SPF Club Handbook: Volume 1
wavelengths & the electromagnetic spectrum
Today’s post is part of Summer Skin School! A monthlong series of educational content, interviews and recommendations to help you and your skin thrive as the days get hotter.
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Hello hello!
Summer Skin School is back in session and there’s no time to waste.
I’ve been working on this SPF Handbook for months and I am so proud to finally start to share it with you. I started by compiling basically every question I’ve ever been asked about sunscreen, answering them with my gold standard of “a bit watered-down but also like I’m an adult with brain cells and enough of an interest in skincare and science to follow along if you give me a chance and explain it logically in bite-size chunks.”
While doing this, I found myself jumping back and forth between answers, referencing the same concepts in different applications. I realized that everything I wanted to explain could be categorized under a few Big Ideas and understanding these Big Ideas would help all the little-r ideas make sense. So that’s how I’ve organized this handbook.
This handbook has multiple volumes that will be published over the next few weeks. I’ll also be publishing a separate list of product recommendations soon, don’t worry!
Today’s Big Idea is Wavelengths & The Electromagnetic Spectrum! Here’s what we’re covering today and a preview of the upcoming issues:
SPF CLUB HANDBOOK TABLE OF CONTENTS
BIG IDEA: Wavelengths
The electromagnetic spectrum
UVA vs. UVB
What is a tan?
Blue Light/HEV
Deciphering the Bottle
SPF
Broad Spectrum/PA Ratings
Water Resistance
Filters
Using & Applying Sunscreen
Myths & Other Stuff
But first…why sunscreen?
The sun is what allows us to exist on this planet, but it is also trying to kill us. UV rays are a known human carcinogen, classified as a Group 1 cancer-causing agent, along with things like plutonium and cigarettes. UV rays are also the primary factor involved in photoaging, such as wrinkles, laxity & discoloration. Sunscreen helps to reduce the amount of UV that enters your skin. Simple as that.
But if that somehow doesn’t convince you, here are some facts to set you straight:
Sun damage is cumulative over your lifetime. Approximately 25% of the total sun damage in your lifetime occurs by age 18, with about 10% more accruing each decade.
Skin cancer is the most common cancer in the US and UV exposure is associated with the vast majority of cases. It’s the most preventable risk factor.
While skin cancer is most common in non-Hispanic white people, it can affect all skin tones. People of color are more likely to have delayed diagnoses, making treatment difficult and mortality rates higher among these populations.
An estimated 80% of skin aging is due to the sun and this risk applies to all skin tones.
Once again, sun damage is cumulative over your lifetime.
BIG IDEA: WAVELENGTHS
Understanding this big idea will help you understand what sunscreen is actually protecting against, the effects of UVA vs. UVB radiation and what that means in terms of the protection you see on a bottle of sunscreen.
The Electromagnetic Spectrum
The electromagnetic spectrum is the range of all forms of energy, which all have their own defining wavelength. This includes radio waves, microwaves, light waves, x-rays and gamma rays. Within the light energy, you can find infrared light, visible light (which we can see) and ultraviolet light.
Longer wavelengths have lower energy, while shorter wavelengths have higher energy. These waves move through air, space and solid objects, which I know sounds crazy and is impossible to visualize because most of them you cannot see, so you just need to take a deep breath, accept it and move on.
Zooming into the light energy portion, you can see that ultraviolet light is divided into 3 types. UVC is filtered out by the ozone layer, so we don’t need to worry about it. (Note that the ranges go from big to small since the wavelengths are decreasing in size.)
Sunscreen contains active ingredients known as filters, which help to prevent the wavelengths from entering your skin.
UVA (Ultraviolet A) vs. UVB (Ultraviolet B)
UVA: A = aging
Lower energy, longer and penetrate deeper into the skin (315-400nm)
Further divided into UVA1 (340-400nm) and UVA2 (315-340nm)
Accounts for about 95% of the rays that reach our skin.
Primarily responsible for photoaging, such as damaging collagen and elastin, stimulating pigment formation and creating highly reactive free radicals that wreak havoc on healthy cells.
UVA rays induce immediate pigment darkening aka tanning.
UVA can penetrate through glass, windows and clouds. It is present and strong all year round, even on the rainiest, cloudiest days.
UVA can cause burning and is linked to cancer, though not as directly as UVB.
UVB: B = burning
Higher energy, shorter wavelengths that don’t penetrate as deeply into the skin (280-315nm).
Primary cause of burning (in as little as 15 mins!) but can cause delayed tanning.
UVB rays directly damage DNA and are most strongly linked to skin cancer.
UVB levels fluctuate throughout the year. They’re strongest in the spring and summer and from 10am-2pm. UVB does not penetrate glass or windows.
What is a tan?
There is no such thing as a healthy tan or base tan. The very presence of a tan indicates sun damage. When unprotected skin is exposed to UVA, it triggers the production of melanin (pigment granules) in an effort to protect what’s inside your cells from damage. A tan is literally your body telling you that it is in defense mode. Melanin is part of what forms our skin color, so darker skin with more melanin does naturally have some sun protection, but that’s only about SPF 15 in the darkest skin. Plus, this damage is all cumulative. Tanning beds use UVA light, which is how they produce the tan and exponentially increase your melanoma risk, even just using once.
