Short answer: UVA and UVB are two bands of the ultraviolet light in sunlight, and both can harm your skin. UVB is the main cause of sunburn, and it also contributes to skin aging and skin cancer. UVA has a longer wavelength, reaches deeper into the skin, contributes to visible aging such as wrinkles and age spots, and adds to skin cancer risk too. UVB varies most with the time of day and the season, and ordinary window glass blocks it. UVA is present all year, comes through cloud and can pass through window glass. Look for Broad Spectrum on a sunscreen label as well as the SPF.
Confusion often starts when the two are lumped together, or split too neatly.
Two bands of the same light
Ultraviolet light sits beyond the violet end of visible light. The part that reaches the ground comes in two bands. UVB has the shorter wavelength, and UVA the longer.
The atmosphere filters out most UVB before it arrives, while most UVA gets through. So most of the ultraviolet reaching your skin is UVA, even though UVB gets more attention because it is the one you can see by evening.
What UVB does
Sunburn. UVB is the primary cause of sunburn, and it acts mainly on the skin’s outer layers.
More than a burn. UVB also contributes to skin aging and skin cancer. Sunburn is a sign of short-term overexposure, while premature aging and skin cancer are effects of prolonged exposure.
It varies a lot. UV levels, and UVB in particular, are highest when the sun is high in the sky: around the middle of the day, in summer and closer to the equator. UV levels also rise with altitude. Water, sand, fresh snow and some surrounding surfaces can reflect UV, adding to your overall exposure, something worth remembering around swimming pools, at the beach, on open water and in snowy environments.
What UVA does
It goes deeper. UVA’s longer wavelength reaches deeper layers of the skin than UVB does.
Visible aging and cancer risk. UVA contributes to wrinkles and age spots, and overexposure to UVA, as to UVB, can lead to skin cancer.
It is there all year. UVA changes less with the seasons than UVB does, so in winter a larger share of the ultraviolet reaching you is UVA. It is present whenever there is daylight.
It arrives without much warning. Because UVB does most of the burning, a day with plenty of UVA can pass without the redness that tells you when you have had too much sun.
That is why the shorthand “UVB burns, UVA ages” is too neat. Both contribute to visible aging, both contribute to skin cancer risk, and protection against both matters. What Sun Damage Looks Like on Your Skin covers the visible signs. Any new, changing or unusual spot belongs with a dermatologist.
Glass and cloud
Window glass. Ordinary window glass blocks UVB. UVA can pass through, and how much depends on the glass, including its type, color, coatings and any laminated layers. A 2016 study of 29 cars found that windshields, which are laminated, blocked most UVA, while side windows blocked less on average and varied widely from car to car. Crepey Skin on the Neck and Décolleté looks at what years of that exposure can mean.
Cloud. UV levels are highest under clear skies, but they can still be high under cloud cover, and how much gets through depends on the cloud. UVB can still burn you on an overcast day, and UVA comes through cloud too.
Do You Need Sunscreen Every Day? explains why sun protection is a daily habit rather than a beach one.
What this means for your sunscreen
SPF is mainly about UVB. The SPF number measures protection against sunburn, which is mostly a UVB effect. What SPF Measures, and What It Does Not explains the number.
Broad Spectrum adds UVA. In the United States, the words Broad Spectrum mean a sunscreen has passed FDA’s test requiring its UVA protection to rise in proportion to its UVB protection. A Broad Spectrum sunscreen with SPF 15 or higher, used as directed with other sun protection measures, decreases the risk of skin cancer and early skin aging caused by the sun.
Sunscreen is one measure. Shade, hats, covering clothing and sunglasses work alongside it.
Cover the places UVA finds. The backs of the hands on a steering wheel, the side of the face nearest the car window, the neck and the chest. Why Hands Can Show Age So Easily covers the hands.
Victoria’s approach
Victoria Nash, founder of Esenté Skincare, has been a licensed aesthetician since 2003. She also owns and operates Younger You Clinique and is an experienced skincare educator. She draws on her professional experience at Younger You Clinique when evaluating skincare products and their performance. Her own preventive habits extend to the car. Victoria applies daily broad-spectrum sunscreen to the backs of her hands, and when driving she wears lightweight white linen gloves to help reduce continued sun exposure. That is her own practice, not a requirement for everyone.
Esenté offers two sunscreens: Derma-Defense SPF 30 in the Legacy Skincare collection and Derma Defense Z-Cote SPF30 in the Elite Skincare collection. Both are labeled Broad Spectrum SPF 30. They are examples among many suitable sunscreens, and the right one depends on your skin, your preferences and your budget. Follow the label directions for whichever you use, including the amount to apply and when to reapply. You can put a question about daily sun protection to Victoria through the Ask Victoria page.
Frequently asked questions
UVB has a shorter wavelength and is the main cause of sunburn. UVA has a longer wavelength and reaches deeper into the skin. Both contribute to skin aging and skin cancer risk, and a Broad Spectrum sunscreen is labeled for protection against both.
Neither is harmless. UVB does most of the burning. UVA is present year-round, through cloud and window glass, and contributes to visible aging. Both add to skin cancer risk, so protecting against only one leaves a gap.
It can. Ordinary window glass blocks UVB, but UVA can pass through, and how much depends on the glass. In one study of cars, windshields blocked more UVA than side windows, which varied widely from car to car.
Yes. Cloud can reduce UV without stopping it, and levels can still be high under cloud cover. UVB can still burn you on an overcast day, and UVA comes through too.
SPF is mainly a measure of UVB protection. To know a sunscreen also covers UVA, look for the words Broad Spectrum, which mean it passed FDA’s test requiring UVA protection in proportion to its UVB protection.
Yes. UVA changes less with the seasons than UVB, so it makes up a larger share of the ultraviolet in winter daylight. Fresh snow also reflects UV. Daily Broad Spectrum sunscreen, applied as the label directs, belongs in a winter routine as much as a summer one.
About the author. Victoria Nash, founder of Esenté Skincare, has been a licensed aesthetician since 2003. She also owns and operates Younger You Clinique and is an experienced skincare educator.
Reviewed by Victoria Nash, licensed aesthetician, founder of Esenté Skincare and owner of Younger You Clinique. This article is educational and is not medical advice. Ask your dermatologist or licensed skincare professional about products appropriate for your own skin and medical history.
