When More doesn’t Mean More – SPF Explained

This summer we are seeing more and more sunscreens proclaiming higher and higher SPF numbers.

No SPF, not even 100+, offers 100 percent protection. SPF only rates UVB protection, but both UVA and UVB radiation can lead to skin cancer, which is why dermatologists now advise using sunscreens with an SPF of at least 30 but look carefully to make sure the sunscreen gives UVA protection too, there is currently no rating system in the US for UVA protection.

The difference in UVB protection between an SPF 100 and SPF 50 is marginal, SPF 100 does NOT offer double the blockage,

SPF 100 blocks 99 percent of UVB rays.

SPF 50 blocks 98 percent of UVB rays.

SPF 30, blocks 96.7 percent of UVB rays,

A sunscreen’s SPF number is calculated by comparing the time needed for a person to burn unprotected with how long it takes for that person to burn wearing sunscreen. So a person who turns red after 20 minutes of unprotected sun exposure is theoretically protected 15 times longer if they adequately apply SPF 15.

A lot of sunscreens rub off  and degrade with time which is why dermatologists advise reapplication every two hours or after swimming or sweating.

Consumers should worry more about wearing enough sunscreen.

Skimp and you lose. To get the SPF advertised, you must use a full shot glass on your body. That’s an ounce, which means a three-ounce tube should last, at most, a few outings.

The biggest danger is people tend to think if they use a sunscreen labeled 100 then they can use half the directed amount and receive a protection factor of 50 but this is not the way the rating works

It turns out that if you apply half the amount, you get the protection of only the square root of the SPF.

Applying a half-ounce of SPF 70 will not give you the protection of SPF 35, but 8.4 and not necessarily give any protection from UVA!

Above SPF 50 lulls consumers into a false sense of security. Put SPF 100 on your kids at dawn, and you might think, “Great, they are covered all day” but the same 2 hour rule still applies no matter how high the SPF number.

Today is the last day of ‘Skin Cancer Awareness Month” but it’s only the beginning of summer.

Which sunscreen do you prefer?


About sunnysleevez

I'm a freckly red head, originally from London now living in LA with 2 pale children. Frankly with our coloring we have no business being such an outdoorsy family and living in southern California. I grew up in a time when factor 8 sun milk was considered the best protection available to combat the hot rays of a holiday in southern Spain, that lead to 3rd degree burns and the rest of the vacation spent in the shade of our apartment. As a family we love to be outside, going to the beach, camping, swimming & hiking. We can't completely avoid the sun (nor should we) but taking sensible measures to avoid unnecessary UV damage is a priority. How do we do that? By staying in the shade in the middle of the day, wearing sun protective clothing (that's why I created Sunny Sleevez), a broad brimmed hat, UV rated sunglasses, using chemical-free broad spectrum sun screen on exposed areas & eating a healthy diet with as few chemicals as possible & lots of antioxidants.
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13 Responses to When More doesn’t Mean More – SPF Explained

  1. BMP says:

    There is a lot of confusion out there about SPF and this is an excellent summary. Great post!

  2. Wow, there’s a lot of maths! But great information – I had no idea about the 2-hour rule. Thanks for this 🙂

  3. kmom14 says:

    Nice article and very informative. I used to use just tanning oil, but with the increase in skin cancers I now use sunscreen, usually Hawaiian Tropics.

  4. Lynette says:

    Great information, thanks! Im passing it onto my readers too. Can’t be too careful with the little’s skin!

  5. Yikes! So much information. It’s like a recipe. I appreciate it though. Very important.

    Sue Bock

  6. Is it true that there’s a difference between European sunscreen/sunblock and American? Seems like I remember hearing a story on the news about this. Which irks me, because we should all be receiving the same quality protection worldwide!

  7. sarahmath97 says:

    Thank you for following my blog about self esteem. Your blog about sunscreens and how to understand SPF ratings. was a shocker. I did not realize I needed sunscreen to protect myself against both UVA and UVB ultraviolet rays. I have fair skin and I get sunburned easily. One of the things I do to protect my skin is to wear a hat.

  8. The only sunscreen I use now is Red Raspberry oil – undiluted. It supposedly has a protection factor of around 25 and I’ve used it since the Spring and haven’t burned at all (I’m fair skinned and live in the South of France). I love it because I hate putting chemical sunscreens on my skin…

  9. So helpful! Thanks for this. I’m also really curious about things I’ve heard through the grapevine about SPF/sunscreen not being safe for kids in particular. I know about the parabens thing but I think it’s something else. Maybe the SPF itself? I’m not sure, but several parents I know request that sunscreen not be put on their children. I’ve been curious why but don’t have the time or energy to do the research and thought you might know, being such a sun protection expert and all.

  10. evestepulremon1978 says:

    Reblogged this on Ginny Bauman Posts.

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