Guinness lists the highest UV index as 43.3—but the measurement is disputed

The highest UV index listed by Guinness World Records is 43.3, recorded on the Bolivian section of Licancabur volcano on 29 December 2003. That remains the entry on its public record page as of 24 October 2026, though Guinness cautions that record changes are not immediately published online. The measurement has also been challenged in the scientific literature, so “Guinness-listed record” is more precise than “undisputed maximum.” Guinness World Records

What was recorded?

Here, “highest UV” means the solar ultraviolet index at Earth’s surface, not total sunlight intensity or a laboratory UV source.

Nathalie Cabrol and colleagues reported the result in Frontiers in Environmental Science. They deployed two Eldonet dosimeters: one at Licancabur’s summit and another at nearby Laguna Blanca, at approximately 4,340 metres. The summit instrument produced the reported 43.3 peak; the lower site showed similar patterns with lower index values. These were instrument observations converted to UVI, not a weather forecast. Original study

Can the UV index go above 11?

Yes. The scale does not stop at 11. WHO describes the index as ranging from zero upward: higher values indicate greater potential for skin and eye damage and less time before harm occurs. A reading of 43.3 therefore does not contradict the UV-index scale. WHO’s UV-index explanation

The number measures UV radiation, not temperature, a percentage, or the probability of sunburn. Nor does this exceptional summit observation establish Bolivia’s typical daily UV level.

Why scientists questioned 43.3

In a 2015 commentary, McKenzie and colleagues argued that the reported extremes would require exceptionally unlikely combinations of low atmospheric ozone and cloud enhancement. They also identified inconsistencies between reported UV-B measurements, derived UVI values and model calculations, and noted that the original paper did not explain the UVI conversion method or quantify measurement uncertainties. They proposed instrument or data-analysis problems as an alternative explanation—not as a demonstrated cause. Scientific critique

The researchers defended their findings in a published response. They cited instrument calibration and similar patterns at the two sites, and argued that cloud scattering combined with localized ozone anomalies could explain the extremes. They also acknowledged that satellite ozone data were unavailable for the exact measurement time and location. Authors’ response

The distinction matters: 43.3 is a published, Guinness-listed record whose measurement validity was disputed. Recognition and scientific agreement are not the same thing.

What this means for a record-breaking forecast

A hypothetical prediction such as “Will the UV record be broken this year?” needs more than a threshold of 43.3. It must specify:

• The authority: a Guinness listing or an accepted scientific measurement?
• The metric: observed surface UVI, rather than a forecast or another radiation measure?
• The deadline: when the observation occurs, when it is published, or when it is recognized?
• The treatment of revisions: what happens if the measurement is later corrected or challenged?

Those rules define different outcomes. This is an example of why an operational definition should come before assigning a probability: forecasting a physical event is not necessarily forecasting its official recognition.