The UV Index

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3 the UV Index at which sun protection is recommended (WHO)

Read the daily curve as a clock: when protection starts, when ultraviolet peaks, and when it falls back below the line.

Settled science

Stasis Home Research Every claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated August 2026

The UV Index measures the strength of sunburn-producing ultraviolet radiation, which is a carcinogen and the most preventable cause of skin cancer. It is not a vitamin D readout. Read it as a band and a clock rather than a score: when it first reaches 3, when it tops out, and when it falls back under 3. Protection starts at 3, whatever the day feels like.

The UV Index: what is it, and where does it come from?

Ultraviolet is the one part of sunlight you cannot feel. Warmth and brightness are their own signals, and the wavelengths that burn skin arrive with neither attached, so a cool day and a low ultraviolet day are not the same day.

The UV Index exists to make that invisible input legible. It weights the current ultraviolet mix by the sunburn action spectrum and reports how strongly it can burn skin at a given place and time, which is the job it was built for and the only job it does well.

Read it as a band rather than a number. Low, moderate, high, very high and extreme are stable enough to plan a day around; the exact value is not, because different forecast services calculate meaningfully different numbers for the same address on the same day.

Then read the band as a clock. Three moments carry the day: when it first reaches 3, when it tops out, and when it falls back under 3. That schedule follows the sun's height, not how warm or bright the day feels, and it is what tells you which hours are the gentler ones.

The UV Index: why does it matter?

Ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer, and none of it registers as sensation while it is happening. That is the entire reason a forecast number earns a glance on the way out the door.

What the index buys you is not certainty. It is a schedule. At 1 or 2 the WHO band calls for no routine protection. From 3 up, protection begins, and the response scales with the band: shade, then covering skin, then sunscreen applied properly, and near the top of the scale, staying out of the open around the peak.

What the index cannot do is tell you about vitamin D. It was built on the sunburn action spectrum, and sunburn-weighted dose is a poor predictor of vitamin D synthesis, so a high reading is a warning and not a yield.

The narrow version holds up well, and it is genuinely useful: find the day's protection window, take your uncovered time outside it where you can, cover up inside it, and stop treating one app's reading as the verdict on the whole day.

The UV Index is defined on the sunburn action spectrum, and sunburn-weighted ultraviolet dose is a poor predictor of vitamin D synthesis.Young et al., 2021 (PNAS; a revised action spectrum for vitamin D synthesis by suberythemal UV radiation exposure in humans in vivo)
The WHO protection categories begin at a UV Index of 3 and rise through moderate, high, very high, and extreme bands.WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide
Correct sunscreen use prevents burning without preventing vitamin D synthesis, so there is no reason to skip sunscreen for vitamin D.Young et al., 2019 (British Journal of Dermatology; correct sunscreen use and vitamin D status)
Above roughly 42 degrees north, winter includes a stretch with no meaningful vitamin D synthesis regardless of the sky; below about 35 degrees there is largely no such stretch.Webb et al., 1988 (J Clin Endocrinol Metab; winter sunlight in Boston and Edmonton does not promote vitamin D3 synthesis in skin)
Ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer.NIH Office of Dietary Supplements, Vitamin D Fact Sheet for Health Professionals

The UV Index: What do the bands mean?

The WHO exposure categories for the UV Index, and the protection each one calls for.
Exposure category	UV Index range	Source
Low (UV Index)	1 to 2 No routine protection required; use ordinary care outdoors	WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide
Moderate (UV Index)	3 to 5 Protection required; seek shade, cover skin, and use sunscreen	WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide
High (UV Index)	6 to 7 Protection required; reduce exposed time and use shade, clothing, and sunscreen	WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide
Very high (UV Index)	8 to 10 Extra protection required; avoid exposed time near the peak	WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide
Extreme (UV Index)	11 or higher Extra protection required; avoid exposed time near the peak and seek shade	WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide

These are internationally agreed categories for sunburn risk, not a legal limit and not a health threshold your report grades you against. The index is a sunburn number, so a band tells you what protection to use, never how long to stay out.

