UVA vs UVB Rays: What’s the Difference and Which Type Is More Harmful to Your Skin?

Plus, how to make sure you’re protected from both.

Author:
Michelle Villett
Updated:
Sep 7, 2024

We all know that UV exposure can be harmful to our skin. UV stands for ultraviolet radiation—a form of invisible electromagnetic energy that comes from the sun, as well as man-made sources like black lights and tanning beds.¹

There are two different types of UV light that reach the Earth and affect us: UVA and UVB.² One is primarily responsible for sunburns, while the other is a trigger for skin cancers and premature aging.

Sunscreens are supposed to protect you from both (that’s why they claim to be “broad-spectrum”), but they don’t always do a good job. In fact, a recent investigation revealed that the most common US sunscreens are inadequate against the most prevalent and dangerous form of UV.³

Shocked? Me too—which is why I created this tutorial. You will learn the difference between UVA and UVB, which one is the most damaging, and what you need in order to protect your skin properly.

What’s the Difference Between UVA and UVB?

Sunlight actually produces three different wavelengths of UV radiation:

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UVA is long-wave radiation between 315 and 400 nanometers (nm)
UVB is short-wave radiation between 280 and 315 nm
UVC is intensive short-wave radiation between 100 and 280 nm²

Fortunately, we don’t need to worry about UVC, since it is filtered out by the ozone layer and never reaches the Earth’s surface.

UVA and UVB are measured on a scale called the electromagnetic spectrum (which also includes visible light and infrared light). According to the scale, UVA is considered low-energy radiation, since it has longer wavelengths (the distance between peaks in a series of waves). In contrast, UVB is high-energy radiation, since it has shorter wavelengths.

What Is UVA?

When you think of UVA, think “A” for aging. Ultraviolet A (UVA) radiation is chiefly responsible for aging your skin and triggering skin cancers. This is because UVA rays have the longest wavelengths, which penetrate the most deeply. In fact, they go all the way into the dermis, the middle layer of skin, where they cause cellular and DNA damage.⁴

“UVA rays are the longer-wave rays that do the most harm,” says Dr. Sharyn Laughlin, dermatologist and co-founder of The Sunscreen Company. “According to studies from the past two decades, UVA is the main driver of skin cancer and photoaging.”

When your skin is exposed to UVA, it causes an immediate tanning effect (which is why it’s the type of UV used in tanning beds). However, at the same time, it initiates more insidious changes—like suppressing your immune system, generating free radicals, and interfering with your body’s DNA repair processes.^5,6,7

This encourages the eventual formation of wrinkles and pigmentation, as well as melanoma and other skin cancers. You just can’t feel or see the damage right away—which is why UVA is often dubbed “the silent killer.”

What Is UVB?

When you think of UVB, think “B” for burning. Ultraviolet B (UVB) is a higher-energy, shorter-wave radiation that is directly responsible for triggering sunburns.

“UVB rays are the shorter wave rays in the UV spectrum, and are Nature’s warning signal to tell people to get out of the sun,” says Dr. Laughlin. “They penetrate to the epidermis [the outermost layer of skin], and initiate early sunburn.”

Anytime a sunburn occurs, your DNA gets damaged. Fortunately, if you get out of the sun right away, your body is capable of repairing it without consequences like aging and skin cancer.⁸

But if you stay in the sun—continuing to expose your skin to not only UVB but also UVA—then the UVA will make things worse. “If the individual remains in the sun and receives ongoing exposure to UVA, the DNA damage continues and is more severe: 94% of the mutations of skin cancer are UVA-induced, whereas only 6% are UVB-induced,” says Dr. Laughlin.

In other words, UVB is the main culprit behind sunburns, but in combination with UVA can also trigger DNA changes that play a role in skin cancers.

Is UVA or UVB More Dangerous?

So, which type of UV is worse: UVA or UVB? According to experts, it’s UVA rays that we should be the most worried about.

First of all, UVA is far more prevalent than UVB. It accounts for 90-95% of ambient sunlight, whereas UVB is only 5-10%.⁴

We also know that up to 90% of the skin changes attributed to “aging” are in fact caused by the sun—and it’s clear that UVA is the main culprit.⁹

“The most damaging rays from the sun are in fact in the UVA spectrum,” says Dr. Howard Murad, dermatologist and founder of Murad. “UVA rays are actually the same strength all year round and can even penetrate through clouds and windows. Plus, they can contribute to premature aging, collagen degradation, and even skin cancer.”¹⁰

What Is the UV Index, and Is It Reliable?

