Level of Protection
To estimate the level of protection offered by an SPF number, the absorbance of the product (how much energy is filtered /absorbed) can be calculated:
A = 1 – 1/SPF
Therefore, if a sunscreen has an SPF 15, it filters 93.3% of UVB. An SPF of 30 filters out approximately 96.7% of UVB rays while an SPF 45 blocks about 97.8% of UVB rays.
No matter how high the SPF number is, no sunscreen protects completely from all UV rays. Several countries such as Canada allow a maximum labeling SPF of 50+ to avoid a false sense of safety and misuse [3,5].
Proper application and reapplication is key no matter the SPF number indicated on the sunscreen.
Elizabeth Hartinger, Ph.D., M.A.Sc., B.Eng.
 Burnett ME, Wang SQ. Current Sunscreen Controversies: a Critical Review. Photodermatol Photoimmunol Photomed. 2011 Apr;27(2):58-67.
 Health Canada. Sunscreen Monograph. Retrieved from:
 Schalka S, Reis VM. Sun Protection Factor: Meaning and Controversies. Br J Dermatol. 2015 Nov;173(5):1345.
 Herzinger T. Sun Protection Factor 50+ : Pro and Contra. Hautarzt. 2017 May;68(5):368-370.
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Sun Protection Factor (SPF)
Daily use of sunscreen is a simple measure that can help protect against photoaging, different skin cancers, or other health problems [1,2].
An ideal sunscreen should have a sun protection factor (SPF) of 30 to 50+ (for extreme conditions) and always provide effective broad-spectrum protection against both UVB and UVA rays, the two types of UV rays that reach the earth and cause damage to the skin. A sunscreen should also be photostable and applied uniformly, frequently, and generously to provide adequate protection.
The SPF is the main information about the level of protection a sunscreen will give against UVB rays and before skin shows erythema (skin reddening) . The SPFj value for a subject, j, participating in a clinical trial, is obtained by measuring the ratio between the minimal erythemal dose (MEDj) of sunscreen-protected skin and the MEDj of unprotected skin. The MED is expressed in energy/surface and corresponds to the minimal UV dose that is required to generate the first clear skin reddening 16h to 24h after UV radiation. SPFj is expressed as follows:
FPSj = MEDj (sunscreen-protected skin) / MEDj (unprotected skin)
The SPF number of a sunscreen is obtained by computing the arithmetic mean value of all valid SPFj obtained from all subjects participating in the clinical study.