Is my MED fixed for life?

No. A tan raises skin's UV tolerance somewhat (at the cost of the damage that created it), some medications lower it dramatically, and children's skin generally burns more easily. Treat any MED-based number as an estimate with a healthy margin, not a personal limit to spend.

How does Sunwise know my MED?

It doesn't measure it clinically, nothing outside a lab does. Sunwise estimates your Fitzpatrick type from a photo using the ITA° colour angle, then applies the typical MED for that type. It's an educational estimate, always presented with that caveat.

Does sunscreen change my MED?

Sunscreen doesn't change your MED, it slows how fast you accumulate dose. Properly applied SPF 50 means roughly one-fiftieth of the erythemal UV reaches your skin, stretching the time to reach your MED accordingly. See how long SPF 50 really lasts.

Minimal erythema dose: the science behind burn times

Every 'how long before I burn' estimate, including Sunwise's, rests on one number from dermatology research: the minimal erythema dose, or MED. It's the smallest dose of ultraviolet radiation that produces visible reddening of the skin a few hours after exposure. Understand MED and the whole burn-time calculation stops being a black box.

What exactly is a minimal erythema dose?

Erythema is the medical term for the redness of sunburn, extra blood flowing to skin that UV has damaged. In phototherapy and sun-protection research, a person's MED is measured by exposing small patches of skin to increasing UV doses and checking, about 8 to 24 hours later, which patch was the first to turn visibly pink. That dose, measured in joules of erythemally weighted UV per square metre (J/m²), is their MED.

MED varies enormously between people, and the biggest factor is pigmentation. Melanin absorbs and scatters UV before it reaches the DNA of living skin cells, so deeper skin tones need several times more UV than very fair skin before they redden.

Typical MED by Fitzpatrick skin type

Research literature reports approximate MED ranges for each of the six Fitzpatrick skin types. Sunwise uses these mid-range values in its estimates:

Approximate literature values; individual MEDs vary widely even within a type.
Fitzpatrick type	Typical MED (J/m²)	Relative to type I
I: Very fair	≈ 200	1×
II: Fair	≈ 250	1.25×
III: Light to medium	≈ 350	1.75×
IV: Olive to tan	≈ 450	2.25×
V: Brown	≈ 600	3×
VI: Deep brown to black	≈ 1000	5×

From MED to minutes: the burn-time calculation

The UV index is a linear scale of erythemal irradiance, the burning power of sunlight, already weighted for how skin responds to different UV wavelengths. One UV index unit equals 0.025 watts of erythemal UV per square metre. Multiply out the units and skin in full sun receives 1.5 × UV index joules per square metre every minute.

That gives the estimate in one line: minutes to burn ≈ MED ÷ (1.5 × UV index). A fair-skinned person (MED ≈ 250 J/m²) at UV 8 receives 12 J/m² per minute, so they reach their burn threshold in roughly 21 minutes. The same person at UV 3 has about 56 minutes. Our burn-time chart runs this calculation for every skin type and UV level.

A related unit: the SED

Some research uses the standard erythema dose (SED), fixed at 100 J/m² regardless of skin type. You may see sunburn risk expressed as 'x SEDs', a very fair person's MED is about 2 SEDs, a deeply pigmented person's about 10. It's the same physics with a standardised yardstick.

What the simple formula leaves out

Reflection. Sand reflects roughly 15–25% of UV, water glint and snow far more (snow up to ~90%), all of it adds to the dose in the forecast number.
Changing UV. The index moves through the day. Sunwise integrates the day's full UV curve rather than assuming the current reading holds; a low-but-rising morning still burns you once the sun climbs.
Individual variation. Medication (some antibiotics, retinoids), recent exposure, and where skin sits within its type all shift a personal MED. Published values are population estimates.
Damage below the threshold. Redness is a late signal. DNA damage begins well before the MED is reached, staying just under your burn time is not the same as avoiding harm.

Frequently asked questions

Is my MED fixed for life?: No. A tan raises skin's UV tolerance somewhat (at the cost of the damage that created it), some medications lower it dramatically, and children's skin generally burns more easily. Treat any MED-based number as an estimate with a healthy margin, not a personal limit to spend.
How does Sunwise know my MED?: It doesn't measure it clinically, nothing outside a lab does. Sunwise estimates your Fitzpatrick type from a photo using the ITA° colour angle, then applies the typical MED for that type. It's an educational estimate, always presented with that caveat.
Does sunscreen change my MED?: Sunscreen doesn't change your MED, it slows how fast you accumulate dose. Properly applied SPF 50 means roughly one-fiftieth of the erythemal UV reaches your skin, stretching the time to reach your MED accordingly. See how long SPF 50 really lasts.

Keep reading

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