Types of sunscreen products:
(1) physical: these agents physically block light; examples include titanium oxide and zinc oxide
(2) chemical: these agents selectively absorb UVA or UVB light over certain wavelengths
Method of determination:
(1) A standard amount of sunscreen is applied over a defined area so that there is 2 mg per square centimeter.
(2) A reference area of the same size is designated.
(3) Testing is done indoors using an artificial light source.
(4) The reaction of the coated area is compared to that of the untreated site.
SPF =
= (minimal erythema dose with the sunscreen) / (minimal erythema dose without the sunscreen)
Using a given sunscreen, under idealized conditions:
duration of sun exposure in minutes before sunburn develops with sunscreen =
= (SPF) * (duration of sun exposure in minutes resulting in sunburn without sunscreen)
Interpretation:
• The American Academy of Dermatology recommends that the minimum SPF should be 15.
• A higher SPF will protect the user better and longer provided it is applied as directed.
• The sunscreen should be reapplied after swimming or heavy sweating.
Limitations:
(1) Most people use significantly less sunscreen than that used for testing, often only 0.5 mg per square centimeter, which reduces the protection significantly.
(2) A user may not reapply the sunscreen after it has been washed off.
(3) Conditions outdoors differ from a controlled indoor environment, and the user may be sweating.
(4) The SPF may not measure the sunscreen's ability to protect against UVA and/or UVB light depending on the testing conditions.
