What Does The Labeling on My Sunscreen Tube Mean
Most of us have some vague sense that we should probably wear higher numbers of SPF and that broad spectrum and water resistant sunscreens are probably better, but you might not know the exact meanings of these terms.
SPF stands for “sun protection factor”. Broadly speaking, it refers to the amount of protection that sunscreen offers against Ultraviolet Type B (UVB) light, which is the type of UV radiation that causes sunburns.
In order to calculate SPF, individuals who are prone to sunburn must sacrifice some of their vampire-like skin. Volunteers are exposed to conditions meant to simulate noontime sun on a certain patch of skin until redness appears. Then, the procedure is repeated using a thick layer of sunscreen. To calculate SPF, researchers divide the amount of time needed to burn with sunscreen by the amount needed without. Thus, SPF is a measure of time, not a measure of strength. SPF 30 is not twice as strong as SPF 15; Sunscreen with SPF 15 blocks 93% of UVB light, whereas SPF 30 blocks 97%.
This has led to the popular simplification that, if it takes ten minutes to burn without sunscreen, it will take 150 minutes with SPF 15. Unfortunately, it is highly unlikely that that is the case. First of all, you’re sweaty and gross, so sunscreen melts off of you when you sit in 90 degree weather. Second of all, the sun is different at different times of day. Third of all, you probably aren’t using enough sunscreen. SPF is calculated using 2 milligrams of product per square centimeter of skin. That is a LOT of sunscreen. That’s about two full ounces to cover your body. Think about how long you have had that eight ounce bottle of sunscreen. Two summers? Three summers? Yeah, you aren’t using enough sunscreen. (Once you factor in reapplications, it almost seems more cost effective to simply hide indoors.)
It is worth noting that not all tests of SPF are in vivo. The FDA does require in vivo tests, but spectrometers measuring the transmittance of sunscreen match up relatively will with the in vivo data.
So, what is the right SPF to use? Dermatologists tend to consistently recommend about SPF 30. Although more is certainly fine, at a certain point it becomes a little silly. Indeed, many countries have banned super high SPF ratings because it gives people unrealistic expectations about their efficacy. The FDA has proposed this law a few times, although legislation obviously has not yet occurred.
The term “broad spectrum” refers to a sunscreen’s ability to protect from both UVA and UVB light.
Although only UVB is responsible for sunburns, there are actually three types of UV light: UVA, UVB, and UVC. These terms refer to the wavelengths of the light. UVA light is the lowest energy, UVB is in the middle, and UVC is the highest energy. Luckily for us, UVC light is absorbed by the atmosphere, meaning we don’t encounter it and thus don’t have to worry about it. UVA and UVB light, however, do enter the atmosphere. UVB light, as I have already mentioned, is associated with sunburns. However, UVA light can also cause skin damage and the dreaded skin cancer.
In the past, companies have been able to label their products “broad spectrum” without much evidence. Thankfully, recently passed regulation has tightened up on what can and cannot be labeled “broad spectrum”. Now, the SPF number will relate to both UVA and UVB rays for broad-spectrum products.
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