UV radiation is a form of electromagnetic energy emitted by the sun that can cause damage to living organisms, ranging from sunburn to DNA mutations. To help people understand and mitigate this risk, the international community developed the standardized Ultraviolet Index (UVI). This index quantifies the strength of the sun’s skin-damaging UV rays at a specific place and time. The UVI was standardized by the World Health Organization (WHO) and the World Meteorological Organization (WMO) in 1994, providing a consistent global metric for sun safety.
How the UV Index Scale Works
The UV Index is structured as an open-ended linear scale that directly correlates with the intensity of sunburn-producing UV radiation. The scale begins at zero and typically extends upward to 11 and higher, with each full number representing a specific level of intensity and associated risk. This standardized scale utilizes a set of categories to communicate the potential danger to the public.
A UVI of 1 to 2 is considered “Low” exposure, while 3 to 5 is categorized as “Moderate” risk. The “High” category falls between 6 and 7, indicating a significant risk of harm from unprotected exposure. Levels of 8 to 10 are designated as “Very High,” and any reading of 11 or greater signals “Extreme” risk. Since the scale is linear, an index of 12 represents twice the intensity of an index of 6, meaning the potential for skin damage increases proportionally with the number.
The Highest Recorded UV Index Value
The highest UV Index value ever recorded on Earth was 43.3 UVI. This extreme reading occurred on December 29, 2003, at the Licancabur volcano, located in the Bolivian Andes. The measurement was taken by an international team of astrobiologists at an altitude of approximately 5,920 meters (19,423 feet). For context, a UVI of 11 is considered the maximum “Extreme” level in most forecasts, demonstrating how far this measurement exceeded typical high-risk conditions.
The exceptional measurement was attributed to a combination of atmospheric and geographical factors. This reading of 43.3 was well above the levels of 25 that are routinely measured in the high Andes region. The high altitude and the timing during the Southern Hemisphere summer contributed significantly to the intensity of this record-breaking event.
Environmental Drivers of Extreme UV
Altitude is one of the most significant environmental drivers of extreme UV levels. For every 1,000-foot increase in elevation, the UV intensity increases by approximately two percent because there is less atmosphere above to absorb and scatter the radiation. This explains why the highest readings are often found in mountainous regions, such as the Andes.
UV radiation is strongest near the equator where the sun’s rays strike the Earth’s surface most directly. The solar angle means the radiation travels through a shorter path of the atmosphere, reducing the natural filtering effect. Depletion of the stratospheric ozone layer, which acts as the planet’s primary UV filter, can also lead to localized spikes in radiation reaching the ground.
Reflective surfaces also play a major role in amplifying exposure, particularly in high-altitude environments. Fresh snow, for example, can reflect between 50 and 88 percent of the UV radiation, creating a double dose of exposure. Even factors like cloud cover can be misleading, as thin or broken clouds can sometimes enhance UV exposure by scattering the rays toward the surface.
Protecting Yourself from High UV Exposure
When the UV Index reaches “Very High” (8-10) or “Extreme” (11+) levels, comprehensive protection is necessary to prevent skin and eye damage. The first line of defense is to seek shade, particularly during the peak sun hours, which typically fall between 10 a.m. and 4 p.m. This midday period is when the sun is highest in the sky and UV intensity is at its maximum.
Physical barriers offer reliable protection. Key protective measures include:
- Wearing long-sleeved shirts and pants made of tightly woven fabric.
- Using a broad-brimmed hat to shield the face, neck, and ears.
- Applying a broad-spectrum, water-resistant sunscreen (SPF 30+) generously to exposed skin 15 minutes before going outdoors.
- Reapplying sunscreen every two hours, or immediately after swimming or sweating.
- Wearing UV-blocking sunglasses to protect the eyes from damaging radiation, which can contribute to conditions like cataracts.