What is the SPF?

I am sure you have seen this number stamped in all sunscreens, the famous SPF. You probably have heard that SPF30 versus SPF50+ is almost equivalent so why bother with SPF50+. I have written a blog post on the subject demonstrating why SPF50+ is far better for the skin and protects the skin twice as much as SPF30 (here). To understand what is the SPF it is important to know how it is determined. 

The SPF number is determined in laboratory conditions with an artificial UV lamp on us. The SPF protocol has ISO guidelines and is accepted in many countries in the world. In the USA the method is almost the same.

Skin phototypes

Only individuals with untanned skin phototype I to III (Fitzpatrick) are eligible for the test. 

Skin phototype I always burn and don’t build a tan

Skin phototype II burns and tans poorly

Skin phototype III burns minimally and build a tan

The lamp

The SPF is not assessed with natural sunlight but with an artificial UV lamp (Xenon arc lamp). The Xenon lamp reproduces the UVs from the sun in lab conditions. It has to be 20% UVB, 20% UVAII and 60% UVAI.

The protocol

The SPF is done on the back divided into 3 areas:

1: with no sunscreen 

2: with the sunscreen to test 

3: with a reference sunscreen used as a control 

Each area is subdivided into 5 subunits exposed to increasing UV doses. Sunscreen formulations are applied at 2mg/cm2. 

20 to 24 hours later a habilitated scientific look at which dose of UV gives a sunburn in each area.

The individual SPF is

SPF: sunburn with sunscreen / sunburn without sunscreen

This protocol is repeated 10 to 20 times. All valid individual SPF is only used to calculate the final SPF mentioned on the bottle. The final SPF of a sunscreen is from the mean of all valid individual SPF.

I am not going to discuss the several controls in the protocol but there are several to ensure that the SPF is accurate. 

So the SPF measures the protection against sunburn. 

What causes sunburn? 

Sunburn is due to UVB and some UVA. Therefore the SPF reflects the protection against UVB and to a lesser extend to UVA. It is a false assumption that sunburns are only due to UVB. UVA also play a small role. But by no means, SPF is sufficient to measure the protection against UVA spectrum. Always remember that whatever the SPF labeled on the bottle a sunburn was always detected. Even with high SPF UV passed and grilled the skin. So don’t forget to wear your sunglasses, hat, and clothes. 

I also have made a video about it. 

Thank you for your time


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