What is the UV index, and who decides the number?
It is a forecast of how strong the sun’s ultraviolet will be at ground level. The World Health Organization built it with three other international bodies, so the number means the same thing everywhere. In the United States the National Weather Service calculates it and the Environmental Protection Agency publishes it. Evidence Evidence Evidence
Why this matters
The EPA calls it a forecast of the expected risk of overexposure. Its scale runs from 1 at the low end to 11 or more at the extremely high end. The calculation starts with satellite readings of ozone across the globe. It then adds the forecast cloud amount, the height of the ground, and the angle of the sun. That angle comes from latitude, the day of the year, and the time of day. The result is weighted for how much damage each wavelength does to skin, then divided by 25 and rounded.
Questions for your dermatologist
- Is there a UV level where you would want me to change what I do?Saving keeps this on your device and needs JavaScript, which is off in this browser.
- Does anything about my treatment change how I should read a forecast?Saving keeps this on your device and needs JavaScript, which is off in this browser.
Why is the same summer stronger in some countries than others?
Because the sun sits at a different angle. The WHO puts it plainly: the closer you are to equatorial regions, the higher the ultraviolet levels. Nothing about your skin or your habits has to change for your exposure to change. Evidence Evidence
Why this matters
Sun elevation is the main driver. The higher the sun sits, the higher the ultraviolet level. So it varies with the time of day and the time of year. Outside the tropics the WHO puts the highest levels at around midday in summer. The EPA uses the same three inputs for the sun’s angle. They are latitude, the day of the year, and the time of day. Ozone matters too, and the WHO notes it varies across the year and even across one day.
Questions for your dermatologist
- I moved from somewhere with very different sun. Does that change anything for me?Saving keeps this on your device and needs JavaScript, which is off in this browser.
- Should I be doing anything differently when I travel?Saving keeps this on your device and needs JavaScript, which is off in this browser.
Does altitude really make a difference?
Yes, and the two bodies we cite do not agree on how much. Thinner air absorbs less ultraviolet, so levels rise as you go up. The WHO puts the increase at 10 to 12 per cent for every 1000 metres. The EPA applies about 6 per cent per kilometre. Evidence Evidence
Why this matters
Both describe the same effect from the same cause. A kilometre and 1000 metres are the same distance. So the gap between the two figures is real, and not a units mismatch. The EPA number is what its published model applies when it works out a forecast. The WHO number sits in the international guide that the US scale is built to match.
Questions for your dermatologist
- I spend time at altitude. Is that worth factoring in for me?Saving keeps this on your device and needs JavaScript, which is off in this browser.
It is overcast, or it is winter. Am I fine?
Not necessarily. Being unable to burn on a cloudy day, and winter ultraviolet not mattering, are both on the WHO’s published list of common misconceptions. The EPA’s model treats clear skies as letting virtually all ultraviolet through. It puts scattered cloud at 89 per cent, broken cloud at 73, and overcast at 31. Evidence Evidence
Why this matters
The WHO says up to 80 per cent of solar ultraviolet can get through light cloud. It adds that haze can even increase exposure. Levels are highest under cloudless skies, but they can still be high under cloud. Scattering adds to the total the same way reflection from a surface does. On winter, the WHO says levels are generally lower, then adds that snow reflection can double overall exposure. It names early spring in particular, when temperatures are low and the sun is unexpectedly strong.
Questions for your dermatologist
- Do I need to think about this in winter, or only in summer?Saving keeps this on your device and needs JavaScript, which is off in this browser.
What about snow, sand and water?
They send ultraviolet back up at you. That reaches skin a hat brim or an awning would otherwise cover. The WHO publishes a reflectance figure for each surface. Evidence
Why this matters
These are the WHO’s figures, and they are why a snowfield and a beach behave differently from a park.
- Fresh snow can reflect as much as 80 per cent of ultraviolet.
- Sea foam reflects about 25 per cent.
- Dry beach sand reflects about 15 per cent.
Questions for your dermatologist
- Are there places or surfaces you would want me to be more careful around?Saving keeps this on your device and needs JavaScript, which is off in this browser.
How do I read this where I am, without an app?
Look at your shadow. The EPA publishes this as the Shadow Rule. If your shadow is taller than you, your ultraviolet exposure is likely to be lower. If it is shorter than you, you are getting more. Evidence Evidence Evidence
Why this matters
The timing behind it is published too. The WHO reports the day’s maximum over the four-hour period around solar noon. Solar noon itself falls between noon and 2pm, depending on where you are and on daylight saving. The forecast for a place is published rather than worked out at home. The National Weather Service calculates it for most US ZIP codes, and the EPA publishes it. The EPA may issue a UV Alert when a forecast is higher than normal. The WHO publishes an action table beside the index, and these are its words rather than ours.
- WHO, 0 to 2: it says you can safely enjoy being outside, and that no protective measures are normally needed.
