What the UV Index measures — and why we need a UVA Index

The UV Index is a genuinely useful number, but it answers one specific question: how quickly will this sunlight burn unprotected skin? Because it's weighted for sunburn, it's dominated by UVB and systematically under-weights UVA — even though UVA is the vast majority of the UV that reaches you.

What the UV Index actually is

The UV Index (UVI) isn't a raw measurement of ultraviolet energy. It's the surface UV spectrum weighted by the erythemal action spectrum — a curve, defined by the CIE, that describes how effective each wavelength is at reddening human skin. The weighted spectrum is integrated and scaled (multiplied by 40 W/m⁻²) to give the familiar 0–11+ scale.

The key feature of that erythemal curve is that it falls off steeply toward longer wavelengths. Skin is far more sensitive to UVB than to UVA when it comes to burning — on the order of a thousand times more sensitive at some wavelengths.

What the UV Index does well

• It's an excellent guide to sunburn risk and how fast you'll redden.
• It tracks the midday and summer peak, when burning UVB is most intense.
• It's standardised, internationally recognised and available everywhere.

For "should I put sunscreen on right now to avoid burning?", the UVI is the right tool.

What it under-represents

The UV Index is a poor proxy for UVA dose, and therefore for the harms UVA drives:

• Photoaging — collagen breakdown, wrinkles and pigmentation are largely UVA effects the UVI barely reflects.
• Deep dermal and indirect DNA damage, and UVA's contribution to melanoma.
• Exposure through glass — in a car or by a window you get almost no UVB (so a low "effective" UVI) yet plenty of UVA.
• Cloudy days and off-peak hours — UVA stays comparatively high when the UVI has dropped, because UVA varies far less with sun angle and cloud.
• Photosensitivity reactions, many of which are UVA-driven.

Why a low UV Index can still mean high UVA

Picture late afternoon, or a hazy/overcast day, or sitting by a sunny window. The UV Index may read low to moderate because the sunburning UVB has fallen away. But UVA — steadier through the day, better at penetrating cloud and glass — can still be delivering a substantial dose to the deeper layers of your skin. The UVI gives you a reassuring number while the UVA exposure quietly continues.

The case for a dedicated UVA Index

A UVA-specific measure fills the gap. Instead of erythemal weighting, it looks at the unweighted UVA band (≈315–400 nm) — the actual long-wave irradiance reaching the surface, in W/m². That tells you about the exposure responsible for aging and deep, longer-term damage, which the UV Index isn't built to convey.

The UVA Index scale

The raw UVA irradiance (in W/m²) is precise but unfamiliar, so we put it on a 0–11+ scale just like the UV Index. The conversion is one constant: UVA Index = UVA irradiance ÷ 6. That divisor is chosen so a clear-sky overhead sun (~66 W/m² in our model) reads about 11 — the same top-of-scale feel as a peak tropical UV Index. The category bands reuse the WHO UV Index colours and words, so a number you already understand carries straight over:

Because the index is strictly proportional to the irradiance, both numbers tell the same story — the index is just the friendlier way to read it at a glance.

Use both together

This isn't "UV Index bad, UVA Index good." They answer different questions and complement each other:

• UV Index → short-term sunburn risk (mostly UVB).
• UVA Index → long-wave UVA dose driving aging and deep, cumulative damage.

Checking both gives a fuller picture of what the sun is doing to your skin than either number alone.