sunscreen: Why It’s Important to Use Broad-spectrum Protection
Since both UVA and UVB rays damage the skin and can cause cancer, and since each type of ray causes a different kind of damage, we need protection against both types of rays.7
Applying sunscreen is one of the most effective approaches to protecting against UV radiation. It is important to select a sunscreen that protects against both UVA and UVB. Because scientists have only recently understood that UVA rays are a concern, sunscreens traditionally contained ingredients that only protected against UVB radiation, but this is no longer the case.
Scientists measure protection against UVB rays using the SPF (Sun Protection Factor) rating that you see on sunscreen containers. SPF is a numerical rating system that indicates how much longer skin protected by sunscreen takes to burn than unprotected skin. In other words, if your skin would normally burn in 30 minutes, an SPF of 4 means it would take 2 hours for your skin to burn. An SPF of 8 means it would take 4 hours, and so forth.
While there is no similar standardized measure for UVA protection, many modern sunscreens contain ingredients that protect against UVA radiation as well. These sunscreens are usually labeled as “broad-spectrum,” or "UVA/UVB protection," meaning that they protect against both UVA and UVB.4 However, because there is no consensus on how much protection these terms indicate, such phrases may not be entirely meaningful.7
A Note about UVA and UVB Rays and Acne
Since many acne patients report that moderate exposure to sunlight appears to improve their symptoms, at least in the short term, let’s look at what the research says in light of the harmful effects of both UVA and UVB rays.8
A 2004 article in the American Journal of Clinical Dermatology reviewed the results of several different studies that investigated whether UV radiation could improve acne. Here is what they found:
- Some studies performed in the lab indicate that UVB rays can kill P. acnes, a strain of bacteria associated with acne. However, these results do not hold up in living people. The authors of this article stated, “Although UVB has the potential to kill P. acnes [in the lab], this seems clinically insignificant since its capacity to penetrate the skin is low and only high doses resulting in sunburn have been shown to induce improvement in acne.”8 What this article does not research is how this kind of overexposure may bite back with a flare of acne in the weeks following a burn as the skin attempts to repair the damage.
- An investigation into whether UVA and UVB radiation could improve acne found little improvement with UV radiation. The authors stated, “In no instance was the number of comedones [clogged pores] appreciably reduced, and only modest improvement in inflammatory acne was observed with UVB and slightly more with the combination of UVA and UVB; UVA alone was the least beneficial. Further studies reached the same conclusion that UVA and UVB treatment has a slight beneficial effect in acne but is not sufficient for therapeutic use considering the potential for [causing cancer] and the inconvenience to patients of repeated visits.”8
Taken together, these studies indicate that UV exposure is not an effective treatment for acne. While getting some sun which does not cause a sunburn may be fine for acne-prone individuals, sun-burning the skin to attempt to heal acne is not wise.