Standard Erythema Dose (SED) and Minimal Erythema Dose (MED)

Standard Erythema Dose (SED) and Minimal Erythema Dose (MED) do not measure the same thing. The SED is a standardised physical unit for erythemally effective UV radiation: 1 SED equals 100 J/m² of erythemally weighted exposure, and it applies regardless of who is being irradiated. The MED is not a fixed unit at all but an individual biological threshold – the smallest dose that turns one particular person's skin visibly red. One quantity describes the radiation, the other describes a human being's response to it. Only together do they tell you whether a given UV exposure is critical for a given person.

Keeping the two apart is not academic hair-splitting; it is the result of a standardisation decision. For years the literature used “MED” inconsistently – sometimes as a physical unit, sometimes as an individual measured value, depending on the study and the author. To remove that ambiguity, the International Commission on Illumination (CIE) recommended in 1997 that MED be reserved exclusively for the individual biological response, and introduced the Standard Erythema Dose as a separate, standardised physical unit (CIE, Standard Erythema Dose – a Review). Since then the two concepts can be separated cleanly and related to one another in a meaningful way.

The distinction matters wherever UV doses are compared or referred to people: in dermatological diagnostics and in medical phototherapy, in the testing of sunscreens, in tanning equipment, in the UV index published by weather services, and in the photobiological assessment of workplaces.

The SED is a unit of measurement for erythemally effective – that is, skin-reddening – UV radiation. The CIE introduced it in 1997 and it is standardised in ISO/CIE 17166 (ISO/CIE 17166:2019). It describes the radiation and nothing else.

• Definition: 1 SED corresponds to an erythemally effective radiant exposure of 100 J/m², equivalent to 10 mJ/cm². The weighting is applied with the standardised erythema action spectrum s_er(λ), which describes how effective each wavelength is at producing skin reddening – short-wavelength UV-B radiation counts orders of magnitude more heavily than UV-A.
• Independent of skin type: The SED says nothing about how a particular person responds. It quantifies the incident, biologically weighted amount of radiation – no more and no less.
• Purpose: It makes different UV sources comparable – sunlight, tanning equipment, therapy devices and test rigs – irrespective of the instrument or measurement method used (Cancer Council Australia, Sunburn).

In measurement terms this means an instrument only delivers trustworthy SED values if its spectral responsivity follows the erythema action spectrum, or if the spectrum of the source is measured and then convolved with s_er(λ) numerically. Where the responsivity of the sensor departs from the action spectrum, a systematic error follows – see spectral mismatch for UV sensors. The underlying quantities, irradiance and radiant exposure, are collected under radiometric quantities.

The MED is not a fixed quantity but a measure of how sensitive one individual is to UV radiation.

• Definition: The MED is the smallest UV dose that produces a just perceptible, sharply demarcated reddening of the skin in a particular person. It is normally read 16 to 24 hours after exposure, because the erythema develops with a delay (PMC/NIH, Minimal Erythema Dose Testing).
• Determined by phototesting: Small adjacent areas of skin are irradiated with a graded series of increasing UV doses. The lowest dose that produces a just recognisable reddening with clear borders on the following day is taken as 1 MED for that person.
• Dependent on skin type: The MED varies considerably with the Fitzpatrick skin type (I to VI), and in addition with age, prior tanning, medication and genetic disposition. The decisive factor is melanin, which absorbs UV radiation before it reaches the sensitive cell layers.

Because the MED is individual, it is no substitute for an exposure measurement: how much erythemally effective radiation a workplace delivers can only be measured, while the effect of that exposure on a particular person depends on their MED. How the two come together in day-to-day practice is described in UV erythema in the workplace.

The two quantities do not exclude one another, they complement one another: an individual MED can be expressed in SED. That makes it comparable independently of the instrument used – and a measured exposure given in SED can be set directly against that person's threshold.

The values below are approximations taken from the literature. The spread within a single skin type is considerable; for a reliable statement about an individual, phototesting remains necessary.

Worked example: A person with skin type II has an MED of roughly 250 J/m², that is 2.5 SED. If a UV source delivers 5 SED, that person has received twice their individual erythema threshold and will show a clearly visible reddening. A person with skin type VI and an MED of around 10 SED would still be below their threshold under exactly the same exposure. Physically identical radiation therefore produces entirely different biological outcomes – which is precisely why the radiometric quantity and the response threshold have to be named separately.

