How Long Does It Take to Burn? MED and UV Index by Skin Type
Your personal 'burn window' isn't a guess, it's set by your skin's minimal erythema dose and how high the UV Index climbs.
The Minimal Erythema Dose (MED) is the smallest UV dose that produces visible redness 24 hours later; fair skin has a low MED, so as the UV Index rises, burn time drops sharply, falling to minutes rather than hours at high UV Index values.
Clinical references describe the MED as a person-specific threshold, explaining that fair-skinned individuals reach it with far less UV than darker skin types, which is why the same midday sun burns some people in minutes and others much more slowly.
Questions we get a lot
What is the minimal erythema dose (MED)?
The MED is the smallest dose of UV radiation that produces just-visible skin redness, typically assessed about 24 hours after exposure. It's a personal threshold that varies with skin type. Fair skin has a low MED, meaning it reaches a burn with relatively little UV.
Why does fair skin burn faster?
Fair skin has less protective melanin and a lower MED, so it absorbs a burning dose of UV more quickly than darker skin. As the UV Index climbs, the time to reach that dose shrinks sharply, which is why fair skin can burn in minutes at high UV levels. Darker skin types have a higher MED but are not immune to UV damage.
Can I calculate exactly how long I can stay out?
There's no perfectly precise countdown, because burn time depends on your MED, the UV Index, altitude, reflection, and how much sunscreen you've applied. The takeaway from the science is directional: the higher the UV Index and the fairer your skin, the shorter your safe window, often minutes at extreme UV. Use sunscreen, shade, and clothing rather than trying to time the exact limit.
r/SkincareAddiction: 'My friend and I were outside the same amount of time and she burned but I didn't. Why does burn time vary so much between people?'