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.



[1] Mancuso JB, Maruthi R, Wang SQ, Lim HW. Sunscreens: An Update. Am J Clin Dermatol. 2017 Oct;18(5):643-650.

[2] Burnett ME, Wang SQ. Current Sunscreen Controversies: a Critical Review. Photodermatol Photoimmunol Photomed. 2011 Apr;27(2):58-67.

[3] Health Canada. Sunscreen Monograph. Retrieved from:http://webprod.hc-sc.gc.ca/nhpid-bdipsn/atReq.do?atid=sunscreen-ecransolaire&..

[4] Schalka S, Reis VM. Sun Protection Factor: Meaning and Controversies. Br J Dermatol. 2015 Nov;173(5):1345.

[5] Herzinger T. Sun Protection Factor 50+ : Pro and Contra. Hautarzt. 2017 May;68(5):368-370.

Information contained in this website is not intended nor implied to be a substitute for professional medical advice, it is provided for educational purposes only. You assume full responsibility for how you choose to use this information.Always seek the advice of your physician or other qualified healthcare provider about any questions you may have regarding a medical condition or treatment.

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)[3] 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) [4]. 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.

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