The Ultraviolet Index (UVI) is an international standard measurement that quantifies the strength of the sun’s ultraviolet (UV) radiation reaching the Earth’s surface. Developed by the World Health Organization, the UVI provides a simple, linear scale to estimate the potential for skin and eye damage from solar exposure. The index is a forecast tool designed to help the public understand the daily risk and determine the appropriate level of sun protection. It is calculated by measuring the intensity of sunburn-producing UV radiation and weighting it according to its effect on human skin.
Understanding the UV Index Scale
The UV Index is classified into five distinct risk categories, which are directly related to the numerical value of the index. A value of 0 to 2 is considered Low, while a Moderate risk category covers a UVI of 3 to 5. The risk of harm becomes substantial at High levels (6 to 7) and at Very High levels (8 to 10). The scale is technically open-ended, but 11 and above represent the Extreme risk category, which is the highest level of conventional reporting. As the UVI number increases, the intensity of the UV radiation rises proportionally, meaning the time it takes for skin damage to occur decreases rapidly.
Record High UV Readings
While the standard UVI scale often caps at 11+ for daily reporting, the actual intensity of UV radiation can soar far higher under specific conditions. The highest UV Index ever recorded was a reading of 43.3, detected on December 29, 2003, at the summit of the Licancabur volcano in Bolivia. This extreme measurement, taken at an altitude of over 19,000 feet, was the result of a “perfect storm” of high-altitude intensity and an unusual depletion of the ozone layer. Regions in the high Andes, particularly in Peru, Bolivia, and Argentina, routinely experience UV Index levels that exceed 20, far surpassing the typical Extreme category. These locations are considered hotspots due to their combination of high elevation and proximity to the equator.
Influences on UV Intensity
Several physical and environmental factors combine to determine the final UV Index number at any given location. Altitude plays a role because the air is thinner at higher elevations, leading to less atmospheric filtering and an approximate 10% to 12% increase in UV intensity for every 1,000 meters of ascent. The angle of the sun, dictated by the time of day and season, causes UV radiation to peak during the four-hour period around solar noon. Geographic latitude heavily influences intensity, and variations in the ozone layer thickness also impact the UVI, since ozone acts as the Earth’s natural shield. Cloud cover is not always protective; while thick clouds block most UV, thin or broken clouds can scatter and even slightly increase the radiation reaching the surface.
Protecting Against Extreme UV Exposure
When the UV Index is rated Very High (8–10) or Extreme (11+), harm to unprotected skin and eyes can occur within minutes. The most effective strategy is to avoid the sun entirely during peak exposure hours, typically between 10 a.m. and 4 p.m., or to seek dense shade. If remaining outdoors is necessary, physical barriers offer the most reliable defense. Protective measures include wearing tightly woven, long-sleeved shirts and pants, ideally with an Ultraviolet Protection Factor (UPF) rating, and a wide-brimmed hat. Broad-spectrum sunscreen with an SPF of 30 or higher should be applied liberally to all exposed skin and reapplied every two hours, or more often if sweating or swimming. Eye protection is also mandatory, requiring sunglasses that block 99% to 100% of both UVA and UVB rays.