Sun protection

How do they test a sunscreen's protection factor?

Not only is the information about a sunscreen product important, but consumers rely on it being correct. Before any sun protection factor figures or information about the UVA protection level can be printed on packaging, the products have to undergo test procedures to validate these values.

Anyone requiring a sunscreen with a high sun protection factor relies on the information on the packaging and needs to be able to trust it. To ensure the printed information reflects reality, there are test procedures that have to be conducted. You will most likely have encountered the names for these types of test already. They play a role in virtually all cosmetic and pharmaceutical products. 


We would like to briefly introduce three types of test used in the world of sun protection:

  • in vivo test

The name is derived from the Latin "vivus" and means "from life" or "living". Reactions are tested on or in a living organism. This requires test subjects, who have the cosmetic formulation applied to their skin. This type of effectiveness test is sometimes very expensive due to the effort involved. There is a global standard for in vivo testing of sunscreens: ISO 24444:2019. This global standard defines and regulates the in vivo measurement of their sun protection or light protection factor.

  • in vitro test

The Latin term "in vitro" translates as “in glass”. This is a laboratory test without the use of a test subject as a biological test base. There is also a standard here for defining the level of protection against skin damage caused by UVA irradiation. ISO 24443:2021 regulates how UVA protection factors are measured. According to the recommendation of the European Commission, UVA protection must represent at least one third of the sun protection factor.

  • in silico test

From the Latin "in silico". It is derived from the chemical element silicon. Results are collected and evaluated by means of computer simulations. Work involving the Sunscreen Simulator is known as an in silico test or an in silico study. The Sunscreen Simulator is now a recognized tool in the industry for determining the sun protection factor (SPF). Its capabilities enable manufacturers to drastically reduce the time to market for their products. Instead of using complex in vivo and in vitro studies, more research can be conducted in the laboratory and online to achieve quicker, better results that are more cost-effective. BASF has conducted many in vivo tests in recent years and compared the calculations of our Sunscreen Simulator with these results. Based on these empirical values, there is a good correlation between in silico and in vivo.

UV protection is tested on the test subject 

The old mnemonic of B=burn, where UVB rays alone were believed to cause damage, is outdated in many respects. Today, we know that both types, UVA and UVB, come with their own risks. The light or sun protection factor describes protection of the skin from direct and visible damage caused by UV radiation, i.e. from sunburn, also known by the medical term "erythema". ISO 24444:2019 is used to determine the SPF level. The in vivo test can definitively determine the sun protection factor, and is the current gold standard for the evaluation of sunscreen products. 

How is this kind of test carried out and what needs to be taken into account? The aim of an ISO 24444 method is to ensure certain preconditions and parameters are respected, so that the results are comparable, no matter the laboratory or where in the world they are measured. These parameters include the quantity to be applied, the type of radiation, the UV range, the skin types of the test subjects and even the ambient temperature in the test environment. Ten to twenty volunteers are required per test series to obtain meaningful results. The same subjects are permitted to participate in such tests several times. However, there must be at least 8 weeks in between tests, and all visible signs of exposure to the sun (tanning, sunburn) must have disappeared. 

Although this test is performed on humans, in vivo is still considered a laboratory method. Using a xenon lamp that acts as a sun simulator, the subject’s back is exposed to the ultraviolet rays that are usually responsible for sunburn. The lamp must illuminate/radiate evenly and there should be no fluctuations between extremely high UV values and extremely low UV values. A quantity of 2 mg/cm2 of the sunscreen to be tested is applied to one area of the back, while another area of the back receives the rays unprotected. A reference agent is applied in the third range in order to be able to check and validate the results.

There are 5 different reference formulations covering SPF 15 up to SPF 60. Whether a reference product with a high or low SPF is selected depends very simply on what sun protection factor is expected for the “new” product and what comparative value is required. 

Test base "human skin" never 100% identical

Over a period of 16 to 24 hours after irradiation, a trained laboratory technician looks at the reactions on the “treated” and “untreated” skin. The individual minimum erythema dose (MED) of the protected and unprotected skin – also known as the threshold dose – is determined based on these observations. The ratio of the two values then gives the SPF for the product. 

Although there is a global standard in place, measurement results may differ from one institute to another. "We have to bear in mind that we test the SPF in vivo, i.e. directly on people. The test basis, the human skin, is never 100% identical from one person to the next. This is one reason for possible variations," explains Marcel Schnyder, Head Global Technical Center Sun Care at BASF. 

In addition, the endpoint of the measurement can lead to variations. The endpoint here refers to erythema, or the redness of the skin. One tester may see redness, but to another tester it may not be visible or insufficient. Minor differences are, however, factored in or permitted. The end result is a uniform value that consumers can rely on. 

Nevertheless, further research is being carried out around the world to develop an in vitro method that minimizes these differences. In addition, an in vitro method is desirable from an ethical point of view, and could also reduce costs for companies and manufacturers. We look at research into in vitro SPF methods in an upcoming article. For that, we will be speaking to our colleagues in Grenzach and getting up to speed on the latest developments.