The most frequently tested parameters referring to the performance (efficacy) of sunscreens include the SPF value, water resistance, and broad-spectrum;

Evaluation of the Sun Protection Factor (SPF)
Determination of the SPF value is performed in vivo, on human subjects as it is required in the OTC monograph. All subjects for the SPF study are typically Fitzpatrick skin types I to III since they are the most susceptible to sunburn.

First, the MED is determined for each subject for unprotected skin. The lowest dose, which produces the first definite redness, is defined as the MED for unprotected skin. Generally, the back is used for this test as usually it does not have high prior sun exposure and is not hypersensitive. To stimulate the correct wavelength of the strength of the UVB radiation, typically a solar simulator is used, which emits light from 290 to 400 nm, similar to midday summer sunlight.

After determining the baseline MED, the sunscreen as well as the reference product are applied to the back in a dose of 2 mg per square centimeter. The test sites are outlined on the back of the subjects, each of which has 4 to 5 subsites. These subsites receive varying dosages of UV radiation. 

At specific points, evaluators assess the back looking for signs of redness, and evaluating it on the scale. The ratio between the baseline MED value and postexposure MED value provides the basis for calculating the SPF value of the product.

Interesting Facts
There were ethical issues of directly exposing the human skin directly to UV light, i.e., solar simulator. Therefore, alternative methods are being developed. One method uses a fiber optic liquid light guide that reflects UV light from the solar simulator, down the tube, and onto the skin site.

The SPF test uses human volunteers, it is sensitive to a certain degree of biological variability among the subjects due to factors such as skin type, skin condition (e.g., sweating), and evenness of product spreading. To rule out significant variations, the OTC monograph requires a test panel with not more than 25 subjects.  From this panel, at least 20 subjects must produce valid data for analysis.

Variability can also result from factors such as solar simulator lamp variation, environment (e.g., temperature, humidity), as well as consistency and accuracy of the MED assessment. Onsite use e.g., product application on the beach, maybe also influenced by factors such as application thickness by users, reapplication rate, presence of any photosensitizing ingredients, substantivity of the product, skin temperature, and wind, among others. Therefore, SPF numbers should be viewed as a guide to protection level and not as an absolute measure.

Water Resistance Testing of Sunscreens
Water resistance refers to the property of a sunscreen to maintain its degree of protection when exposed to water, such as sweating or swimming. Intensive sweating and water immersion are common phenomena occurring when using sunscreens; therefore, sunscreens must maintain their efficacy under such conditions.

Water resistance testing is an in vivo test, performed similarly to the SPF value determination. According to the OTC monograph of sunscreens, for a product claimed to be "water resistance", the label SPF should reflect the SPF measured after 40 min of water immersion. During the test, the subjects are immersed in water two times for 20 minutes separated by a 20-minute rest period without toweling. The SPF is determined after the second 20-minute water immersion. 

For a product claiming to be "very water resistance", its labeled SPF should reflect the SPF value determined after 80 minutes of water immersion. For the test phases of 20-minute water immersion, a 20-minute resting without toweling is repeated four times and the SPF is determined after the water immersion.

Broad Spectrum Testing
The currently required by the FDA to determine whether a sunscreen protects against UVA radiation, in addition to UVB protection, is an in vitro test. This broad-spectrum test procedure measures a product's UVA protection relative to its UVB protection.

This method is referred to as the critical wavelength method, which is a pass/fail test.

The critical wavelength must be greater than 370 nm as its standard for acceptable broad-spectrum protection. In this test, the sunscreen product is applied to an optical grade polymethylmethacrylate (PMMA) plate suitable for UV transmittance measurements at a dose of 0.75 mg per square centimeter.

The UV light emitted is in the UVB-UVA range (290 to 400 nm range).

The UV absorbance of the sunscreen measured is plotted against wavelength. The width of the curve plotted on the graph shows how broadly effective a sunscreen is across the UV spectrum. The area under the curve (AUC) is measured, and a line can be drawn at the wavelength. Therefore, the higher the critical wavelength, the better the UVA protection provided by the sunscreen.