- WHO, 3 to 7: it says to seek shade during midday hours, and to wear a shirt, sunscreen and a hat.
- WHO, 8 and above: it says to avoid being outside during midday hours, and calls shade, a shirt, sunscreen and a hat a must.
Questions for your dermatologist
- Would you want me to change anything on high-index days?Saving keeps this on your device and needs JavaScript, which is off in this browser.
- Is there a forecast you would rather I used?Saving keeps this on your device and needs JavaScript, which is off in this browser.
Evidence and update context
This optional layer shows the evidence boundary reviewed for this page as of 2026-08-21.
- What this evidence supports
- The EPA calls it a forecast of the expected risk of overexposure. Its scale runs from 1 at the low end to 11 or more at the extremely high end. The calculation starts with satellite readings of ozone across the globe. It then adds the forecast cloud amount, the height of the ground, and the angle of the sun. That angle comes from latitude, the day of the year, and the time of day. The result is weighted for how much damage each wavelength does to skin, then divided by 25 and rounded.
- What it does not establish
- It is a forecast for a place, not a measurement of your skin. Neither body measures skin that has lost its pigment. No source registered here pairs an index value with what happens to a depigmented patch. The index also describes sunlight. It says nothing about a prescribed course of narrowband UVB, where your clinician sets the light and the schedule.
Evidence behind this page
Sources
Each evidence badge opens the source and its limits. The full list stays available here.
- World Health Organization, with the World Meteorological Organization, the United Nations Environment Programme and the International Commission on Non-Ionizing Radiation ProtectionGuideline · Regulatory / guideline, tier 1Independent source
- Published
- SteadySkin last checked
What this source can and cannot tell you
What this source supports
Supports that the Global Solar UV Index is a measure of the ultraviolet level at the Earth’s surface. It names six things that change that level. The first is sun elevation. The higher the sun, the higher the level, so it varies with time of day and time of year. Outside the tropics the highest levels come at around solar noon in the summer months. The second is latitude, and the closer to equatorial regions the higher the levels. The third is cloud cover. Levels are highest under cloudless skies, but can still be high under cloud, because scattering can act like reflection from a surface. The fourth is altitude. A thinner atmosphere absorbs less, and levels rise by 10% to 12% for every 1000 metres. The fifth is ozone, which varies over the year and even across a day. The sixth is ground reflection. Fresh snow can reflect as much as 80% of ultraviolet, sea foam about 25%, and dry beach sand about 15%. It supports that the ultraviolet region covers 100 to 400 nm. It divides into UVA at 315 to 400 nm, UVB at 280 to 315 nm and UVC at 100 to 280 nm. All UVC and about 90% of UVB are absorbed by ozone, water vapour, oxygen and carbon dioxide. It supports that the maximum level falls in the four-hour period around solar noon. Solar noon falls between local noon and 2 p.m., depending on location and on daylight saving. It supports an international presentation in five categories. They are low at 1 and 2, moderate at 3 to 5, high at 6 and 7, very high at 8 to 10, and extreme at 11 and above. It supports that up to 80% of solar ultraviolet can penetrate light cloud cover. It supports that haze can even increase exposure. It supports that water offers only minimal protection, and that reflections from water can enhance exposure. It supports that levels are generally lower in winter. It supports that snow reflection can double overall exposure, especially at high altitude. It names early spring, when temperatures are low and the rays are unexpectedly strong. It carries a table of common misconceptions. That table includes a suntan being healthy, a tan protecting you, being unable to burn on a cloudy day, being unable to burn while in the water, and winter ultraviolet not being dangerous.
What it does not support
It is an educational and reporting guide for authorities and media, not a trial. It was published in 2002, and its reflectance and altitude figures are stated without a measurement range. It never mentions vitiligo. It measures nothing about skin where pigment has been lost, so it supports no statement about depigmented patches. It sets no exposure time for any person and predicts nothing for one. Its sun-protection categories are recommendations this site reports rather than issues. Nothing on it describes a prescribed course of phototherapy, and nothing on it is used here to weigh one.
Claim-specific review for this source is still in progress. Only the source-level evidence and limits are shown here.
- World Health OrganizationPatient education · Patient education, tier 5Independent source
- Published
- SteadySkin last checked
What this source can and cannot tell you
What this source supports
Supports that the index is a measure of the level of ultraviolet radiation, with values from zero upward. The higher the value, the greater the potential for damage to skin and eye, and the less time it takes for harm to occur. It supports that the index was developed by the World Health Organization with the United Nations Environment Programme, the World Meteorological Organization and the International Commission on Non-Ionizing Radiation Protection. It supports that reported values are the maximum daily level, which occurs during the four-hour period around solar noon. It supports that solar noon takes place between noon and 2 p.m., depending on geographical location and on daylight saving. It supports that some countries issue sun protection times when levels are forecast to be 3 or higher. It carries an action table. At 0 to 2 it says you can safely enjoy being outside. At 3 to 7 it says to seek shade during midday hours, and to slip on a shirt, slop on sunscreen and slap on a hat. At 8 and above it says to avoid being outside during midday hours, and that shade, a shirt, sunscreen and a hat are a must. It supports that below a value of 2 the risk is limited even for very sensitive fair-skinned people, and that no protective measures are normally needed. It supports that the index is reported with the weather forecast in many countries. This is the current restatement by the same body of the 2002 guide registered alongside it.
