UVA vs UVB: Understanding Sun Protection

A high SPF number on the front of a bottle tells you one thing: how well the product stops you burning. It says almost nothing about the wavelength doing most of the ageing damage. That gap — between what SPF measures and what people assume it covers — is where a lot of sun protection goes wrong, and it’s why “broad-spectrum” on a label matters more than the SPF number itself.

Two wavelengths, two different jobs

UV radiation that reaches the skin splits into two bands that behave quite differently.

UVB (280–315nm) is higher energy and shorter wavelength. It’s largely absorbed by the epidermis, is blocked by ordinary window glass, and varies a lot by season, time of day and latitude — more intense at midday, more intense in summer. It’s the wavelength behind sunburn, and it’s the leading driver of the DNA damage linked to most sun-related skin cancers. It also triggers vitamin D production and the immediate tanning response.

UVA (315–400nm) is lower energy but longer wavelength, which lets it penetrate deeper into the dermis. Unlike UVB, it’s present at a fairly constant intensity throughout the day and across seasons, and it passes through glass — meaning a desk by a window or a long commute can add up over years. UVA is the wavelength most associated with photoageing: collagen and elastin breakdown, textural changes, and a contribution to pigmentation. It also plays a role in skin cancer risk and appears to have some immunosuppressive effect on skin.

UVB gets the attention because its damage is visible and immediate — you know when you’ve caught the sun. UVA works more quietly, accumulating over years without an obvious warning sign. Worth saying plainly: a tan, whichever wavelength caused it, is a visible marker of DNA damage. Melanin production is the skin’s defensive response to injury that’s already happened, not evidence of “safe” sun exposure.

What SPF actually measures

SPF (Sun Protection Factor) is a UVB-specific measure — how much longer skin can tolerate UV exposure before burning, compared with unprotected skin. Roughly: SPF 15 blocks around 93% of UVB, SPF 30 around 97%, SPF 50 around 98%. The jump from 30 to 50 is small in percentage terms, though SPF 50 can help compensate for the under-application that’s extremely common in practice — more on that below.

What SPF doesn’t tell you is anything about UVA coverage. A product can carry a high SPF number and still offer thin UVA protection, because the two aren’t automatically linked. That’s the specific gap “broad-spectrum” labelling exists to close.

Reading UVA protection on a label

A few systems exist, and they don’t all appear on UK products, so it’s worth knowing what you’re looking at:

• UVA-PF (Europe/UK): for the UVA circle logo to appear, UVA protection must be at least a third of the labelled SPF.
• PA system (used across Japan and Korea, and on some imported products): PA+ through PA++++, indicating increasing UVA protection.
• Critical wavelength (US): a product must protect up to 370nm to claim “broad-spectrum.”
• Star ratings (the Boots system, common in the UK): 1–5 stars, where five stars means UVA protection roughly matches UVB protection.

For UK shopping, the practical shortlist is: broad-spectrum claim, the UVA circle logo, and ideally four or five stars.

Chemical and mineral filters

Sunscreen actives fall into two broad categories, and neither is inherently better — they trade off differently.

Chemical (organic) filters — avobenzone, octinoxate, octisalate, homosalate, octocrylene, and newer ones like Tinosorb S/M and Mexoryl SX/XL — absorb UV radiation and convert it to heat. They tend to sit elegantly on skin without white cast and can reach high SPF in a thin layer. The trade-offs: some people react to specific filters, a few older ones (like avobenzone alone) are photounstable and degrade in sunlight unless stabilised, there are environmental concerns around reef safety for some, and these filters are absorbed into the bloodstream at detectable levels — though what that means for health, if anything, isn’t established either way.

Mineral (physical) filters — zinc oxide and titanium dioxide — mostly scatter and reflect UV rather than absorbing it. Zinc oxide in particular gives strong, genuinely broad-spectrum coverage including UVA. Mineral filters are photostable, work as soon as they’re applied, tend to suit reactive or sensitive skin better, and aren’t absorbed into the bloodstream. The trade-off is cosmetic: potential white cast (more visible on deeper skin tones), a heavier or chalkier feel, and formulations that need more careful, even application. Micronised and tinted versions have narrowed this gap considerably.

Most modern sunscreens blend both filter types, aiming for broad coverage with a texture people will actually tolerate daily — which matters more than it sounds, since a sunscreen left in the drawer protects nobody.

Getting the application right

The best-formulated sunscreen underperforms with typical real-world application. The commonly cited amounts are around a quarter of a teaspoon for the face and roughly 35ml (a shot glass) for the body — and most people apply somewhere between a quarter and a half of that, which meaningfully cuts the protection actually delivered versus what’s stated on the bottle.

Chemical filters generally benefit from being applied 15–30 minutes before sun exposure to bind properly; mineral filters work immediately but still settle better given a few minutes. Reapplication every two hours during sun exposure, and after swimming or heavy sweating, matters regardless of filter type or SPF number — no sunscreen lasts a full day on one application. Commonly missed spots: ears, neck, the backs of hands, a thinning scalp, and lips (an SPF balm covers that last one).

Two things worth knowing even outside obvious “sun exposure” days: UVA passes through glass, so time spent near windows or driving adds up cumulatively over years, and even brief outdoor moments — walking to the car, a lunchtime errand — contribute to lifetime UV exposure, which is the logic behind a daily habit rather than only applying sunscreen on beach days.

Sunscreen isn’t the only layer

Sunscreen works best as one part of a broader approach rather than the sole line of defence: seeking shade during the strongest midday hours, UV-protective or simply covering clothing, and sunglasses for the eyes and surrounding skin. None of this replaces sunscreen — it just reduces how much protective work the product alone has to do.

Common questions

Do I need sunscreen on cloudy days? Yes — up to around 80% of UV radiation still penetrates cloud cover, so overcast skies aren’t meaningful protection on their own.

Does darker skin need sunscreen? Melanin does provide some inherent UV protection, often estimated at roughly the equivalent of SPF 13, but that’s well short of adequate protection on its own — skin cancer and photoageing still occur in deeper skin tones, and post-inflammatory hyperpigmentation in particular tends to be worse with unprotected UV exposure.

Does sunscreen block vitamin D production? Realistically, no — achieving that would require near-perfect, constant application that most people simply don’t manage. For most people, typical sunscreen use isn’t the limiting factor in vitamin D status.

Is a higher SPF worth chasing? The jump from SPF 30 to SPF 50 is modest in real terms. Applying enough product and reapplying on time will do more for actual protection than swapping to a higher number.

Does a “base tan” offer any protection? It provides the rough equivalent of SPF 2–4 — negligible, and it represents damage that’s already occurred rather than a protective adaptation worth pursuing.

The bottom line

UVB causes the burning and drives much of the cancer risk; UVA drives the quieter, cumulative ageing damage and passes through glass. SPF on its own only tells you about UVB — broad-spectrum labelling, the UVA circle, or a strong star rating are what confirm UVA coverage. Chemical and mineral filters both work; which suits you depends more on skin type and tolerance than on which is “better.” And the product matters less than how it’s used: enough of it, reapplied on schedule, on a habit that holds up even on the days the sun doesn’t feel like a threat.

Daily, broad-spectrum sun protection remains one of the better-evidenced things in skincare — arguably more consequential for both visible ageing and skin cancer risk than most of the actives discussed on this site, layering guide included.