Minimal Erythema Dose (MED) is one of the most important concepts in dermatology and ultraviolet (UV) research. Yet outside scientific literature, it’s rarely explained properly.
If you’ve ever wondered how researchers measure sunburn, compare sunscreen performance, or determine safe UV exposure during phototherapy, the answer is usually MED.
In this guide, we’ll explain exactly what Minimal Erythema Dose means, how it’s measured, why it differs between individuals, and why it’s only one piece of the puzzle when thinking about healthy sun exposure.
What does MED stand for?
MED stands for Minimal Erythema Dose.
It is defined as:
The lowest dose of ultraviolet (UV) radiation that produces clearly visible redness (erythema) on the skin with well-defined borders, typically assessed 16–24 hours after exposure.
In simple terms, MED is your personal sunburn threshold.
Every person has a different MED depending on their skin type, genetics and recent sun exposure.
What is erythema?
Erythema is the medical term for redness of the skin caused by increased blood flow in the small blood vessels near the surface.
After excessive UV exposure, inflammation develops within the skin. This inflammatory response causes the familiar redness, warmth and tenderness we call sunburn.
The redness itself isn’t the damage — it’s your body’s response to UV-induced injury.
How is Minimal Erythema Dose measured?
In research and dermatology clinics, MED is determined using a controlled UV light source.
Several small areas of skin — usually on the back — are exposed to progressively higher UV doses. For example:
| Test area | UV dose |
|---|---|
| 1 | 20 mJ/cm² |
| 2 | 25 mJ/cm² |
| 3 | 30 mJ/cm² |
| 4 | 35 mJ/cm² |
| 5 | 40 mJ/cm² |
The skin is examined around 24 hours later.
The first area showing clear, uniform redness is considered that person’s Minimal Erythema Dose.
This approach is widely used in phototherapy, sunscreen testing and dermatology research.
Why does MED differ between people?
There is no universal MED. Several factors influence how much UV radiation your skin can tolerate before burning. These include:
- Fitzpatrick skin type
- Natural skin pigmentation
- Genetics
- Recent tanning or sun exposure
- Body location
- Age
- Certain medications
- The wavelength of UV used
Someone with very fair skin may reach their MED after a relatively small UV dose, while someone with darker skin may tolerate several times more UV before visible redness develops.
This is why researchers often describe exposures as a fraction or multiple of each participant’s MED rather than giving everyone the same UV dose.
MED is mostly a measure of UVB
Although both UVA and UVB reach the Earth’s surface, UVB is far more effective at causing erythema. That means MED primarily reflects your skin’s response to UVB exposure.
This distinction matters because:
- UVB is the major cause of sunburn.
- UVA penetrates deeper into the skin but produces relatively little immediate redness.
- Different biological effects of sunlight occur at different wavelengths.
A high UVA exposure may have very little effect on your MED despite delivering a substantial UV dose.
Does reaching your MED mean you’ve received the benefits of sunlight?
No. This is one of the biggest misconceptions surrounding UV exposure.
MED measures the threshold for visible sunburn, not the threshold for beneficial biological effects.
Many positive responses occur well below one MED, including:
- Vitamin D production
- Release of nitric oxide from the skin
- Changes in immune signalling
- Circadian light signalling
Researchers often use sub-erythemal doses (below MED) specifically because they can study these effects without intentionally causing sunburn.
Visible redness should not be viewed as a marker that you’ve received “enough” sunlight.
MED vs SED: what’s the difference?
You’ll often see another unit called Standard Erythema Dose (SED). Although they sound similar, they’re different.
| MED | SED |
|---|---|
| Individual-specific | Standardised physical unit |
| Depends on your skin | Same for everyone |
| Measures your sunburn threshold | Measures erythemally weighted UV received |
Think of it like this:
- SED tells you how much erythema-effective UV was delivered.
- MED tells you how your own skin responds to that dose.
Because everyone has a different MED, the same SED may cause sunburn in one person but not another.
Why dermatologists use MED
Minimal Erythema Dose testing has several important clinical applications. It is used to:
- Determine safe starting doses during UV phototherapy
- Investigate photosensitivity disorders
- Evaluate sunscreen performance
- Study skin responses to ultraviolet radiation
- Compare UV sensitivity between individuals
Because MED provides a personalised measure of UV sensitivity, it remains one of the most useful tools in dermatology research.
The bottom line
Minimal Erythema Dose is an important scientific measurement, but it should not be confused with an ideal amount of sunlight.
MED simply tells us the minimum UV dose required to produce visible sunburn in a particular individual. Many beneficial effects of sunlight occur below this threshold, while repeated exposure above it increases the risk of skin damage.
Understanding MED helps put common advice about sun exposure into context. Rather than treating sunlight as universally harmful or universally beneficial, it reminds us that the relationship between UV and health depends on dose, wavelength, timing and individual biology.
Frequently asked questions
What is a normal MED?
There is no single normal value. MED varies widely depending on skin type, pigmentation, genetics and recent sun exposure.
Is MED the same as sunburn?
Not exactly. MED is the minimum UV dose required to produce visible sunburn. More UV beyond your MED generally causes a more severe sunburn.
Is MED mainly caused by UVA or UVB?
MED is primarily determined by UVB radiation, which is much more effective at producing erythema than UVA.
Can you produce vitamin D before reaching your MED?
Yes. Vitamin D synthesis begins at UV doses well below the level required to produce visible redness.
Why do researchers use MED?
Using MED allows researchers to personalise UV exposure for each participant instead of exposing everyone to the same absolute dose. This produces fairer comparisons between individuals with different skin sensitivities.
