How UVA and UVB Differ
How UVA and UVB Differ — two bands of ultraviolet light that reach the skin in different ways, with UVA penetrating deeper and driving aging while UVB acts more on the surface and causes burning. Both contribute to skin cancer, which is why broad-spectrum protection covers the full range.
Key points
- UVA has a longer wavelength, penetrates into the dermis, and is the main contributor to photoaging and wrinkles.
- UVB has a shorter wavelength, affects the outer layers, and is chiefly responsible for sunburn.
- UVA levels stay relatively constant through the day and year, while UVB peaks midday and in summer.
- Broad-spectrum sunscreens are formulated to shield against both bands, since SPF alone measures mainly UVB protection.
The two bands, property by property
| Property | UVA | UVB |
|---|---|---|
| Wavelength range | Roughly 315 to 400 nm | Roughly 280 to 315 nm |
| Depth reached | Through the epidermis into the dermis | Largely absorbed in the epidermis |
| Main chromophore | Flavins, porphyrins, mitochondrial components | DNA bases and urocanic acid |
| Damage route | Indirect, via reactive oxygen species | Direct photochemistry on adjacent DNA bases |
| Share of ground-level UV | Commonly cited as around 95 per cent | The remaining few per cent, after ozone |
| Daily and seasonal pattern | Near-constant through daylight and across the year | Peaks near solar noon and in summer |
| At window glass | Largely transmitted | Almost entirely blocked |
Why wavelength sets everything else
Every other difference follows from photon energy and what absorbs it. Shorter wavelengths carry more energy per photon and are scattered and absorbed sooner, so UVB deposits most of its energy in the outermost fraction of a millimetre. Longer UVA is absorbed less on the way in, so a meaningful share reaches the dermis.
Absorption also fixes the chemistry. DNA peaks in the shorter ultraviolet, so UVB photons are taken up by the bases themselves and react between them directly. UVA is barely absorbed by DNA but well absorbed by flavins, porphyrins and mitochondrial chromophores, which pass energy to oxygen and damage DNA at second hand.
What the difference means for real exposure
Because ozone removes most UVB but little UVA, the mix reaching the ground is dominated by UVA whenever the sun is up. UVB rises and falls sharply with solar angle, so burning is a midday and summer event; UVA changes little, so a winter morning still delivers it.
The same physics explains indoor exposure. Ordinary glass blocks nearly all UVB but transmits much of UVA, so a seat by a window supplies the ageing wavelengths without the warning sign of redness. SPF is measured by erythema, a UVB endpoint, so it says little about the UVA side.
Where the simple split breaks down
The mnemonic that UVB burns and UVA ages is useful but oversimplified. UVA contributes to redness too, far less efficiently per photon but present in far greater numbers. UVB also drives photoageing, and both bands are established causes of DNA damage; neither is the safe one.
The 315 nm boundary is a convention, and some sources place it at 320 nm. Within UVA, the shorter UVA2 region behaves partly like UVB, while the long UVA1 tail reaches deepest and passes glass most freely.
Frequently asked
A helpful way to remember the difference between UVA and UVB?
Many people use the mnemonic that UVA is for aging and UVB is for burning, though both types damage DNA and increase skin cancer risk.
Does a high SPF protect against UVA?
SPF primarily reflects UVB protection, so you need a product labeled broad-spectrum to ensure meaningful defense against UVA as well.
Why does UVA reach deeper into the skin than UVB?
Longer wavelengths scatter and are absorbed less on the way in, so more survives the epidermis. UVB is shorter and higher in energy, so it is taken up almost entirely in the outer layers.
Why is there so little UVB at ground level?
Stratospheric ozone absorbs strongly at shorter wavelengths, removing UVC entirely and most of the UVB band. UVA sits outside that absorption, so it arrives largely intact and makes up most of the ultraviolet reaching us.
Can UVA cause sunburn on its own?
It can, but very inefficiently — per photon it is orders of magnitude weaker at producing redness. Almost all ordinary sunburn is a UVB effect, so a lack of burning says nothing about the UVA dose.
Is the UVA from a tanning bed the same as the UVA from sunlight?
It is the same range of wavelengths, though the balance of UVA to UVB differs and the intensity can be much higher. The biological route is unchanged, so it is not a lower-risk substitute for the sun.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
