How long does it take for a sunburn to show up?
A sunburn usually shows up 2 to 6 hours after sun exposure, is reddest between 12 and 24 hours, then fades in 3 to 7 days (DermNet, 2025). Yet the skin's DNA is damaged in less than a second: the redness is the skin's delayed response.
Schematic drawing, not to scale. Dotted curves are estimates or calculations; redness and prostaglandin E2 were measured up to 72 hours in 32 volunteers (Rhodes, 2009).
How long does it take for a sunburn to show up?
Two hours in the sun, seen in cross-section inside the skin, then the week that follows. Choose a situation, then tap a label on the drawing to find out what it is.
Sources: timing of redness and peeling (DermNet, 2025); redness, prostaglandins and white blood cells hour by hour in 32 volunteers (Rhodes et al., FASEB J, 2009); DNA welded by UVB (Schreier et al., Science, 2007; Young et al., J Invest Dermatol, 1998); tanning (Brenner and Hearing, Photochem Photobiol, 2008); amount of sunscreen (Faurschou and Wulf, Br J Dermatol, 2007; Bech-Thomsen and Wulf, 1992; Petersen and Wulf, 2014). Drawing not to scale; scene timings: estimates; UVB shares and minutes: calculations.
Keyboard: 1 to 3 situations, Space pause, L slow motion, R restart, ← → previous or next step.
Frequently asked questions about sunburn
How long does a sunburn last?
A mild sunburn usually lasts 3 to 7 days: the redness builds during the first 12 to 24 hours, then slowly fades (DermNet). The skin may peel between day 4 and day 7. A strong sunburn lasts longer: in volunteers exposed to 4 times the dose that just causes redness, the redness was still almost at its peak after 72 hours (Rhodes, 2009). According to the NHS, most sunburns get better within 7 days.
Does a tan protect against sunburn?
A little, but much less than people think: a tan from the sun protects about like an SPF 3 sunscreen (Brenner and Hearing, 2008). In one study, two weeks of small UV doses gave a protection of 1.4 to 2.3 depending on skin type and dose (Sheehan, 1998). A tan only appears 2 to 3 days after the sun, so too late for the same day. A tan from UVA, as in a tanning bed, is not considered protective (Brenner and Hearing, 2008).
How much sunscreen should you apply?
To get the SPF written on the tube, you need 2 mg of sunscreen per cm² of skin, the amount used for testing (Faurschou and Wulf, 2007). In real life, people often apply 2 to 5 times less, between 0.39 and 1 mg per cm² (Petersen and Wulf, 2014). And protection drops fast: with half the sunscreen, an SPF 30 protects about like an SPF 5 to 6 (calculation). Cancer Council counts about one teaspoon per arm, per leg, for the front, for the back and for the face with neck and ears, so 7 in all. A second coat within the hour also helps (Petersen and Wulf, 2014).
When should you see a doctor for a sunburn?
It is best to see a doctor quickly if the skin blisters or swells, for high fever, chills, extreme tiredness, dizziness, nausea, headache or cramps, and always for a baby or a child under 5 (NHS). These signs can also come from heatstroke. For a simple sunburn: a cool shower, after-sun lotion, drinking water, acetaminophen or ibuprofen if needed; no ice or petroleum jelly, and do not pop blisters (NHS).
Does a sunburn raise the risk of melanoma?
Yes, sunburns raise the risk of melanoma, and not only in childhood: an analysis of 51 study groups finds a higher risk at all ages (Dennis, 2008). In 108,916 American nurses followed for 20 years, 5 or more blistering sunburns between ages 15 and 20 went with a 1.8 times higher risk of melanoma (Wu, 2014). The good news: in an Australian trial in 1,621 adults, applying sunscreen regularly went with half as many melanomas, 11 vs. 22, a difference at the edge of chance (Green, 2011).
True or false?
Black people don't get sunburned.
False. Melanin protects, but only like an SPF of about 1.5 to 4: very dark skin can burn too, just more slowly (Brenner and Hearing, 2008).
You can see a sunburn right away.
False. DNA is damaged in a tiny fraction of a billionth of a second (Schreier, 2007), but the redness only shows up 2 to 6 hours later (Huang, 2025).
Putting on a second coat of sunscreen improves protection.
True. People often apply much less sunscreen than the tested amount; a second coat gets closer to the protection on the label (Petersen and Wulf, 2014).
This page as text
The same information as the charts, to read without animation: for each chart, the values at the main moments, then the notes with their level of evidence.
