Minimal erythemal dose

In practice, human skin without sunscreen is exposed to UV solar-simulated radiation until erythema is observed. This dose, called the minimal erythemal dose (MED), varies with skin color as well as from volunteer to volunteer of the same skin color. In practice, the same volunteers have had adjacent skin areas exposed, which are protected with the test sunscreen until erythema could be observed. The ratio of the protected to the unprotected dose is the SPF. 

The most obvious reaction of human skin towards an overexposure by UV light is the development of an erythema, which is an inflammation reaction of the skin well known as sunburn. Experiments with monochromatic radiation showed that the dose necessary for an erythemal response (1 minimal erythemal dose = 1 MED) depends strongly on the wavelength. The inverse of this dose as function of wavelength is called the erythemal action spectrum. It is used in a normalized form, such that the maximum value becomes unity. In 1987 McKinley and Diflfey reviewed a comprehensive amount of experimental data, and on this basis published an averaged erythemal action spectrum, which became a broadly accepted reference published by the CIE (International Comission on Illumination) and is shown in Fig. 2. ... 

Equation (3) refers to a static value of the SPF without consideration of transmission changes due to photoinstabilities in the course of the irradiation. These changes can be taken into account with equation (6), where tMED refers to the time, after which one minimal erythemal dose (1 MED) is transmitted. 

The SPE is defined as the ratio of the time required to produce a perceptible erythema on a site protected by a specified dose of the uv protectant product to the time required for minimal erythema development in the unprotected skin. An SPF of 8 indicates that the product allows a subject to expose the protected skin 8 times as long as the unprotected skin to produce the minimum erythemal response. The measurement can be quite subjective unless skin color and the history of reactions to sun exposure of the test subjects are taken into account. The MED range for Caucasians at 300 nm averages 34 mJ/cm. The range is 14—80 mJ/cm. Perspiration or the use of artificial irradiation devices can create additional problems. 

---