Question: What's the difference between SPF and PPD?
SPF stands for sun protection factor. It measures a sunscreen's amount of protection from UVB rays. PPD, on the other hand, stands for persistent pigment darkening. It measures a sunscreen's amount of protection from UVA rays. UVA rays are the ones responsible for aging and UVB rays are the ones responsible for burning. You want to protect yourself from both of these UV rays by using a broad-spectrum, photostable sunscreen.
That doesn’t necessarily mean you need a sunscreen with the highest SPF possible. As you go up in SPF, the higher the SPF number, the less difference it makes in sun protection. For example, you get a bigger increase in protection moving from SPF 15 to SPF 30 than you would if you moved from SPF 50 to SPF 65. In fact, many sunscreens in Europe are not allowed to be labeled with a SPF number exceeding 50, as to not be misleading in a sunscreen product's level of protection.
SPF numbers measure how long a sunscreen will protect you from burning compared to how quickly your skin turns red without sunscreen. For instance, if your skin starts burning 30 minutes after being out in the sun with no protection, a sunscreen with SPF 20 would prevent you from burning for 600 minutes (30 min x 20) or 10 hours. This, of course, is dependent on the sunscreen's stability and whether you correctly wear and re-apply it in the first place.
PPD values aren’t measured as precisely as SPF numbers. In fact, there is no standardized system for determining them, so there is some ambiguity in their assessment. PPD is also known as PA in some countries with plus signs (ex. PA++) that signal UVA protection strength. The higher the PPD number or the greater the number of plus signs, the better a sunscreen protects against UVA rays.
SPF and PPD are different, but they are both necessary factors to have in a good, strong sunscreen. For more information about sunscreen, click here. To find out your sunscreen’s PPD, check out this helpful website.
Last updated: September 25, 2012
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