Why skin burns
Ultraviolet B radiation damages the DNA of skin cells. Within a few hours, the skin reacts with redness, warmth and pain: a sunburn, or erythema. Each sunburn, especially in childhood, increases the long-term risk of skin cancer, including melanoma. Knowing how quickly you burn helps you plan protection.
The UV index
The UV index, developed by the World Health Organization and partners, measures the strength of sunburn-producing UV at a given place and time. Each unit equals 25 mW/m² of erythemally weighted irradiance. It depends on the sun’s height, cloud cover, altitude and reflection from snow, sand or water. It usually peaks around solar noon.
| UV index | Category |
|---|---|
| 0 to 2 | low |
| 3 to 5 | moderate |
| 6 to 7 | high |
| 8 to 10 | very high |
| 11 or more | extreme |
Skin types
The Fitzpatrick scale classifies skin by how it reacts to the sun. Each type has an approximate minimal erythema dose (MED), the UV dose that causes just-visible redness:
| Type | Description | Approximate MED |
|---|---|---|
| I | always burns, never tans | 200 J/m² |
| II | usually burns, tans with difficulty | 250 J/m² |
| III | sometimes burns, tans gradually | 350 J/m² |
| IV | rarely burns, tans easily | 450 J/m² |
| V | very rarely burns | 600 J/m² |
| VI | almost never burns | 1,000 J/m² |
The calculation
Time to burn (minutes) = MED ÷ (UV index × 0.025 W/m²) ÷ 60
The calculator also shows the dose per hour in standard erythema doses (SED), where 1 SED = 100 J/m².
Example
At a UV index of 8, a person with skin type II, who usually burns, reddens after about 21 minutes of unprotected exposure. At the same UV index, skin type IV would take about 38 minutes.
Protecting yourself
- Check the daily UV index and plan outdoor activities for early morning or late afternoon.
- Seek shade between 10 a.m. and 4 p.m.
- Wear a wide-brimmed hat, UV-blocking sunglasses and clothing.
- Apply a broad-spectrum sunscreen of SPF 30 or more generously, about one ounce for the whole body, and reapply every two hours and after swimming or sweating.
- Be extra careful with children, at altitude and near snow or water.
Limits
These times are estimates. Individual sensitivity varies, and some medications, such as certain antibiotics and acne treatments, make skin more sensitive to UV.