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.
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.)
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.
|Let's be real here. This is just silly.|
The term "broad spectrum" refers to a sunscreen's ability to protect from both UVA and UVB light.
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.
Many countries (not the US) require specific information about a sunscreen's UVA protection. One common way to express this is using the PA system, or the Protection Grade of UVA rays.
|Persistent Pigment Darkening on some girl's belly.|
The PA system is based on a test called Persistent Pigment Darkening (PPD). It is similar to the in vivo SPF test, except that instead of burning, it measures skin darkening. PA+ indicates a UVA protection factor between two and four, PA++ is between four and eight, and PA+++ is more than eight.
You may remember having sunscreens that were advertised as "waterproof" or "sweatproof". Sadly, such things do not exist. Thus, the new FDA guidelines have eliminated them. Instead, they have replaced them with the term "water resistant". Water resistant products must remain effective after 40 minutes or 80 minutes of swimming, and this information must be written on the bottle.
In order to be labeled "water resistant", the SPF value that is written on the bottle must still be in effect after this amount of water exposure. After this period of time you may begin to see a decline in protection.