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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UV Index vs UVA Index: what each one measures
The UV Index is erythemally weighted, so it is dominated by UVB and barely registers UVA. What each index measures, the situations where a low UV Index sits alongside high UVA, and how to read the 0–11+ UVA Index scale.
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The UV Index scale explained — and what it leaves out
What the UV Index numbers mean, where the 0–11+ scale comes from, the WHO band colours and advice, how high it can actually go — and the erythemal weighting that makes it under-report UVA.
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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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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 time is UVA highest? The daily curve
UVA peaks at solar noon — but its curve is far flatter than the UV Index's, so mornings and late afternoons carry proportionally much more UVA. Hour-by-hour numbers, why the shadow rule fails, and why solar noon is rarely 12:00.
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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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UVA around the world: why latitude matters less than you think
Ozone barely absorbs UVA, so the equator-to-pole gradient is far shallower for UVA than for the UV Index. Peak clear-sky UVA Index for cities worldwide, why the southern hemisphere runs hotter, and what actually drives local differences.
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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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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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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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Does clothing block UVA? UPF ratings and what actually works
UPF is erythemally weighted, so like SPF it is really a UVB measure — which is why the European standard adds a separate UVA limit. Fabric transmission by material, colour and weave, what happens when cloth gets wet or stretched, and how hats and shade really perform.
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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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How much sunscreen you actually need — and why UVA suffers most
Sunscreen is tested at 2 mg/cm² and applied at a quarter of that, and protection falls exponentially with thickness. Why under-application and photodegradation hit UVA protection hardest, plus the teaspoon and two-finger rules.
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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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UVA and skin aging: how photoaging actually happens
UVA reaches the dermis and drives the enzymes that dismantle collagen. The mechanism from photon to wrinkle, the trial showing daily sunscreen halts measurable aging, the evidence from truck drivers and twins, and what genuinely repairs the damage.
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Does UVA tan you? Tanning, burning and the base-tan myth
UVA darkens skin within minutes by oxidising melanin you already have — a tan that offers almost no protection. The three distinct tanning responses, why UVA barely burns, and what that means for tanning beds and the base-tan idea.
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UVA and vitamin D: all of the cost, none of the benefit
Vitamin D synthesis is driven by a narrow slice of UVB around 295–300 nm and stops entirely below 315 nm. UVA produces none of it — which makes winter sun, window light and sunbeds pure exposure with no return.
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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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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 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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How to measure UVA yourself: meters, cards and phone myths
What it takes to actually measure UVA — spectroradiometers, broadband radiometers, cheap consumer meters and photochromic cards — where each fails, why no smartphone can measure ultraviolet, and how to sanity-check any reading.
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