What helps against The UV Index?

3 approaches below reduce The UV Index in a home, ordered from the most direct fix to the smallest change in habit. The most direct is Read the daily UV curve as a protection window. Each entry also names what it cannot do, because no single fix covers every exposure route.

Free and behavioral

Read the daily UV curve as a protection window
Use the first crossing of 3, the peak band, and the drop below 3 as the day's practical clock rather than reacting to one isolated number.
Forecast services disagree on the exact value. The band and the timing are the resolution that holds; the decimal is not.
Use shade and clothing during the protection window
Reduce exposed time when the UV Index is 3 or higher by moving into shade and covering skin.
Shade is part of protection, not a guarantee of zero ultraviolet exposure.
Use sunscreen correctly at 3 and above
Correct sunscreen use prevents burning without preventing vitamin D synthesis.
Sunscreen is one layer of protection, not permission to stay in the open. No sunscreen makes ultraviolet exposure harmless.

The UV Index is one of the few environmental numbers that already works, and the way people use it is where it goes wrong. It gets read as a score for the day, quoted to a decimal that three different services will not agree on, and then pressed into answering a question it was never built for.

We use it the way it was designed: a band, a start time, a peak, and an end time. That is a smaller claim than the apps make, it is the part that survives contact with reality, and it is enough to tell you which hours to take and which to cover up for.

Common questions about The UV Index

What is a high UV index, and what UV index is dangerous?

A high UV Index is 6 to 7, while 8 to 10 is very high and 11 or higher is extreme. Protection starts earlier, at 3, because ultraviolet radiation is carcinogenic and the most preventable cause of skin cancer. Treat the category as a protection band, not as a precise measure of personal harm.

Is the UV Index measuring the ultraviolet that is there, or the ultraviolet that burns?

The UV Index measures the ultraviolet that burns. Each wavelength is scored by how efficiently it burns skin, and the weighted total is reported as one number, so the index answers a sunburn question rather than counting raw ultraviolet. That is the useful weighting, because ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer. It is a different weighting from the one vitamin D synthesis follows.

What is the difference between the UV Index and UV-B?

UV-B is a band of wavelengths, roughly 290 to 320 nanometres, and the UV Index is a number built on top of the ultraviolet arriving at a place. One is part of the spectrum. The other is a score that weights the ultraviolet mix by how strongly it burns skin. So UV-B is the physical thing, the index is the sunburn readout, and neither is a vitamin D measure.

Does a UV Index of 3 mean the same thing in every country?

A UV Index of 3 means the same thing in every country. The scale comes from a guide issued jointly by the World Health Organization, the World Meteorological Organization, the UN Environment Programme and the International Commission on Non-Ionizing Radiation Protection, so the bands and the protection they call for are internationally agreed. What varies between forecasts is the number behind the band, not the meaning of the band.

What time of day is UV strongest?

The UV Index is strongest around the daily peak in sun height, not necessarily when the day feels brightest or hottest. Read your forecast curve as a clock: note when it reaches 3, when it enters its highest band, and when it drops below 3. The exact clock time changes with place, season, and day.

Does a high UV index mean more vitamin D?

A high UV Index does not tell you how much vitamin D your skin will make. The index follows the sunburn action spectrum, and sunburn-weighted dose is a poor predictor of vitamin D synthesis. Correct sunscreen use prevents burning without preventing vitamin D synthesis, so there is no reason to skip sunscreen for vitamin D.

What UV index do you need sunscreen for?

Use sun protection when the UV Index reaches 3, the start of the WHO moderate band. Protection includes shade, clothing, and correctly used sunscreen. Ultraviolet radiation is carcinogenic, and correct sunscreen use prevents burning without preventing vitamin D synthesis, so vitamin D is not a reason to skip it.

Why do different apps show different UV numbers?

Different apps can show different UV numbers because their forecast sources and calculations differ. That disagreement is why the UV Index is best read as a WHO band and a daily curve, not as a decimal. Compare when each forecast crosses 3, reaches its peak band, and falls below 3 rather than treating one exact value as ground truth.