UVA light is a type of ultraviolet radiation with wavelengths between 315 and 400 nanometers, making it the longest-wavelength UV light and the closest to visible light on the electromagnetic spectrum. It accounts for roughly 95% of all UV radiation that reaches the Earth’s surface, and unlike the shorter-wavelength UVB rays that cause sunburn, UVA works more quietly, penetrating deeper into skin and causing damage you often can’t feel happening.
Where UVA Fits in the UV Spectrum
Ultraviolet radiation is divided into three bands: UVA (315 to 400 nm), UVB (280 to 315 nm), and UVC (100 to 280 nm). UVC is the most energetic but gets absorbed entirely by the atmosphere, so it never reaches you outdoors. UVB makes up only 5 to 10% of the UV radiation at ground level, and it’s responsible for sunburn and plays a direct role in vitamin D production. UVA dominates what actually hits your skin on any given day.
Because UVA wavelengths are longer, they carry less energy per photon than UVB. That lower energy is precisely why UVA doesn’t cause the immediate redness of sunburn. But it also means UVA passes through skin tissue more easily, reaching layers that UVB can’t.
How Deep UVA Penetrates Your Skin
Your skin has two main layers: the epidermis (the thin outer barrier) and the dermis (the thicker layer beneath it, packed with collagen, elastin, and blood vessels). UVB radiation is mostly absorbed within the epidermis, where it triggers DNA mutations and inflammation. UVA, by contrast, travels through the epidermis and into the dermis, and can even reach the hypodermis, the fatty layer below that.
This deep penetration is what makes UVA particularly relevant to skin aging. Research comparing UVA and UVB effects on skin tissue found that UVA exposure increased the thickness and disorganization of the dermis, broke down collagen fibers, and caused significant DNA oxidation in dermal cells. UVB exposure, meanwhile, mainly sped up cell turnover in the epidermis without obvious changes deeper down. In other words, UVB hits the surface hard. UVA quietly damages the structural layer underneath.
UVA and Skin Aging
Photoaging, the premature aging of skin caused by sun exposure, is largely a UVA story. When UVA reaches the dermis, it generates reactive oxygen species, which are unstable molecules that damage cells and proteins. This oxidative stress triggers your body to produce enzymes called matrix metalloproteinases that break down collagen and elastin, the two proteins responsible for keeping skin firm and elastic.
Over time, this process leads to wrinkles, fine lines, and skin laxity. It also triggers an inflammatory response, releasing signaling molecules that amplify the oxidative damage and drive further collagen degradation. The effect is cumulative. Years of UVA exposure gradually diminish the skin’s capacity to repair itself, and the structural damage becomes visible as sagging and loss of firmness. This is why dermatologists emphasize daily sun protection even on cloudy days or during short commutes, since UVA intensity stays relatively consistent throughout daylight hours and across seasons compared to UVB.
UVA, DNA Damage, and Cancer Risk
UVA doesn’t cause DNA damage the same way UVB does. UVB directly strikes DNA molecules and creates mutations. UVA works indirectly: it interacts with light-absorbing molecules inside your cells, generating reactive oxygen species that then damage DNA, proteins, and lipids. One particularly concerning finding is that UVA-induced oxidation can disable DNA repair proteins, meaning your cells lose some of their ability to fix the damage that’s occurring. This combination of causing damage while simultaneously impairing repair may help explain UVA’s link to melanoma and other skin cancers.
UVA Passes Through Glass
One of the most practical things to know about UVA is that standard glass blocks very little of it. Tempered glass, the type used in most car side windows and many building windows, can allow up to 79% of UVA radiation to pass through. Smooth building glass transmits around 70%. So sitting by a sunny window at work or driving with your windows up does not protect you from UVA exposure.
Laminated glass is the exception. Car windshields are made from laminated glass (a plastic layer bonded between two panes), and they block about 98% of UVA. Laminated building glass also eliminates UVA transmission almost entirely. If you spend long hours near windows or in a car, the type of glass between you and the sun matters more than you might expect.
Effects on Your Eyes
UVA doesn’t just affect skin. Both UVA and UVB contribute to cataract formation, and UV radiation is a risk factor for retinal damage, particularly in children whose lenses are more transparent and filter less UV. Sunglasses labeled with UV400 protection block wavelengths up to 400 nm, covering the full UVA range. Cheaper sunglasses that darken your vision without proper UV filtering can actually increase risk, since your pupils dilate behind dark lenses, letting in more UV if the coating is inadequate.
UVA Does Not Produce Vitamin D
A common misconception is that any sun exposure helps your body make vitamin D. In reality, vitamin D synthesis is triggered exclusively by UVB radiation. UVB converts a cholesterol compound in your skin into pre-vitamin D3, which your body then processes into active vitamin D through heat. UVA plays no part in this reaction at all. So tanning beds that primarily emit UVA, or sun exposure filtered through regular glass (which blocks UVB but transmits UVA), will not boost your vitamin D levels. You get the aging and cancer risk without the vitamin D benefit.
How UVA Protection Works
The SPF number on sunscreen measures protection against UVB, the rays that cause sunburn. It tells you nothing about UVA protection. For UVA coverage, look for “broad spectrum” on the label if you’re in the United States, which means the product has passed a test for UVA filtering. In many Asian and European markets, a more specific system called the PA rating is used, where PA stands for “Protection Grade of UVA.” It’s based on how well a product prevents persistent pigment darkening, the tanning response triggered by UVA.
The scale runs from PA+ (some protection) through PA++++ (extremely high protection). The U.S. FDA has not adopted this system, determining that its own broad-spectrum test is sufficient, but you’ll see PA ratings on imported sunscreens. A product can have a high SPF and still offer poor UVA protection if it isn’t labeled broad spectrum, so checking for both is important.
Medical Uses of UVA
Despite its damaging potential, UVA is used therapeutically in controlled doses. Phototherapy with UVA light treats skin conditions like psoriasis, eczema, and vitiligo. In a common version of this treatment called PUVA, patients first take or soak in psoralen, a plant-derived chemical that makes skin more sensitive to UVA. They then receive carefully measured UVA exposure in a light booth. The treatment suppresses the overactive immune response in the skin that drives these conditions. Sessions are typically brief, and the cumulative dose is tracked carefully, since prolonged UVA therapy carries an increased risk of skin cancer over time.