Understanding SPF: How Sun Protection Factor is Measured and Verified

Written by Rachelle at Vivoderm on April 1, 2025. Posted in Fundamentals, Natural Skincare, Skincare Research, Sun Protection

How Sun Protection Factor is Measured and Verified

Determining a sunscreen’s Sun Protection Factor (SPF) is essential to assess its effectiveness in shielding the skin from harmful ultraviolet (UV) radiation. SPF quantifies the level of protection against UVB rays, which are primarily responsible for sunburn and can contribute to skin cancer. Accurate determination of SPF involves both in vivo (on human subjects) and in vitro (laboratory-based) testing methods.

HOW DO YOU KNOW YOUR SPF IS ENOUGH?

In Vivo Testing

The traditional method for determining SPF is the in vivo test, as outlined in ISO 24444:2019. This procedure involves applying the sunscreen to the skin of human volunteers and measuring the time it takes for erythema (redness) to develop compared to unprotected skin. The SPF value is calculated by dividing the amount of UV exposure that causes erythema on protected skin by the amount that causes erythema on unprotected skin. While this method has been the standard, it raises ethical concerns due to the intentional exposure of participants to potentially harmful UV radiation.

In Vitro Testing

To address these concerns, in vitro methods have been developed. These laboratory-based techniques measure the sunscreen’s ability to absorb or reflect UV radiation without involving human subjects. One such method is specified in ISO 23675:2024, which provides guidelines for determining SPF by analyzing the spectral absorbance characteristics of sunscreen products. This approach is applicable to various formulations, including emulsions and alcoholic solutions, but excludes powders and sticks.

Recent Developments

In early 2025, the International Organization for Standardization (ISO) approved two new SPF testing methods aimed at providing more ethical and less invasive alternatives to traditional in vivo tests. These methods seek to deliver reliable evaluations of UV protection while minimizing risks to human participants. The adoption of these new standards marks a significant advancement in sunscreen testing, promoting both accuracy and ethical responsibility.

Expected SPF Based on Sunscreen Agents

While in vitro and in vivo testing are necessary for precise SPF determination, formulators often estimate the expected SPF based on the type and concentration of active sunscreen agents used. For instance:

Avobenzone: Provides UVA1 protection. Allowed use levels vary by region (e.g., USA 3%, EU 5%). Does not significantly increase SPF on its own.
Homosalate: Offers UVB protection. Allowed up to 15% in the USA. Specific SPF contribution data may not be available.
Octocrylene: Protects against UVB. Usage levels differ depending on combination with other sunscreens.
Octyl Methoxycinnamate (OM-Cinnamate): Provides UVB protection. In the USA, allowed up to 7.5%. Approximately 1% concentration yields an SPF of about 1.5.
Oxybenzone: Offers protection against UVB and UVA2. Allowed up to 6% in the USA. Approximately 1% concentration yields an SPF of about 1.5.

It’s important to note that these figures are approximate, and the actual SPF of a final product can only be accurately determined through standardized testing methods.

See the last chart here by https://www.makingcosmetics.com/

Sunscreen	UV Protection*	Allowed Use Level	Expected SPF Increase
Avobenzone	UVA1	USA 3%, EU 5%, Brazil 5%, Japan 10%, Australia 5%	None
Homosalate	UVB	USA 15%, EU 10%, Japan, 10% Australia 15%	No data available
Octocrylene	UVB	USA 10% if used alone and 7-10% if used with other sunscreens	No data available
OM-Cinnamate	UVB	USA 7.5 %, EU 10%, Japan 20 %, Australia 10%	1% gives a SPF of about 1.5
Oxybenzone	UVB, UVA2	USA 6 %, EU 10%, Japan 5%, Australia 10%	1% gives a SPF of about 1.5
Titanium Dioxide	UVB, UVA2	USA 25%, Japan no limit	1% gives a SPF of about 1.3
Titanium Dioxide (micronized)	UVB, UVA2		1% gives a SPF of about 2.5
Zinc Oxide	UVB, UVA2, UVA1	USA 25%, EU 25%, Japan no limit, Australia 20%	1% gives a SPF of about 1
Zinc Oxide (micronized)	UVB, UVA2, UVA1		1% gives a SPF of about 1.5
Zinc Oxide (micronized & coated)	UVB, UVA2, UVA1		1% gives a SPF of about 1.5
Zinc Oxide (micronized) plus Titanium Dioxide	UVB, UVA2, UVA1		5% each gives a SPF of about 12-19
OM-Cinnamate plus Titanium Dioxide (micronized)			5% each gives a SPF of about 12-19
OM-Cinnamate plus Octyl-Salicylate			5% each gives a SPF of about 10-18

*UVA1 = 340-400 nm, UVA2 = 320-340 nm, UVB = 290-320 nm

Zinc Repairing Facial Cream

Conclusion

Accurate SPF determination is crucial for ensuring the efficacy and safety of sunscreen products. While estimations based on active ingredients provide a preliminary understanding, standardized in vivo and in vitro testing methods are essential for precise measurement. The recent advancements in SPF testing methodologies reflect a commitment to ethical practices and scientific accuracy in the cosmetic industry.

NOTE: SPF testing must be done by special laboratories that have a lot of expertise and experience. Here are laboratories that offer this service: