SPF stands for Sun Protection Factor. Sun Protection Factor is not a measure of the time that someone spends in the sun, instead it is a measurement of solar energy (according to the FDA). Protection against this sun energy is the primary purpose of most sunscreens. Using the FDA’s website and other sites, including Wikipedia, the goal of this article is to better explain what SPF is.
SPF ranges from 2 to 60 (according to this site: How to Understand SPF Numbers | HealthGuidance.org), however we at Peeq have seen and used SPF 100. The key to the numbers is that they represent a logarithmic scale, not a linear one. That means that an SPF 2 prevents the absorption of 50% of the ultraviolet rays, SPF 15 absorbs 94%, and an SPF 30 absorbs 97%. If you were wondering, an SPF 100 absorbs 99% of ultraviolet rays. Importantly, not all sunscreen is broad spectrum – meaning that some sunscreens, and particularly sun tan lotions, only block some bands of ultraviolet rays. This does allow the person wearing the sunscreen to absorb solar energy – causing tans and burns in some cases.
Overall, the SPF number is a factor of how much absorption there is, but the amount of solar energy is also not linear. Instead, solar energy exposure varies based on three primary conditions.
First, your geography is critical. Across the United States, individuals in warm southwestern states have higher solar exposure (see above map for UV radiation in the US).
Second, the time of day has a dramatic effect on the amount of solar exposure, as well as the time of year. (see the World Health Organization’s UV Index by month and time of day above). Lastly, your skin shade effects how much radiation is absorbed by your skin. Darker skin absorbs less radiation at a given time.
Thus, a fair skinned person who stands outside at noon in Phoenix, Arizona in the month of July receives more exposure to the sun than a darker skinned person who stands outside in Minnesota at 4pm in January.
The goal of sunscreen is to protect your skin from solar energy, and it serves a broad geography and diverse population. Therefore, it is critical that the person using the sunscreen understands what it can and cannot do. The SPF is not a measurement that is intended to represent the hours that one can spend in the sun before re-application, but instead a more general measurement of the amount of solar radiation that can be “blocked.” The higher the SPF, the more solar energy absorption is prevented.
The FDA recommends sunscreen of at least SPF 15.
For more information, check out these sites: