This index must be marked on all products marketed as sunscreens. Common is that we find two types of abbreviations to indicate the protection index: IP (assigned to Europe) and SPF (assigned to America). But, do we know what this sun protection index is based on? Sun protection index is the relationship that exists between two types of skins: a protected and the other not. This relationship is expressed with the acronym DEM which is the minimal eritematogena dose, i.e., the time that has to pass the exposed skin to show erythema. The relationship, therefore, will remain this way: with no sunscreen sunscreen/DEM DEM = IP, or SPF. For example, 150 minutes / 10 minutes = 15. The IP of the product would be 15. Now however, it is important to educate people about the solar protective term.
The protection is not permanent. This IP only tells us that if the person takes 10 minutes to develop the rash after exposure, if the product is applied it will protected 15 times more time (150 minutes), with what after that time the person remains unprotected and solar Erythema is effective. The general conclusion to all this is that the IP is a multiplier factor of natural protection that gives us our own skin. Currently, the pharmaceutical industry has developed powerful protectors with IP very superior as the + 50 or 90. This in theory, leave protected the individual for many hours without having that concerned by the application of a new dose, but in practice this is not the case and prolonged exposure should be avoided. This is because each company bases its IP in their own products or clinical studies presented by what the final results will be highly variable between each other. There are brands that can be + 25 but the real IP of the product is + 15, for example.