SPF
Outline: Sun Protection Factor Assay (SPF)
Final Monograph CFR 64 No. 98, May 21, 1999
Objective: To determine the Sun Protection Factor (SPF) of sunscreens products for over-the-counter use.
Subjects: Not more than 25 healthy, fair-skinned male or female human subjects of skin types I, II and III (sensitive and normal) are used. A "screening" test of 5 subjects may be performed to determine whether or not the test article is delivering the anticipated level of sun protection.
Timing: Start test within 2 weeks of receipt of samples
Finish test within 2-3 weeks
Report issued within 2 weeks of study completion
Quantity: Approximately 4 oz. (125 mL or g)
Standard: The FDA Monograph recommended standard sunscreen control, 8% homosalate (HMS), with a mean SPF value of 4.47 + 1.279, is provided by the testing facility.
Light Source: A 150 watt Xenon Arc Solar Simulator, equipped with electronically-controlled shutter release and dosage control system (Solar Light Co., Phila, PA), having a continuous emission spectrum in the range of 290 to 400 nanometers.

Methodology:
Clinical assessment of dermal response by a trained evaluator

Procedures:
The FDA, Sunscreen Drug Products For Over-the-Counter Human Use: Final Monograph, as described in the Federal Register, Vol. 63, No. 98, May 21, 1999 pp 27666-27693.

MED Determination
Prior to SPF testing, each subject's sensitivity to UV-B on their unprotected skin is determined, ie, the Minimal Erythema Dose (MED).


Application and Test Sites
Test areas (10 cm x 5 cm) are delineated on the subject's back between the beltline and the scapulae and 100 mL of the test article are evenly applied at a density of 2 mL or mg per square centimeter. After allowing the test article to dry for 15 minutes, sub-test areas are irradiated with one of a geometric series of UV-B exposure doses. These exposure doses are based upon each subject's MED and the expected SPF of the test article.


Response Determination

Immediately after exposure, any skin responses are recorded.

Twenty-two to 24 hours after irradiation, all exposure sites are examined by an evaluator who is unaware of which formulation or UV radiation was administered to which site.

Evaluations are conducted under a warm white fluorescent light bulb providing illumination of 450-550 lux.


Determination of SPF

To obtain an SPF value, the erythema effective exposure (Joules/cm2) (MED) for protected skin is divided by the erythema effective exposure for unprotected skin:
 

MED (Protected Skin)

SPF = __________________
  MED (Unprotected Skin)

The mean SPF values and the standard deviation(s) are determined. The upper 5% point from the t distribution table with N-1 degrees of freedom is calculated (t). The ts is computed (A). The label SPF equals the largest whole number less than x - A.


Labeling Designations
FDA/OTC monograph-labeling designations are:
Mean SPF Value Recommended SPF Claim Sunscreen Product Category Designation
2 to 11 2 - 11 Minimal Sunburn Protection
12 to 30 12 -30 Moderate Sunburn Protection
30 or above 30 - (plus/+) High Sunburn Protection


Rejection of Data

If an MED response is not elicited either on the treated or unprotected skin sites, or a response is randomly absent, or the subject is non-compliant, test data will be rejected.