Do you still need protection at UV 4?

The published schemes say yes at any reading of 3 or above: the WHO guide states that “above the threshold value of 3, protection is necessary”, and the EPA publishes “Protection needed” across its whole 3-to-7 range. Neither lowers the recommendation between 3 and 7.

WHO category: Moderate (3 to 5)

How long does it take to burn in UV 4?

At UV index 4, the erythemal-dose model computes a first-perceptible-reddening reference dose after roughly 33 minutes of continuous unprotected exposure for Fitzpatrick skin type I, and roughly 2 hours 47 minutes for type VI. The full type I–VI table is below, with the arithmetic and the sources.

Computed from a dose rate of 3.6 standard erythema doses per hour at UV index 4. Reference values, not measurements of any individual.
Fitzpatrick skin type	Reference dose to first erythema (SED)	Minutes to that dose at UV 4	0.6 × that figure (min)
Type Ialways burns, never tans	2	33	20
Type IIusually burns, tans minimally	2.5	42	25
Type IIIsometimes mild burn, tans uniformly	3.5	58	35
Type IVburns minimally, always tans well	4.5	75	45
Type Vvery rarely burns, tans easily	6	100	60
Type VIdeeply pigmented, never burns	10	167	100

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 4 question. The app answers yours.

Real days do not sit at 4 — 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 4 means

Four sits in the middle of WHO’s Moderate band. It is the least remarked-on part of the scale and the part where the index moves fastest: on a clear mid-latitude day the reading climbs through 4 rather than settling on it, because solar elevation — the dominant factor — is changing quickly at that time of the morning and afternoon.

Because the index is linear in erythemal irradiance, the arithmetic is exact rather than approximate: a reading of 4 accumulates erythemal dose one third faster than a reading of 3, so every threshold in the table below is exactly three quarters of the corresponding UV 3 figure.

Where and when UV 4 actually occurs

Four belongs to a sun that is climbing or falling rather than one at its peak, which is why it is more often a time of day than a place. Readings of 4 typically come from:

Clear-sky midday at mid-latitudes in mid-spring and early autumn.
Roughly two to three hours either side of solar noon on a mid-latitude summer day.
Midsummer midday at high latitudes, roughly 50–60°, where the sun never gets high enough to push the index much further.
A location that would read 3 at sea level, lifted by roughly 3000 metres of altitude at WHO’s published 10–12% per 1000 metres.

Four is a transit level in temperate latitudes. WHO notes that about 60% of a day’s UV is received between 10 a.m. and 2 p.m.; a reading of 4 in summer typically falls outside that window, on the climb in or the fall out of it.

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

What the published guidance says at UV 4

The US EPA groups 4 in the range it labels “Moderate to High” and publishes for it: “Protection needed. Seek shade during late morning through mid-afternoon. When outside, generously apply broad-spectrum SPF-15 or higher sunscreen on exposed skin, and wear protective clothing, a wide-brimmed hat, and sunglasses.”

WHO’s own note on cloud is relevant at this level, because a reading of 4 is often a cloudy-day reading: “UV radiation levels are highest under cloudless skies but even with cloud cover, UV radiation levels can be high”, and over 90% of UV can penetrate light cloud.

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 4, an hour of continuous exposure delivers 4 × 0.9 = 3.6 standard erythema doses. Fitzpatrick type II’s reference dose is 2.5 SED, so 2.5 ÷ 3.6 × 60 = 42 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 does it take to burn in UV 4?: At UV index 4 the model computes a first-erythema reference dose in about 33 minutes of continuous unprotected exposure for Fitzpatrick type I skin and about 167 minutes for type VI. These are computed model outputs for a reference dose, not durations of demonstrated safety.
Do you still need protection at UV 4?: The published schemes say yes at any reading of 3 or above: the WHO guide states that “above the threshold value of 3, protection is necessary”, and the EPA publishes “Protection needed” across its whole 3-to-7 range. Neither lowers the recommendation between 3 and 7.

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 →