Chapter 6
Understanding MED and MMD

· Minimal Erythemal Dose
· Minimal Melanogenic Dose

Two terms commonly used in the indoor tanning industry is Minimal Erythemal Dose (MED) and Minimal Melanogenic Dose (MMD). Both terms seem to be self-explanatory, however, the true definition of each term is necessary for a clear understanding of the science of tanning.

Understanding MED and MMD
MED is the Minimal Erythemal Dose and is defined as the threshold dose that may produce sunburn. MMD on the other hand is the Minimal Melanogenic Dose and is equal to the lowest dose required to develop a visible suntan.

Even though the terms MED and MMD seem to be self-explanatory at first glance, the translation of these values in the daily practice of indoor tanning often leads to misunderstandings and wrong interpretations, especially when it comes to determining exposure times based on MED and MMD values.

How Threshold Dosages are Determined
Assume that unprotected skin has been exposed to UV radiation for the first time. In order to determine the MED, the reaction of the skin will be recorded 24 hours after exposure. The minimal dose that induces any visible reddening at that point is defined as one MED.

Redness that occurs immediately after exposure, however, and disappears during the following three to five hours is mainly caused by heat and is not comparable with real UV erythema. This is the reason why the reading is not taken until 24 hours later.

For users of tanning units, the MED provides important information about the sunburning effect of the equipment, since an even perceptible reddening is the first sign of a sunburn reaction. In order to prevent possible acute or long term risks due to indoor tanning, the MED should not be exceeded during a session.

The MMD is determined in a very similar manner. In contrast to the MED examination, however, the readings are taken seven days after exposure instead of 24 hours. The minimal dose required to produce an even noticeable tan, which can be observed seven days later, is defined as one MMD. The interval between exposure and reading is necessary to permit the occurrence of new melanin biosynthesis (melano-genesis), which only becomes evident after several days of UV application.

Why Standard Values?
To better understand MED and MMD, it should be said that both are individual values. The lowest effective dose developing a sunburn as well as the value of producing a suntan depend distinctly on the skin sensitivity of the person (i.e. skin types).

In order to eliminate these individually influencing factors, MED and MMD have been standardized. With standardized MED and MMD values, sunlamp products can be characterized and specified and become comparable with respect to their biological capabilities.

Such information based on these standard values is of greater meaning than statements about the physical data such as UVB/UVA ratios or UVB percentages.

Standard MED and MMD Compared
By comparing MED and MMD values of tanning units, it may be surprising that the required exposure time for reaching one MMD is usually longer than the corresponding time for one MED. This seems to indicate that it is impossible to tan without first developing red skin. At the same time, the question comes up: How can we achieve tanning slowly, progressively, and safely without producing a sunburn?

To shed some light on this question, consider the following: As mentioned above, MED and MMD are standardized values and valid for unprotected and untanned Skin Type II. This means that such given values are basically only valid before undergoing the first exposure.

The effect on the skin of a melanogentic dosage will become evident only three to five days after exposure, at the earliest. Further, Melanogenesis is a long lasting process, therefore single doses work cumulatively. In other words, the skin does not forget the induced pigment effects and accumulates these single pigment-producing dosages over time.

Besides Melanogenesis there exists the ‘IPD’, an immediate pigment darkening effect which is a rather superficially effective tanning mechanism. IPD is a transient reaction induced by the photochemical oxidation of preformed melanin pigments by long-wave UV, darkening the skin during exposure.

By the use of a sunbed, for example, which is characterized by an exposure time of 20 minutes for one MED and 45 minutes for one MMD. Melanogenesis can be induced in two different and, at least theoretically, conceivable ways.

MED/MMD-Based Schedule
Consider the given MMD exposure time of 45 minutes. Although the applied melanogenetic dose is high enough to produce new pigments, an exposure of this duration cannot be recommended because the MED would be exceeded more than twice during such a session.

It is better to get a suntan by starting an exposure schedule consisting of three applications of 15 minutes each during one week. The advantage of such a procedure is twofold. The applied dose per session does not reach the limit of one MED, however, at the same time the skin has received a total melanogenetic dose of one MMD. This means that the process of new pigment formation will be induced without the risk of sunburn. Furthermore, at the beginning of each session series, this exposure schedule is in agreement with FDA regulations. In additions to Melanogenesis, even during the first exposure session the skin will be tanned immediately if the horny layer contains some weakly colored, preformed pigments which then can be darkened by IPD. Generally, human skin has some pigment pre-stages available (except Skin Type I). In this context, it may be helpful to know that with most of the commonly used sunlamps, the threshold dose to initiate IPD will be reached quicker than 1 MED.

Depending on the amount of available pigment (and skin types), the effect of IPD usually remains only for hours, at the most a few days. With an increasing number of sessions, the amount of pre-stage pigment will be enhanced.

Talking about indoor tanning as well as outdoor tanning, the mechanisms of ‘immediate pigment darkening’ and of ‘pigment formation’ (Melanogenesis) interact so that a clear differentiation between them is often impossible. As a rule, it can be established that IPD is more important during the first sessions while Melanogenesis comes more and more into play during the following exposures.

By using suntanning units, both mechanisms are utilized. At the beginning, the tanning results are mainly caused by IPD. With increasing sessions, the obtained suntan becomes darker and deeper due to further melanin synthesis. Further, with well-tanned skin, the required exposure time to develop erythema will be prolonged, and thus offers an effective sun protection.