The UV index is an international measurement system that predicts the level of UV radiation on any given day.¹¹ It takes the following factors into account to estimate the strength of UV radiation reaching the Earth (and your skin):

Time of day: UV rays are at their daily peak between 10 a.m. and 4 p.m. The time between noon and 1 p.m., known as “solar noon,” is when they are the most intense.
Time of year: UV is the strongest during the spring and summer months, because the sun is high in the sky and its rays shine down more directly.
Latitude and altitude: The closer you are to the equator, the more UV exposure you get. The same goes for higher elevations—the higher up you are, the stronger the UV.
Clouds and haze: Although UV rays can penetrate through thin clouds and haze, thick and heavy cloud cover can block most UV from getting through.
Ozone: The ozone layer, which is continuously fluctuating, absorbs UV radiation, making it less intense once it reaches Earth.
Reflection: UV light can bounce off surfaces such as snow, ice, sand, water and concrete.

The higher the UV index, the more precautions you’re meant to take:

Low (1-2): The best time to be outside, with low risk of sunburn.
Moderate (3-5): Take routine precautions—like wearing SPF 30 and covering up unprotected skin.
High (6-7): You’ll need to protect your skin from burning. Seek shade at midday in addition to wearing (and re-applying) sunscreen and sun-protective clothing.
Very High (8-10): Unprotected skin can burn quickly, so consider avoiding the sun during peak hours. Otherwise, cover up and wear sunscreen.
Extreme (11+): This means that unprotected skin (and eyes) can burn within just a few minutes, and sun exposure during peak hours should be avoided altogether.

What’s Wrong With the UV Index?

The UV index is useful if you want to know when you’re most likely to burn. But if you’re worried about signs of aging, you should still wear sunscreen—even when the UV index is low.

“It’s really a vehicle for public awareness,” says Dr. Cara McDonald. “We don’t see much sunburn and we don’t see much increased risk of skin cancer if the UV index is below 3. However, we do still see some effects of UV light, particularly on our signs of aging. From my point of view, I don’t want to have to check the time of the day and where I am and whether or not the UV index has gone above 3. So, personally, I would just suggest that we wear [sunscreen] all the time, particularly in those areas that are frequently exposed, like the face.”¹¹

“UVA is ubiquitous and constant all year round and does not vary according to latitude or time of day,” says Dr. Laughlin. “So everyone should wear sunscreen at any time of day, in any season, in any location on the planet. The linchpin of photoprotection is to apply a sunscreen every day, first thing in the morning, regardless of your intended activity.”

What Is “Broad-Spectrum” Sunscreen, and What Should You Look For?

Dermatologists always recommend “broad-spectrum” sunscreen. This term means that the sunscreen will protect your skin against both UVA and UVB rays.

According to Dr. Laughlin, a truly broad-spectrum sunscreen needs to be SPF 30 or higher, with a UVA Protection Factor (UVA-PF) of at least 10-20.

The problem is, the FDA and Health Canada don’t require UVA Protection Factor testing, even though it is the best way to determine how well a sunscreen defends against UVA. Instead, they allow manufacturers to infer the level of UVA protection (and therefore make the claim “broad-spectrum”) based on another, less accurate test called the Critical Wavelength test.

“The word ‘broad-spectrum’ as it appears on North American labels is inaccurate: 90% of available sunscreens use 3% avobenzone, or zinc oxide at less than a 14% concentration, and can only achieve a UVA-PF of 5-8,” says Dr. Laughlin. “This has been proven by a computer model developed over the past 20 years, and further confirmed by the gold standard of testing on skin (in-vivo). In North America, UVA is inferred from a method called Critical Wavelength, which can be very misleading. Two sunscreens with the same Critical Wavelength may have widely different UVA-PF values.”

So it’s not surprising that a recent study discovered that the majority of sunscreens provide significantly lower UVA protection than implied. Researchers found that the average unweighted UVA-PF values were only 24% of the labelled SPF—which means the typical SPF 30 might have a UVA-PF of only 7, instead of the 10-20 that is needed.³

What to Look For in a Broad-Spectrum Sunscreen

Clearly, we can’t trust the term “broad-spectrum” on sunscreen labels. And since brands aren’t required to perform the UVA-PF test or publicize their results, we can’t rely on that. So what should you look for to make sure your sunscreen protects against both UVA and UVB?

