UVA Knowledge Base
Short, plain-English guides to ultraviolet A radiation — what it is, how it differs from UVB, why it matters for your skin, and why the familiar UV Index doesn't tell the whole story.
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What is UVA radiation?
UVA is the long-wavelength band of ultraviolet light (315–400 nm). Learn where it sits in the UV spectrum, why ~95% of the UV reaching the ground is UVA, and how it behaves through the day, through clouds and through glass.
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UVA vs UVB: what's the difference?
UVA vs UVB compared: wavelength, how deep each penetrates skin, sunburn vs photoaging, vitamin D, skin-cancer roles, and how clouds, glass, time of day and season affect each.
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The dangers of UVA radiation
UVA's health effects: photoaging and wrinkles, indirect DNA damage via reactive oxygen species, skin-cancer and melanoma risk, eye damage, immune suppression and photosensitivity — plus practical ways to cut exposure.
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What the UV Index measures — and why we need a UVA Index
The UV Index is erythemally weighted, so it's dominated by UVB and under-represents UVA. Learn exactly what the UV Index does and doesn't capture, and why a dedicated UVA Index is a useful complement.
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How the UVA Index is calculated
The full method behind our UVA Index: how surface UVA irradiance (315–400 nm, W/m²) is derived from solar geometry plus live cloud, aerosol and altitude data, the exact formulas and coefficients used, and which factors we deliberately leave out.
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Does glass block UVA? Windows, cars and UV film
Ordinary window glass stops almost all UVB but lets a large share of UVA through. How much passes through home, office and car glass, why windshields differ from side windows, what the driver skin-cancer studies show, and how to choose UV film.
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UVA on cloudy days: how much gets through
Cloud dims UVA far less than it dims sunlight — and broken cloud can briefly push UVA above clear-sky levels. Transmission by cloud type, why UVA penetrates cloud better than UVB, the cloud enhancement effect, and how the UVA Index models it.
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Why UVA changes far less with the seasons than UVB
Ozone absorbs UVB strongly and UVA barely at all, so a low winter sun collapses UVB while UVA falls only modestly. Why winter and early-morning UV is disproportionately UVA, with the numbers — and why a low UV Index never means low UVA.
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Reflected UVA: snow, sand, water and concrete
Ground reflectance adds to the UVA you receive — and UV albedo is not visible albedo. Reflectance values by surface, why skiers and boaters burn under the chin, why shade fails on a beach, and how the UVA Index models surface albedo.
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UVA at altitude: mountains, hiking and flying
UV rises with elevation — but UVA rises more slowly than the often-quoted 10% per 1000 m, which is an erythemal figure. Why the altitude gradient differs by wavelength, how snow and clean air compound it, and what the cockpit UVA and pilot melanoma research shows.
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UVA indoors: nail lamps, tanning beds and what your lights really emit
Gel-manicure lamps are almost pure UVA, tanning beds emit heavily UVA-shifted spectra, and screens emit no UV at all. What each indoor source actually puts out, how it compares to midday sun in W/m², and where the real risks are.
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UVA, melanin and pigmentation: melasma, hyperpigmentation and skin tone
UVA and visible light drive melasma and post-inflammatory hyperpigmentation far more than UVB — which is why tinted, iron-oxide sunscreens outperform clear ones. What melanin does and doesn't protect against, across Fitzpatrick types.
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UVA and your eyes: sunglasses, UV400, cataracts and pterygium
How UVA reaches and damages the eye, what UV400, EN ISO 12312-1 and ANSI Z80.3 actually guarantee, why dark lenses without UV filtering may be worse than none, and why frame shape matters more than lens darkness.
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Photosensitivity: medications and conditions that make UVA dangerous
Most drug photosensitivity is UVA-driven, which is why it happens through car and office windows and on overcast days. Phototoxic vs photoallergic reactions, the main drug classes, PMLE, lupus and porphyria, and what protection actually works.
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UVA sunscreen labels decoded: PA+++, the UVA circle, stars and critical wavelength
SPF measures UVB almost exclusively. How to read actual UVA protection: the EU circled-UVA logo, the Japanese PA system, Boots stars, the US broad-spectrum test, avobenzone photostability, and which UVA filters are available where.
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