No UV index is safe on its own. The index is a rate of sunburning UV; a burn is that rate multiplied by minutes, judged against your skin type. At UV 2, very fair skin reddens in about an hour. At UV 9, dark skin has about two hours. The 3+ protection rule is about minutes, not the number.

One unit of UV index is 25 milliwatts per square meter of erythemally weighted ultraviolet, meaning UV weighted by how well each wavelength reddens skin. That is a rate, like speed. A burn happens when the accumulated dose crosses your minimal erythemal dose, the MED, which is a quantity, like distance. Fair type I skin reddens at about 200 joules per square meter; dark type VI skin at about 1,560. Time to burn is the MED divided by the rate. Nothing about "safe" lives in the index alone.

What the 3+ threshold is for

The WHO scale, adopted in 2002, calls 1 to 2 low, 3 to 5 moderate, 6 to 7 high, 8 to 10 very high, and 11 and above extreme. The WHO and the US EPA both advise sun protection from 3 upward. That line is a public-health convenience: a single number that a fair-skinned person can use to decide whether an unplanned day outdoors will end in a burn.

It was never a line between safe and dangerous. Below 3, hours outside still add up. Above 3, twenty minutes on brown skin does not come close to the threshold. The band tells you how fast the clock runs. Your skin type and your watch tell you the rest.

Minutes to a burn, by skin type

The table gives the time to one MED, the first perceptible pinkness that shows 8 to 24 hours later, at a steady index with skin uncovered and unshaded. Fitzpatrick types run from I (very fair, always burns) to VI (dark brown or black, very rarely burns).

Skin type UV 3 UV 6 UV 9
I 44 min 22 min 15 min
II 67 min 33 min 22 min
III 93 min 47 min 31 min
IV 133 min 67 min 44 min
V 209 min 104 min 70 min
VI 347 min 173 min 116 min

These are model figures, and the direction of the errors matters. Snow reflects up to 80 percent of UV back at you, sea foam about 25 percent, dry sand about 15 percent, none of which the index includes. Elevation adds roughly 7 to 10 percent per 1,000 meters. A fresh tan raises your MED; skin that has been indoors since October sits at its annual minimum. And a burn is not a tally of DNA damage. It is the skin's adaptive stress response, triggered through damaged RNA (Vind 2024), when the dose arrives faster than the tissue can accommodate.

If you take a photosensitizing medication, a tetracycline antibiotic, a thiazide diuretic, isotretinoin, or St John's wort among others, or you have lupus or a photosensitive condition, the times above do not apply to you.

"It said UV of 1 or 2 and that I was safe for the day"

That reviewer was burned by the difference between a rate and a dose. UV 2 for a very fair person means about 67 minutes to one MED. Six hours at UV 2 is more than five MED. The app did not lie about the index; it answered "safe right now" to someone who heard "safe today". The same trap catches people on cool overcast days, where the warmth signal is gone and the UVB is mostly still there, and on the water, where reflection adds to what the sky sends down.

UV 3 is also the vitamin D floor

The same threshold works in both directions. UVB is the band that both reddens skin and converts 7-dehydrocholesterol into vitamin D3, and it only comes through the ozone layer in useful amounts once the sun stands roughly 30 degrees above the horizon. On a clear day at sea level, that is about where the index crosses 3. Below it there is little to burn you and little to make vitamin D. Above it, both begin.

So the useful question is not which index is safe but which dose, at which time, on your skin. A quarter to a half of your MED, taken when UVB is present, builds melanin and vitamin D without a burn. That is timing and modulation, not maximization.

The index says nothing about UVA

UVA, 315 to 400 nanometers, rises when the sun passes about 10 degrees of elevation, long before the index leaves 1. It darkens existing pigment within the hour, releases nitric oxide from stores in the skin, passes through window glass, and drives most photoageing. The erythema weighting counts it at about one thousandth of UVB, so an index of 1 at 8 AM can coincide with a large UVA dose. An index that reads "low" is not telling you the light is weak, only that it will not burn you quickly. What the number actually measures, and what we show instead, is in the questions below.

What to do

  • Learn your Fitzpatrick type.
  • Read the index as a clock. Take the burn time for your type at the current index and plan to stop at half of it.
  • Time it. Midday for UVB and vitamin D, with the shorter times in the table. Morning and late afternoon for UVA and circadian light, with almost no burn risk.
  • Shade, cloth and a hat after your dose.
  • On snow, water or above 1,500 meters, cut the table times by a third.

Sunlight is Life shows the burn time for your skin type next to the current UV.

Take the sunlight quiz for a plan matched to your skin and lifestyle →

The through line

The UV index was built to keep fair-skinned people from burning on unplanned afternoons, and for that job 3+ is a fine rule. For you, with a known skin type and a watch, it is a rate. Multiply it by minutes, compare it with your threshold, and the question answers itself: any index is safe for the right length of time, and none is for long enough.