What it does not support
It is a public question-and-answer page, not a trial and not a measurement. It never mentions vitiligo and measures nothing about skin that has lost its pigment. Its action table is a recommendation from the World Health Organization, reported here in its own words and never issued by this site. It gives no exposure time for any person. It does not describe or address phototherapy prescribed by a clinician. It cannot say what any value will do to one person.
Claim-specific review for this source is still in progress. Only the source-level evidence and limits are shown here.
- U.S. Environmental Protection AgencyRegulatory · Regulatory / guideline, tier 1Independent source
- Published
- SteadySkin last checked
What this source can and cannot tell you
What this source supports
Supports that the UV Index scale used in the United States conforms with the international guidelines for reporting established by the World Health Organization. It supports three published bands with labels. They are 1 to 2 low, 3 to 7 moderate to high, and 8 and above very high to extreme. It reports what it recommends at each band. At low it says no protection is needed and you can safely stay outside using minimal sun protection. At moderate to high it says protection is needed, and to seek shade during late morning through mid-afternoon. At very high to extreme it says extra protection is needed and to be careful outside during those same hours. It supports the Shadow Rule as an easy way to tell how much ultraviolet exposure you are getting. If your shadow is taller than you are, in the early morning and late afternoon, exposure is likely to be lower. If your shadow is shorter than you are, around midday, you are being exposed to higher levels.
What it does not support
It is consumer guidance from a regulator, not a trial and not a measurement of any person. It never mentions vitiligo and measures nothing about skin where pigment has been lost. Its protection recommendations carry numbers, including an SPF floor, and each is reported here as the agency saying it rather than issued by this site. The Shadow Rule is offered as an easy indicator, not as a measurement of the index. It says nothing about clinician-directed phototherapy. It is a page that is revised in place, so it is registered as mutable and re-verified rather than pinned.
Claim-specific review for this source is still in progress. Only the source-level evidence and limits are shown here.
- U.S. Environmental Protection AgencyRegulatory · Regulatory / guideline, tier 1Independent source
- Published
- SteadySkin last checked
What this source can and cannot tell you
What this source supports
Supports that the UV Index is a forecast of the expected risk of overexposure to ultraviolet radiation from the sun. It supports that the National Weather Service calculates the forecast for most ZIP codes across the United States, and that the EPA publishes it. It supports a scale from 1 at low to 11 or more at extremely high. It supports that a UV Alert may be issued for an area when the index is forecast to be higher than normal. It supports how the number is produced. The calculation starts with measurements of total ozone over the entire globe, obtained from two satellites operated by the National Oceanic and Atmospheric Administration. A model uses that ozone forecast and the incident angle of sunlight to calculate ground-level strength. It supports that the sunlight angle is determined by latitude, day of year, and time of day. It supports that strength is calculated across 280 to 400 nm, covering UVB at 280 to 314 nm and UVA at 315 to 400 nm. It supports that the result is weighted for the sensitivity of human skin using the McKinlay-Diffey erythema action spectrum, because shorter wavelengths cause more damage at the same intensity. It supports that the total is then adjusted for elevation and cloud. Ultraviolet intensity increases about 6% per kilometre of elevation above sea level. Clear skies allow virtually 100% of ultraviolet through, scattered clouds transmit 89%, broken clouds transmit 73%, and overcast skies transmit 31%. It supports that the total is divided by 25 and rounded to the nearest whole number to give the index. It supports that ozone depletion and seasonal and weather variation change how much ultraviolet reaches the Earth.
What it does not support
It is a description of a national forecasting model, not a trial and not a measurement on any person. It never mentions vitiligo and measures nothing about skin where pigment has been lost. Its worked example uses hypothetical wavelength values, which it says plainly, so no figure in that example is a real reading. Its elevation figure of about 6% per kilometre is what the model applies, and it differs from the 10% to 12% per 1000 metres in the World Health Organization guide registered alongside it. This site publishes that difference rather than choosing between them. Its cloud transmission percentages are model inputs, not a reading of the sky above a reader. It says nothing about clinician-directed phototherapy. It is a page that is revised in place, so it is registered as mutable and re-verified rather than pinned.
Claim-specific review for this source is still in progress. Only the source-level evidence and limits are shown here.