What is the difference between SPF 30 and SPF 50?

First used in 1934, it was Franz GREITER, an Austrian scientist and avid mountaineer, who introduced the concept of sun protection factor (SPF). This is a measure that attests to the effectiveness of a sunscreen in preventing sunburn. It actually measures the rate of filtering of erythemal UV rays by a sunscreen, which are composed of 85% UVB and 15% UVA. To date, there are four levels of SPF.

How is it measured?

The SPF is determined by tests in vivo, during which volunteers have either sunscreen or a control solution applied to their backs at a rate of 2 mg/cm2, and are then exposed to a UV lamp simulating sunlight. It is thus defined as the ratio between the minimum dose of UV rays required to produce erythema on skin protected by sunscreen and the minimum fraction of UV necessary to cause the appearance of the same redness on unprotected skin.

According to the recommendations of the French Agency for the Safety of Health Products (AFSSPS), this value must be spelled out in full on the packaging of sunscreen creams to indicate their ability to block erythemal UV rays. However, the sun protection index can be displayed under different acronyms that all represent the same meaning: Sun Protection Factor (SPF) in English, Facteur de protection solaire (FPS) or even Indice de Protection (IP) in its French translation.

Each sun protection factor corresponds to a percentage of erythemal UV rays blocked. The higher the SPF, the greater the photoprotection action. However, SPF values are not proportional. The difference between SPF 30 and SPF 50 is the amount of UV radiation that reaches the body. An SPF 30 will filter up to 96.67% of erythemal UV rays, allowing approximately 3.33% of UV rays to pass through, while an SPF 50 will block about 98% and allow 2% to pass through. The gap between these two percentages is not significant: SPF 50 allows 1.33 times fewer erythemal UV rays to pass through than SPF 30. However, in real-world usage conditions, this statement is incorrect.

Indeed, several studies have shown that consumers do not apply the correct amount of sunscreen in practice, using between half and a quarter of the recommended amount. The standard for SPF in vivo tests is based on an application of 2 mg/cm2. However, most people only apply 0.5 to 1.0 mg/cm2 of the product, which reduces the effectiveness of sun protection. Indeed, using a lesser amount of sunscreen will result in a decrease in protection, almost linearly: it would therefore be reduced by more than twice.

• MARLOWE E. & al. Commentary on 'UVB-SPF': the SPF labels of sunscreen products convey more than just UVB protection. Photodermatology, Photoimmunology & Photomedicine (2008).

• HERZOG B. & al. In vitro measurements of sunscreen protection. Photochemical & Photobiological Sciences (2010).

• RUVOLO E. & al. Measuring sunscreen protection according to the FDA final rule. Principles and Practice of Photoprotection (2016).