14 September 2008

SPF measures the length of time a product protects against erythema or skin reddening from UV B, compared to how long the skin takes to redden without protection.

Minimal erythermal dose (MED) is the least amount of UV radiation that will illicit redness of the skin. MED number varies from person to person.

Sunscreen Effectiveness in the UVA and UVB Regions

Ethylhexyl methoxycinnamate++
Octyl salicylate++
Zinc oxide+++
Titanium Dioxide+++

SPF effectiveness is determined by radiating the skin of human panelists with a solar ultraviolet stimulator. When choosing human subjects for SPF efficacy, the clinical labs must use the following inclusion and exclusion criteria.

  • Skin types I always burn easily, never tans
    Skin types II always burn easily, tans minimally
    Skin types III burns moderately, tans gradually
  • Male and female must be included
  • Ages are between 18 to 70 years
  • Persons must be in a good health
  • Exclusion: known allergy to sunscreens, UV light, and cosmetics. Any preexisting conditions that could interfere with the test. Pregnant or nursing women, chronic medication deoendence, etc.
  • The total number of panelists should be no more than 25 human subjects; at least 20 subjects must have valid data.
  • Two different SPF ddeterminations are made, called static and water resistant.
Static SPF Method (US Regulation)

One week prior to start of the study, at least 20 subjects are evaluated for their MED for unprotected skin using this method with an untreated site.

Day 1
  • The investigational drug and reference standard are applied to test sites on the back following a computer generated randomization scheme.
  • A single topical application (2mg/cm2) is applied to an area about 60 cm2 on the back of each subject. Total dose is 120 mg applied.
  • An untreated control is placed as well.
  • After 15 minutes, the treated sites are subdivided into seven area, all of which are exposed to increasing doses of UVR.
  • The treated sites are exposed to increasing doses of radiation based on their expected.
  • The untreated sites are exposed to increasing doses of UVR determine each subject’s inherent MED.
  • The dose selected are a geometric series represented by 1.25n where each exposure is 25% greater than the previous one.
  • These provide secondary confirmation values for the MED for unprotected skin that are used to calculate the SPF.
Day 2
  • All sites are evaluated for edema and erythema.
  • Erythema is graded on a five point scale of 0, 0.5,1 ,2, and 3.
  • The amount of erythema effective energy that produces a grade of 1 used as the MED.
  • A standard 8% homosalate sunscreen is included in the test, which is prepared according to the monograph and must have an SPF of 4 for the test to be valid.
Calculation and Lamp Characteristic

The SPF is a ratio of the UV dosing time to produce MED on sunscreen treated skin vs the dosing time needed to produce MED in untreated skin. The longer the UV exposure before producing erythema, the higher the SPF. Remember that SPF reflects protection against UVB predominantly.

The exposure dose is expressed in joules per square meter :

Dose (J/m2) = irradiance (W/m2) x time (sec)

MED is the minimal erythema dose of radiation required to produce a barely visible erythema on the site. The irradiance is a constant, and the radiation flow from the lamp is in watts per meter square. The dose of radiation is in joules per meter square. Time exposure is therefore the variable controlled and measured during the study.

To minimize variability caused by different light sources, regulatory agencies define specifications for solar simulators. US FDA requires solar stimulations to be filtered do that they provide a continuous emission spectrum from 190 to 320 nm, similar to sunlight at sea level with the sun at a zenith angle 100. The simulator has less than 1% of its total energy output contributed by nonsolar wavelengths shorter than 290 nm, and it has not more than %of its total energy output contributed by wavelength over 400 nm.

Water Resistant SPF Method

This SPF method is a variation of the static method that involves exposure of sunscreen treated skin to water soaking for a period, followed by determination of SPF as above.

Day 1
  • Sunscreen and a water resistant sunscreen control were applied to individual areas on the subject’s back.
  • A single topical application (2mg/cm2) was applied to an area about 60 cm2 on the back of each subject. Total dose is 120 mg applied.
  • After 15 minutes, the subject entered a whirlpool bath and remained in the circulating water for 20 min.
  • The subject than rested out of the water 20 min, returned to the whirlpool bath for 20 min, and then exited the bath for the conclusion of the water test, allowing the test sites to air dry.
  • The standard sunscreen control (8% homosalate) was then applied to a designated site on the upper portion of the subject’s back.
  • The site were divided into subsites, about 1 cm2 in area, each of which was exposed to increasing doses of UVR.
  • The dose selected are a geometric series represented by 1.25n where each exposure is 25% greater than the previous one.
  • The middle subsite received a dose expected to yield the SPF of the product. The exact series of exposures administered was previously determined by the MED of unprotected skin.
  • The MED for unprotected skin was repeated and used to calculate the SPF.
Day 2

The test sites were evaluated based on the erythema grading scale.


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