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What a Sunscreen's SPF Really Means

by Will / November, 13 2013 01:00

SPF stands for Sun Protection Factor so you'd think it measures how well a sunblock protects you. However, that's only partially true...

What is SPF

There are three kinds of ultraviolet rays that come from the sun: UVA, UVB and UVC. UVC rays don't reach the earth's surface so UVA and UVB are the one's you have to protect yourself from (for a deep discussion see our article "UVA, UVB and UVC Rays: What They Are and How Sunscreen Protects You").

What does this have to do with SPF? Well SPF on it's own only measures how well a sunblock protects you from UVB rays. It doesn't give you the full scoop on protection from UVA rays and unfortunately both types of rays cause skin aging, skin cancer, eye damage and more. It's interesting to note that UVB rays are what cause sunburn so UVA rays do their damage over time.

What is the Connection Between Broad Spectrum Protection and SPF

How do you get UVA protection? You need a "broad spectrum" sunscreen like those from Block Island Organics. New US Food and Drug Administration (FDA) regulations specify that only sunblocks with an SPF of 15 or more that protect against both UVA and UVB can be labeled as such. The physical active ingredients in sunscreens, zinc oxide and titanium dioxide, protect against both with zinc generally giving even broader protection than titanium. There are active chemical sunscreen ingredients that will protect you too. Our UVA vs UVB vs UVC article has the full list.

What Does the SPF Number Mean

Let's get back to SPF. We know what it measures and what it doesn't but how does an SPF 15 compare to an SPF 30 and so on? A lot of info on the web says SPF is a measure of the length of time you can stay in the sun. For the sake of example, let's say you can stay in the sun for 10 minutes without sunblock before you burn (this time varies widely from person-to-person). Around the web many articles will then say with an SPF 15 you will be protected for 150 minutes (SPF 15 x 10 minutes) before burning. It sounds pretty simple but it's not entirely true. For example using an SPF 30 doesn't mean you can stay in the sun twice as long as with an SPF 15. SPF measures how much UVB radiation a sunscreen blocks; it does not exactly measure how long you can stay in the sun when using a sunscreen.

As far as SPF as a measure of UVA protection, the FDA says a broad spectrum sunscreen's SPF rating measures UVA protection in proportion to UVB protection. This means the higher the SPF rating on a broad spectrum sunblock, the better the UVA protection should be but it doesn't mean it will have the same level of UVA and UVB protection. A little confusing but better than nothing we think.

Should You Get the Highest SPF Possible

Like everything, yes and no. Past a certain SPF level (we'd say SPF 40 or 50) you're not getting much additional UVB protection. Plus, according to the Environmental Working Group (EWG) there are serveral problems with SPFs over 50 ranging from consumer misuse to marginally better protection to poorer balance.

Here's a handy SPF vs UVB protection chart that clear shows the diminishing returns with high SPF sunscreens:

SPF and UVB Protection Levels

Once you hit SPF 40 you block out 97.5% of UVB radiation and SPF 50 blocks 98%. Beyond that you get diminishing returns for your dollar - SPFs over 40 or 50 are done for marketing purposes in our opinion. More important than using a super high SPF is using enough sunscreen. 1 fl oz is the recommended amount - check out our article "How Much Sunscreen Should You Use?" to see how that should be used across your body.

What Does This All Mean for You

Here's what we recommend you look for in a sunblock no matter what brand you choose:

  1. Be sure to use a sunscreen labeled "Broad Spectrum" (like ours) to get UVA and UVB protection as SPF on it's own is only a measure of UVB protection. SPFs on broad spectrum sunblocks measure UVA protection in proportion to UVB protection according to the FDA.
  2. Above SPF 40 or 50 you're spending extra money for something you don't need. You're not getting much additional protection and in fact, the FDA says testing of SPFs above 50 may be unreliable.
  3. We recommend a minimum SPF 15 but many organizations, including the Skin Cancer Foundation, recommend SPF 30 at a minimum.
  4. Be sure to use enough sunscreen. Most people don't use the recommended 1 fluid ounce (about a shot glass full) to cover your whole body.

Hopefully that was helpful for you. Let us know what you think in the comments below. For more check out our suncare smarts series or deep dive on our SPF articles.

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