How much stronger is UV 9 than UV 3?

Exactly three times, in dose-rate terms. The index is linear in erythemal irradiance by definition, so a reading of 9 delivers erythemal dose three times as fast as a reading of 3 and reaches any given reference dose in a third of the time.

WHO category: Very high (8 to 10)

How long to burn in UV 9?

At UV index 9, the erythemal-dose model computes a first-perceptible-reddening reference dose after roughly 15 minutes of continuous unprotected exposure for Fitzpatrick skin type I, and roughly 74 minutes for type VI. The full type I–VI table is below, with the arithmetic and the sources.

Computed from a dose rate of 8.1 standard erythema doses per hour at UV index 9. Reference values, not measurements of any individual.
Fitzpatrick skin type	Reference dose to first erythema (SED)	Minutes to that dose at UV 9	0.6 × that figure (min)
Type Ialways burns, never tans	2	15	9
Type IIusually burns, tans minimally	2.5	19	11
Type IIIsometimes mild burn, tans uniformly	3.5	26	16
Type IVburns minimally, always tans well	4.5	33	20
Type Vvery rarely burns, tans easily	6	44	27
Type VIdeeply pigmented, never burns	10	74	44

The fourth column is a fixed 0.6 multiple of the third — an arithmetic margin for the roughly ±30–50% spread in published reference doses within one phototype. It is what the app reports.

“Reporting burn times is not recommended.”

— WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide, WHO/SDE/OEH/02.2 (2002)

The guide gives its reasoning directly: “people tend to interpret burn times to mean that there is a safe level of unprotected sun exposure”, and relating index values to a time to burn therefore “sends out the wrong message”.

Why we publish the figure anyway →

This page answers the UV 9 question. The app answers yours.

Real days do not sit at 9 — the index climbs to a peak and falls again. Safe Sun Window takes your skin type and the index where you actually are, and runs this same calculation on those inputs. Free, no account, no ads. Return visits are counted with a random first-party id that goes to no one else. If you turn on location, your device rounds your position to about 10 km before it reaches our weather provider for the UV reading, with no id attached.

Open Safe Sun Window →

What UV 9 means

Nine sits in the middle of WHO’s Very high band. It is another exact multiple: 9 is three times 3, so at UV 9 erythemal dose accumulates at precisely three times the UV 3 rate and every modelled threshold below is exactly a third of its UV 3 value.

Nine is also roughly where total column ozone starts to be a visible day-to-day influence rather than a background one. WHO lists ozone among the six factors that set the index and notes that ozone levels “vary over the year and even across the day” — which is why two clear days at the same place and season can report different maxima.

Where and when UV 9 actually occurs

Nine takes the same conditions that produce 8 with one of the six factors pushed further — usually latitude, altitude or ozone. Readings of 9 typically come from:

Clear-sky midday in high summer in the subtropics and in the drier, less cloudy parts of the southern mid-latitudes.
Tropical latitudes at solar noon across most of the year.
High-altitude sites in mid-latitude summer — at WHO’s published 10–12%-per-1000-metres gradient, roughly 1000 to 1250 metres is enough to turn a sea-level 8 into a 9.
Days with unusually low total column ozone, which shifts the whole day’s curve upward rather than changing its shape.

At 9, the day’s peak is narrow and the shoulders are still substantial: a location peaking at 9 will typically still be reading in the High band an hour or two either side of solar noon, so the elevated part of the day is longer than the peak reading alone suggests.

The six factors WHO lists as setting the index, with its own figures →

What the published guidance says at UV 9

The US EPA publishes for 8 and above: “Extra protection needed. Be careful outside, especially during late morning through mid-afternoon. If your shadow is shorter than you, seek shade and wear protective clothing, a wide-brimmed hat, and sunglasses, and generously apply a minimum of SPF-15, broad-spectrum sunscreen on exposed skin.”

WHO’s guide records a companion approach developed in Australia in 2000, which reports the hours of the day during which the index is above a threshold rather than the peak value alone — on one day the index may exceed 3 for half an hour, on another for several hours, and the peak number alone does not distinguish them.

The published scheme is binary, not graded: the same measures apply from UV 3 upward, and what changes with the index is the rate at which erythemal dose accumulates. Why the scheme is not graded →

Check the arithmetic

At UV 9, an hour of continuous exposure delivers 9 × 0.9 = 8.1 standard erythema doses. Fitzpatrick type II’s reference dose is 2.5 SED, so 2.5 ÷ 8.1 × 60 = 19 minutes. Every row in the table above is that division with a different reference dose.

The 0.9 factor is exact, and follows from the definition of the index; the reference doses are not. Where the 0.9 comes from, and what the reference doses are →

Questions

How long to burn in UV 9?: At UV index 9 the model computes a first-erythema reference dose in about 15 minutes of continuous unprotected exposure for Fitzpatrick type I skin and about 74 minutes for type VI. These are computed model outputs for a reference dose, not durations of demonstrated safety.
How much stronger is UV 9 than UV 3?: Exactly three times, in dose-rate terms. The index is linear in erythemal irradiance by definition, so a reading of 9 delivers erythemal dose three times as fast as a reading of 3 and reaches any given reference dose in a third of the time.

Other UV levels

All levels, the full method, and why burn times are contested →

Sources for the figures above

World Health Organization, World Meteorological Organization, United Nations Environment Programme and International Commission on Non-Ionizing Radiation Protection. “Global Solar UV Index: A Practical Guide.” WHO/SDE/OEH/02.2, 2002.
ISO 17166:1999 / CIE S 007/E-1998. Erythema reference action spectrum and standard erythema dose.
United States Environmental Protection Agency. “UV Index Scale.” epa.gov, last updated 15 June 2026; verified 3 August 2026.
Fitzpatrick TB. “The validity and practicality of sun-reactive skin types I through VI.” Archives of Dermatology, 1988;124(6):869–871.
Minimal-erythema-dose values per Fitzpatrick phototype — working reference values, range-checked against ISO 17166:1999; the six-value progression itself is not traced to a named primary document.

Full annotated source list, with what each one is used for →