What happens in the skin, from hour 1 to day 7
DNA is hit right away, redness follows within hours, tanning in 2 to 3 days, then the skin peels. Redness and prostaglandin E2 were measured; the other curves are estimated. Note: the axis changes scale at 24 hours.
| Time after sun, UV index or amount of sunscreen | Damaged DNA (estimate) (dotted) | Redness (measured up to 72 h) (dotted) | Prostaglandin E2 (measured at 18 and 24 h) (dotted) | Tan (estimate) (dotted) | Peeling skin (estimate) (dotted) | What happens |
|---|---|---|---|---|---|---|
| sun | 100 | 0 | 0 | 0 | 0 | The end of sun exposure |
| 4 h | 90 | 44 | 15 | 0 | 0 | after the sun |
| 12 h | 75 | 83 | 32 | 1 | 0 | after the sun |
| 24 h | 55 | 100 | 100 | 5 | 0 | Day 1 after the sun |
| 3 d | 25 | 89 | 20 | 70 | 0 | Day 3 after the sun |
| 5 d | 14 | 45 | 5 | 95 | 80 | Day 5 after the sun |
| 7 d | 8 | 15 | 0 | 100 | 80 | Day 7 after the sun |
Key points
- DNA is hit right away (Up to 6 h, Shown in humans)
UVB rays weld two neighboring DNA letters together (two thymines), which bends it. The weld forms in about a picosecond, a thousandth of a billionth of a second. Source: Schreier et coll., « Thymine dimerization in DNA is an ultrafast photoreaction », Science, 2007 - The redness comes from DNA (Up to 12 h, Shown in humans)
The light that damages skin DNA the most, around 300 nm, is also the light that causes the most redness: DNA would be the starting point of sunburn. Source: Young et coll., « The similarity of action spectra for thymine dimers in human epidermis and erythema suggests that DNA is the chromophore for erythema », J Invest Dermatol, 1998 - Visible after 2 to 6 hours (From 1 h à 12 h, Shown in humans)
Redness usually shows up 2 to 6 hours after sun, is strongest between 12 and 24 hours, then fades in 3 to 7 days. Source: Huang, « Sunburn », DermNet, 2025 - Measured hour by hour (From 4 h to day 3, Shown in humans)
In 32 volunteers exposed to 4 times the dose that just causes redness, redness was clear at 4 h, almost at its peak at 18 h, highest at 24 h, and still strong at 72 h. Source: Rhodes et coll., « The sunburn response in human skin is characterized by sequential eicosanoid profiles that may mediate its early and late phases », FASEB J, 2009 - Messengers that open the vessels (From 12 h to day 3, Shown in humans)
Prostaglandins, such as PGE2, rise with the redness during the first 24 to 48 hours and widen the small blood vessels. White blood cells (neutrophils) peak around 24 h. Source: Rhodes et coll., « The sunburn response in human skin is characterized by sequential eicosanoid profiles that may mediate its early and late phases », FASEB J, 2009 - Cells that sacrifice themselves (From 12 h to day 4, Shown in humans)
Cells whose DNA is too damaged die cleanly (apoptosis): these are the "sunburn cells." Source: Huang, « Sunburn », DermNet, 2025 - Tanning comes after 2 to 3 days (From day 2 on, Shown in humans)
Melanocytes make more melanin. This tan protects little: about like an SPF 3 sunscreen. Source: Brenner et Hearing, « The protective role of melanin against UV damage in human skin », Photochem Photobiol, 2008 - The skin peels after 4 to 7 days (From day 4 on, Shown in humans)
The damaged top layer comes off. Most sunburns get better within 7 days. Source: Huang, « Sunburn », DermNet, 2025 - Curves partly estimated (Throughout, Hypothesis)
Redness measured at 4, 18, 24 and 72 h and PGE2 at 18 and 24 h, for a strong sunburn (4 times the dose that causes redness). The rest of the curves are estimated, as relative levels.
Minutes in the sun before redness, by UV index and skin type
A calculation, as on the vitamin D page: the dose that just causes redness, divided by the dose received each minute. At a UV index of 8, very fair skin turns red in about 17 minutes, olive skin in 38 minutes. These are rough figures.