✓ At Least 15% Zinc Oxide

Here in North America, zinc oxide is the only available UV filter that offers extensive protection against both UVA and UVB.¹³ Look for a minimum 15% concentration up to the maximum of 25%.

✓ SPF 30 or Higher


Sunscreen Filter	Maximum SPF Units Per 1%
Zinc Oxide	1.6
Titanium Dioxide	2.6

SPF is a measure of how well a sunscreen guards against UVB. Rather than relying on the number claimed (which is often inaccurate), you can estimate it yourself.³

According to industry sunscreen simulation tools, every 1% of zinc oxide is equivalent to a maximum of 1.6 SPF units.^14,15 So a 20% zinc oxide sunscreen would be about SPF 32.

Often, you’ll see zinc oxide in combination with titanium dioxide. Since every 1% of titanium dioxide is equivalent to a maximum 2.6 SPF units, a sunscreen with 15% zinc oxide and 5% titanium dioxide would be about SPF 37.

Conclusion + Further Reading

Now you know the difference between UVA and UVB, and the best way to protect your skin from their damaging effects. It all comes down to wearing a truly broad-spectrum sunscreen with strong UVA protection every day.

“The most important reason to use a sunscreen with high UVA protection is to prevent skin cancer over your lifetime, but I find that people are sometimes more motivated by preserving the look of their skin than statistics relating to skin cancer,” says Dr. Laughlin.

“In my clinic, patients sometimes invest thousands of dollars into improving their skin with laser treatments and injectables, but I always tell them their investment goes out the window if they do not preserve it with a high-UVA sunscreen. Studies have shown that the daily use of a truly broad-spectrum sunscreen over a lifetime can shave up to 20 years off the look of your skin.”

Wait—wearing the right sunscreen can turn back the clock two decades?! I don’t know about you, but hearing that statistic makes me extra-motivated to be diligent about UV protection, no matter what.

^{How much sun is too much? (2018, November 29). National Library of Medicine.}
^{Radiation: Ultraviolet (UV) radiation. (2016, March 9). World Health Organization.}
^{Andrews, D. Q., Rauhe, K., Burns, C., Spilman, E., Temkin, A. M., Perrone-Gray, S., Naidenko, O. V. & Leiba, N. (2022). Laboratory testing of sunscreens on the US market finds lower in vitro SPF values than on labels and even less UVA protection. Photodermatology, Photoimmunology & Photomedicine. 2022 May; 38(3): 224-232.}
^{D’Orazio, J., Jarrett, S., Amaro-Ortiz, A. & Scott, T. (2013). UV Radiation and the Skin. International Journal of Molecular Sciences. 2013 Jun; 14(6): 12222–12248.}
^{Kripke, M. L. (1979). Effects of UV radiation on the immune system: consequences for UV carcinogenesis. Advances in Experimental Medicine and Biology. 1979; 121(A): 589-98.}
^{Tyrrell, R. M. (1995). Ultraviolet radiation and free radical damage to skin. Biochemical Society Symposium. 1995; 61:47-53.}
^{Cleaver, J. E. & Crowley, E. (2002). UV damage, DNA repair and skin carcinogenesis. Frontiers in Bioscience. 2002 Apr 1; 7:d1024-43.}
^{Brash, D. E., Ziegler, A., Jonason, A. S., Simon, J. A., Kunala, S. & Leffell, D. J. (1996). Sunlight and sunburn in human skin cancer: p53, apoptosis, and tumor promotion. The Journal of Investigative Dermatology. 1996 Apr; 1(2): 136-42.}
^{Health Effects of UV Radiation. (2022, January 4). United States Environmental Protection Agency.}
^{Lister, R. (2019, June 30). This is Why You Burn so Easily – Even When Slathered in Sunscreen. Women’s Health.}
^{A Guide to the UV Index. United States Environmental Protection Agency.}
^{Watts, G. (2020, September 23). We Asked a Dermatologist to Bust Some Sunscreen Myths. Glow Journal.}
^{Sabzevari, N., Qiblawi, S., Norton, S. A. & Fivenson, D. (2021). Sunscreens: UV filters to protect us: Part 1: Changing regulations and choices for optimal sun protection. International Journal of Women’s Dermatology. 2021 Jan; 7(1): 28–44.}
^{BASF Sunscreen Simulator.}
^{DSM Sunscreen Optimizer.}

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