Rationale and brief history

The SPF procedure as described in ISO 24444:2019 and the quite comparable procedure in the US as defined by the FDA for SPF measurement are labor-intensive and complex human tests. The UV challenge to the subjects is on a small area but can bear considerable dosages, which is why there has long been a desire for alternative methods. Moreover, the complexity of the procedure and some intrinsic variabilities of this biological test (human subjects, human sunscreen application and human readout) create, for validation purposes, a relatively large random variability, in particular, between different laboratories performing these tests.

Skin cancer incidence rates are still growing around the globe and there are not many silver streams on the horizon that this trend diminishes. The SPF value, measured on human skin, represents a number that is intrinsically linked with the protection against DNA damage in human skin and thus it is connected with the protective ability of sunscreens against skin cancer, although, application of sunscreens should not be the only strategy against the disease. An alternative SPF test method should therefore deliver at least equivalent results as the current SPF in vivo in terms of accuracy and there is on top the desire, that the new method shows improvements in precision (repeatability and reproducibility), is easy to adapt by laboratories and cost-effective to perform.

Over the last two to three decades alternative methods such as for example, in vitro transmission tests in silico calculation, or non-invasive in vivo testing (using negligible UV dose on humans) have emerged and been developed to a stage that makes them promising alternatives.

The desire to understand the precision behavior of the alternative methods demand sophisticated statistical tools to analyze and, in particular, to characterize them for a general public use. These statistical tools aim to deliver key features to rate accuracy and to measure reproducibility, degree of systematic errors against different categories of sunscreens. The accuracy, repeatability and reproducibility should be known to decide on the release of a method measuring protective abilities of sunscreens that are linked to skin cancer prevention.

Comparing alternative SPF data against the gold standard requires a solid set of in vivo SPF data and thus represents a significant experimental effort that should not be repeated for each alternative method. This is how, in 2018, the main idea for the ALT-SPF project was born.

More information about the initiative can be found in the project synopsis.