Minimal Erythema Dose (MED) – Definition and Relevance
The Minimal Erythema Dose (MED) is the lowest dose of UV radiation that produces a visible reddening of the skin. It is a key measure of individual UV sensitivity.
Things worth knowing about "Minimal Erythema Dose"
The Minimal Erythema Dose (MED) is the lowest dose of UV radiation that produces a visible reddening of the skin. It is a key measure of individual UV sensitivity.
What is the Minimal Erythema Dose?
The Minimal Erythema Dose (MED) is defined as the lowest dose of ultraviolet (UV) radiation required to produce a just-visible, uniform reddening of the skin (erythema) when assessed 16 to 24 hours after exposure. It is a fundamental parameter in dermatology and photobiology used to quantify individual skin sensitivity to UV light.
The MED is expressed in millijoules per square centimetre (mJ/cm²) and varies significantly depending on skin type, body site, and the UV wavelength used (UVA or UVB).
Significance and Applications
Determining the Minimal Erythema Dose is relevant across several medical and scientific fields:
- Dermatology: Assessment of UV sensitivity in patients, particularly those with conditions such as psoriasis, vitiligo, or photodermatoses.
- Phototherapy: When treating skin conditions with UV light (e.g., narrowband UVB therapy), the individual MED serves as the reference starting dose to ensure safe and effective treatment.
- Sunscreen research: The MED forms the basis for calculating the Sun Protection Factor (SPF) of sunscreen products.
- Radiation protection: It serves as a benchmark for protecting against UV-induced skin damage in occupational and everyday settings.
Dependence on Skin Type
The MED is strongly influenced by skin type, which is classified according to the Fitzpatrick scale into six types (I to VI):
- Skin type I (very fair skin, always burns): very low MED, approximately 200–300 mJ/cm²
- Skin type II (fair skin, often burns): MED approximately 250–350 mJ/cm²
- Skin type III (medium skin, sometimes burns): MED approximately 300–500 mJ/cm²
- Skin type IV (olive skin, rarely burns): MED approximately 450–600 mJ/cm²
- Skin types V–VI (dark to very dark skin): considerably higher MED
Additional factors such as age, immune status, certain medications, and genetic background can also affect an individual's MED.
How is the Minimal Erythema Dose Determined?
The MED is clinically determined through a procedure known as phototesting or UV test irradiation. Incremental UV doses are applied to small, defined areas of skin. The lowest dose that produces a visible erythema after 16 to 24 hours is recorded as the MED.
In phototherapy, the individual MED is used to set the initial treatment dose, typically starting at 70–80% of the MED. This approach minimises the risk of burns while still achieving a therapeutic effect.
MED and Sun Protection
The Sun Protection Factor (SPF) of a sunscreen indicates the factor by which the time to sunburn is extended compared to unprotected skin. This calculation is based on the individual MED: a product with SPF 30 theoretically extends the time it takes to reach the MED by a factor of 30.
It should be noted that the SPF is determined under laboratory conditions using UVB radiation. In practice, the actual protection may be reduced by insufficient application or by sweating and water exposure.
Clinical Relevance and Risks
A single exposure exceeding the MED results in sunburn. Repeated UV exposures above the MED increase the long-term risk of:
- Premature skin ageing (photoaging)
- DNA damage in skin cells
- Development of skin cancer (e.g., basal cell carcinoma, squamous cell carcinoma, malignant melanoma)
Medical and public health organisations therefore recommend keeping daily UV exposure consistently below the personal MED and using appropriate sun protection during prolonged sun exposure.
References
- Menter JM, Hatch KL. Skin Aging and Sun Damage. Photochemistry and Photobiology. 2004;80(3):447–450.
- World Health Organization (WHO). Ultraviolet radiation and the INTERSUN Programme. www.who.int/uv (accessed 2023).
- Diffey BL. What is light? Photodermatology, Photoimmunology and Photomedicine. 2002;18(2):68–74.
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