• Determining the sun protection factor (SPF): The MED of unprotected skin and the MED of the same skin with sunscreen applied are both determined; the ratio of the two gives the sun protection factor (CLINUVEL, Photoprotection and the significance of MED testing). The test method itself is laid down in ISO 24444.
• Finding the dose in phototherapy: In psoriasis, atopic dermatitis or vitiligo, the individual MED serves as the starting point for the initial dose, which is then increased step by step over the course of treatment. The technologies involved and the quantities behind them are described under medicine and phototherapy.
• Diagnosing photodermatoses: Phototesting with MED determination helps to identify and classify photosensitive conditions – polymorphic light eruption, for instance, or drug-induced photosensitisation.
• UV index and public communication: The UV index is defined as the erythemally effective irradiance in W/m² multiplied by 40 m²/W. At a UV index of 8 the erythemally effective irradiance is therefore 0.2 W/m², and 1 SED is reached after roughly 500 seconds – a little over eight minutes. A person with skin type II would be at their erythema threshold after about 21 minutes.
• Tanning and irradiation equipment: Erythemally effective output is stated in SED so that equipment from different manufacturers can be compared and exposure times limited on a traceable basis.
• Occupational safety: Exposure limit values for incoherent optical radiation do not work with the SED but with the effective radiant exposure H_eff and a weighting function S(λ) of their own; the daily limit of 30 J/m² goes back historically to the minimal erythema dose of sensitive skin types. The regulations that set it out are summarised in guidelines, norms and standards in UV.

Does the SED measure the same thing as the MED?
No. The SED is a fixed physical unit for erythemally effective radiation (1 SED = 100 J/m²); the MED is the individual dose at which a particular person's skin turns red. The SED measures the cause, the MED describes the sensitivity to the effect.

How do you convert an MED into SED?
Divide the MED value in J/m² by 100 J/m². An MED of 250 J/m² therefore corresponds to 2.5 SED. This assumes the MED value is erythemally weighted and not an unweighted radiant exposure.

Is 1 SED harmful?
1 SED lies below the erythema threshold of every skin type – the lowest typical MED values start at around 2 SED, so visible reddening is not to be expected at 1 SED. From an occupational and radiation-hygiene point of view that is not a free pass: the doses of a single day add up, and workplace exposure limit values are considerably lower.

How can erythemally effective exposure be measured?
With a radiometer whose sensor is filtered to the erythema action spectrum, or with a spectroradiometer that measures the spectrum and then convolves it with s_er(λ). The second route is the more robust one because it stays independent of the source spectrum; selection of UV sensors gives guidance on choosing an instrument.

The essential difference between SED and MED lies in their nature: the SED is a fixed physical measure of the radiation, while the MED describes the individual biological response of the skin to it. Treating the two as interchangeable means comparing values that are not comparable; keeping them cleanly apart allows a measured UV exposure in SED to be related directly to one person's threshold. It is this interplay that makes statements in dermatology, sun protection and UV safety defensible.

• CIE: Standard Erythema Dose, a Review – the reasoning behind separating SED from MED, cie.co.at.
• ISO/CIE 17166:2019, Erythema reference action spectrum and standard erythema dose – the normative definition of the action spectrum and the unit, iso.org.
• Cancer Council Australia: Sunburn – Standard Erythema Dose and Minimal Erythema Dose in context, cancer.org.au.
• PMC/NIH: Minimal Erythema Dose (MED) Testing – method and reading time of phototesting, pmc.ncbi.nlm.nih.gov.
• CLINUVEL: Photoprotection and the significance of Minimal Erythema Dose (MED) Testing – the role of the MED in determining the sun protection factor, clinuvel.com.

Whether a UV source really reaches the erythemally effective dose you assume it does only becomes clear once it is measured. We measure the spectrum, irradiance and dose of your source and advise on the right instrument – from the RMD Pro radiometer, available with an erythemally weighted sensor, through the flat UVpad spectroradiometer to the SR900 laboratory spectroradiometer for spectral evaluation. Traceability of the readings is provided by our accredited calibration laboratory. Send us your question.