UVA vs UVB: what's the difference?
UVA and UVB are the two bands of ultraviolet light that reach Earth's surface. They sit next to each other in the spectrum, but they behave differently in the atmosphere and in your skin. The short version: UVB burns, UVA ages — and both contribute to skin cancer.
At a glance
| UVA | UVB | |
|---|---|---|
| Wavelength | 315–400 nm (longer) | 280–315 nm (shorter) |
| Energy per photon | Lower | Higher |
| Share of surface UV | ~95% | ~5% |
| Skin penetration | Deep — into the dermis | Shallow — mainly the epidermis |
| Main short-term effect | Immediate tanning, little burning | Sunburn (erythema) |
| Main long-term effect | Photoaging, wrinkles, pigmentation | DNA damage, most skin cancers |
| Vitamin D | No | Yes — triggers synthesis |
| Through glass / clouds | Largely passes through | Mostly blocked |
| Time of day / season | Fairly constant | Peaks at midday and in summer |
1. Wavelength and energy
UVB has shorter wavelengths and therefore more energy per photon, which makes it more directly damaging photon-for-photon. But it's also more strongly absorbed by atmospheric ozone, so only a small fraction reaches the ground. UVA's longer wavelengths slip through the atmosphere almost unimpeded, which is why it makes up the vast majority of surface UV.
2. How deep they go
Penetration depth scales with wavelength. UVB is largely stopped in the epidermis, the outer layer — which is why its damage shows up there as sunburn. UVA reaches the dermis, the deeper layer that holds collagen, elastin and blood vessels. That's the structural foundation of skin, and it's why UVA is the main driver of long-term photoaging: wrinkles, sagging, leathery texture and uneven pigmentation.
3. Burning vs aging
UVB is the chief cause of sunburn. It damages DNA directly, your skin reddens, and you get a clear warning that you've had too much sun. UVA rarely burns — instead it drives the immediate tanning response and, over years, silent cumulative damage. Because there's no sting and no redness, UVA exposure is easy to under-count.
4. Skin cancer
UVB causes direct DNA damage (it's absorbed by DNA and forms lesions such as pyrimidine dimers) and is the principal driver of common non-melanoma skin cancers. UVA works mostly indirectly: it generates reactive oxygen species that damage DNA, proteins and lipids at one remove. UVA is increasingly implicated in melanoma and contributes to overall photocarcinogenesis. The practical takeaway is that both bands matter for cancer risk — which is why good sunscreens are labelled "broad spectrum."
5. Vitamin D — and the trade-off
UVB has one clear benefit: it triggers vitamin D synthesis in the skin. UVA does not. This is the genuine tension in sun-exposure advice — the same UVB that helps make vitamin D also burns and damages DNA. UVA, by contrast, offers no comparable benefit while still carrying long-term risk.
6. Clouds, glass, time and season
This is where the two bands diverge most in everyday life:
- Glass: standard windows block most UVB but let much UVA through — so you can age your skin behind a car or office window.
- Clouds: light cloud cuts UVB more than UVA; a meaningful UVA dose gets through on overcast days.
- Time and season: UVB is highly concentrated around solar noon and in summer, because a low sun forces it through more ozone. UVA is comparatively flat across the day and year.
These differences are a big reason the single, sunburn-weighted UV Index can be misleading about your real UVA exposure — and the case for tracking UVA on its own.
Estimate the UVA Index where you are →