| Time after sun, UV index or amount of sunscreen | Very fair skin (I) (dotted) | Fair skin (II) (dotted) | Fair to olive skin (III) (dotted) | Olive skin (IV) (dotted) | What happens |
|---|---|---|---|---|---|
| UV 3 | 44 | 56 | 78 | 100 | UV index 3: skin I 44 min, skin II 56 min, skin III 78 min, skin IV 100 min (calculation) |
| UV 4 | 33 | 42 | 58 | 75 | UV index 4: skin I 33 min, skin II 42 min, skin III 58 min, skin IV 75 min (calculation) |
| UV 6 | 22 | 28 | 39 | 50 | UV index 6: skin I 22 min, skin II 28 min, skin III 39 min, skin IV 50 min (calculation) |
| UV 8 | 17 | 21 | 29 | 38 | UV index 8: skin I 17 min, skin II 21 min, skin III 29 min, skin IV 38 min (calculation) |
| UV 10 | 13 | 17 | 23 | 30 | UV index 10: skin I 13 min, skin II 17 min, skin III 23 min, skin IV 30 min (calculation) |
| UV 12 | 11 | 14 | 19 | 25 | UV index 12: skin I 11 min, skin II 14 min, skin III 19 min, skin IV 25 min (calculation) |
Key points
- What is the UV index? (Whatever the UV index, Shown in humans)
It tells how strong the skin-burning sun is. From 3 to 7: seek shade at midday, wear a T-shirt, sunscreen and a hat. At 8 and up: avoid being outside around midday. Source: WHO, « Radiation: The ultraviolet (UV) index », questions and answers - A calculation, not a measurement (Whatever the UV index, Hypothesis)
Minutes = dose that just causes redness ÷ dose received per minute. A UV index of 1 delivers about 1.5 J/m² per minute. Same doses as the vitamin D page: 200, 250, 350 and 450 J/m² for skin types I to IV (to be checked). Source: Lehmann et coll., « Public health messages associated with low UV index values need reconsideration », Int J Environ Res Public Health, 2019 - Dark skin burns too (Whatever the UV index, Shown in humans)
Melanin protects about like an SPF 1.5 to 4 sunscreen. On Black or brown skin, redness is hard to see, but the sunburn is there. Source: Brenner et Hearing, « The protective role of melanin against UV damage in human skin », Photochem Photobiol, 2008 - Skin types V and VI (Whatever the UV index, Hypothesis)
With 600 and 1,000 J/m², the calculation gives about 50 and 83 minutes at a UV index of 8 (to be checked). - In real life, it varies (Whatever the UV index, Hypothesis)
Shade, water, sand, snow, altitude and body position change the dose received. The calculation only gives a rough figure.
How much UVB still gets through, by amount of sunscreen?
A sunscreen's SPF is measured with 2 mg per cm² of skin. People often apply much less, and protection drops faster than the amount. Curves calculated from a study done with an SPF 4 sunscreen.
| Time after sun, UV index or amount of sunscreen | SPF 15 (dotted) | SPF 30 (dotted) | SPF 50 (dotted) | What happens |
|---|---|---|---|---|
| 0 | 100 | 100 | 100 | mg of sunscreen per cm²: UVB still getting through, 100% with SPF 15, 100% with SPF 30, 100% with SPF 50 (calculation) |
| 0.5 | 51 | 43 | 38 | mg of sunscreen per cm²: UVB still getting through, 51% with SPF 15, 43% with SPF 30, 38% with SPF 50 (calculation) |
| 1 | 26 | 18 | 14 | mg of sunscreen per cm²: UVB still getting through, 26% with SPF 15, 18% with SPF 30, 14% with SPF 50 (calculation) |
| 1.5 | 13 | 8 | 5 | mg of sunscreen per cm²: UVB still getting through, 13% with SPF 15, 7.8% with SPF 30, 5.3% with SPF 50 (calculation) |
| 2 mg/cm² | 7 | 3 | 2 | 2 mg of sunscreen per cm²: UVB still getting through, 6.7% with SPF 15, 3.3% with SPF 30, 2% with SPF 50 (calculation) |
Key points
- The SPF is measured with a lot of sunscreen (Whatever the amount, Shown in humans)
The SPF on the tube is measured with 2 mg of sunscreen per cm² of skin. Source: Faurschou et Wulf, « The relation between sun protection factor and amount of suncreen applied in vivo », Br J Dermatol, 2007 - People apply 2 to 5 times less (From 0.4 to 1 mg per cm², Shown in humans)
In real life, people apply 0.39 to 1 mg per cm². On a beach, 42 people applied 0.5 mg per cm² on average. Source: Petersen et Wulf, « Application of sunscreen, theory and reality », Photodermatol Photoimmunol Photomed, 2014 - Half the sunscreen, far less protection (Whatever the amount, Shown in humans)
In 20 volunteers, protection followed a curve: with 1 mg per cm², the SPF dropped to its square root; with 0.5 mg, to its fourth root. Source: Faurschou et Wulf, « The relation between sun protection factor and amount of suncreen applied in vivo », Br J Dermatol, 2007 - For SPF 30, a calculation (Whatever the amount, Hypothesis)
2 mg per cm²: SPF 30, 3% of UVB gets through. 1 mg: SPF about 5.5, 18% gets through. 0.5 mg: SPF about 2.3, 43% gets through. Rule measured with SPF 4, applied here to 15, 30 and 50. - A second coat helps (Whatever the amount, Shown in humans)
Applying sunscreen before going out, then a second coat within the next hour, makes up for some of the missed spots. Source: Petersen et Wulf, « Application of sunscreen, theory and reality », Photodermatol Photoimmunol Photomed, 2014 - For the whole body (Whatever the amount, Shown in humans)
Cancer Council counts about one teaspoon per arm, per leg, for the front, for the back and for the face with neck and ears, about 35 mL in all (a bit over 1 fl oz). Source: Cancer Council Australia, « How to apply sunscreen » - Sunscreen and melanoma (Whatever the amount, Shown in humans)
In an Australian trial in 1,621 adults, those who applied sunscreen regularly had half as many melanomas (11 vs. 22), a difference at the edge of chance. Source: Green et coll., « Reduced melanoma after regular sunscreen use: randomized trial follow-up », J Clin Oncol, 2011
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