Blue Light/HEV
On the electromagnetic spectrum, blue/purple light is at the end of the visible light spectrum, closest to where the UVA range starts. It’s sometimes referred to as high-energy visible light (HEV). While UV rays are the most damaging light energy from the sun, it does also emit some visible light energy like blue light. In recent years, more studies have been conducted to learn about the effects of blue light and the skin, though it’s still an active area of research and we don’t know everything yet. Here’s what we do know:
HEV can produce free radicals, though much less than UV radiation and it doesn’t seem to cause the same type of damage (Blue Light & The Skin)
HEV can cause pigmentation, particularly in those with darker skin (Fitzpatrick 4-6). This is the biggest potential concern with HEV exposure, though it requires significantly more exposure to happen compared to pigmentation from UV radiation.
You don’t really need to worry about the blue light emitted from screens–it’s a minuscule amount, but if you’re really worried you can sit further away and turn down the brightness. (Blue Light & Screens)
The levels of HEV from the sun vary depending on the time of year and location.
Iron oxides (colored pigments) are able to absorb visible light and can help protect your skin from visible light damage, which can be particularly beneficial for darker skin tones or those prone to pigmentation, like melasma. We’ll talk more about iron oxides in an upcoming post.
Looking at the electromagnetic spectrum, you can start to understand why blue light is associated with pigment, as it’s closest to the UVA wavelength, which is what causes pigmentation.
DECIPHERING THE BOTTLE
SPF
SPF = Sun Protection Factor. The SPF rating only refers to protection from UVB! It tells you much UV radiation it would take to cause skin reddening (sunburn) while using that product compared to not wearing sunscreen (when using the proper amount).
Put differently, if it would take 10 minutes to burn without the sunscreen and 300 minutes to burn with the sunscreen, 300 is divided by 10 to get an SPF value of 30. If it takes 500 minutes, the SPF value is 50. Since UVB levels fluctuate, this does not necessarily mean you can stay out 30x longer, but it does mean you can receive 30x more radiation before burning.
This is a bit tricky to conceptualize, so let’s look at it a few different ways.
SPF 15 = allows the skin to receive 15 times more UV radiation before burning.
SPF 30 = allows the skin to receive 30 times more UV radiation before burning.
SPF 50 = allows the skin to receive 50 times more UV radiation before burning.
Now, let’s look at it in terms of percentages.
SPF 15 = provides 93% protection, allowing ~7% of UVB rays to reach your skin.
SPF 30 = provides 97% protection, allowing ~3% of UVB rays to reach your skin.
SPF 50 = provides 98% protection, allowing ~2% of UVB rays to reach your skin.
So, how much of a difference is this really? It doesn’t seem like much, until you realize that SPF 30 is allowing 50% more rays to hit your skin compared to SPF 50. This is a helpful graphic. (Further reading here.)
Broad Spectrum
When sunscreen was first developed, the dangers of UVA weren’t widely known, so sunscreen (and the SPF value) only included UVB protection. As more was learned about UVA, companies started adding ingredients specifically to protect against UVA rays and the term “broad spectrum” was introduced. Remember the electromagnetic spectrum—this literally means that it protects against a broader spectrum of wavelengths, encompassing more of the UVA range. But, broad spectrum doesn’t necessarily mean it protects all the way up to 400nm. UVA protection requirements vary by region and country.
In order to be labeled as “broad spectrum” in the US, a sunscreen only needs to meet one criteria (the critical wavelength of 370nm, which is complicated and you don’t need to understand but physics nerds can read this). Broad spectrum does mean that the UVA protection is proportional to the UVB protection (so higher SPF = higher UVA protection), but beyond that there’s no way to know the level of UVA protection a product has.
By contrast, in the EU and Australia, in order to get broad spectrum labeling, a product must meet the critical wavelength requirement and UVA protection must be at least ⅓ of the SPF value. Because of this requirement, many sunscreens labeled broad spectrum in the US wouldn’t meet the criteria to be labeled broad spectrum in other countries. Yikes! In the EU, sunscreens that meet these criteria will have the UVA Circle Logo instead of “broad spectrum.”
PPD/PA Ratings
Outside the US, countries use rating systems to indicate the level of UVA protection, based on a product’s Persistent Pigment Darkening (PPD) score. PPD looks at UVA-induced pigmentation (tanning) on human volunteers and calculates a numerical value. You won’t always see the exact PPD value, but in the EU you can usually find it on the packaging.
The Protection Grade of UVA (PA) score is based on PPD ratings. It was developed in Japan and used throughout Asia, but is increasingly gaining global recognition. The PA score will be displayed right next to the SPF value. Though not officially recognized by the FDA, some US brands have started paying for independent PA testing and including it on their products!
PA+ = PPD 2-4 (some protection)
PA++ = PPD 4-8 (moderate protection)
PA+++ = PPD 8-16 (high protection)
PA++++ = PPD 16+ (highest level of protection)
(Further reading here and here.)
Water Resistance
Some sunscreens are labeled water-resistant, which means that they have been tested to maintain their level of protection after immersion in water or sweating, up to 40 or 80 minutes. You still need to reapply after this period, or after toweling off. Importantly, it is illegal to call a sunscreen “waterproof” because there is no such thing as a completely waterproof sunscreen.
There are some companies that have developed technology to increase water-resistance and sweat-resistance, helping to ensure consistent protection levels. For example, Shiseido’s products use their WetForce technology, which increases water resistance when wet, meaning the film becomes stronger and more uniform. You still have to reapply, but you just don’t need to worry as much about your protection being compromised by water or sweat. (There are also some other high protection sunscreens that have been tested to stay effective for hours, which I’ll get into in another post.)
Alrighty, that was a lot, but you did it! Go interpret your sunscreen labels and feel smarter.
If you missed our first lesson in Summer Skin School, get caught up on vitamins here.
Class dismissed! See you next session (or in the comments if you have any questions).
xx,
Jolie
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Jolie the physicist!!! Loved this, sending it to everyone <3
this was a great read! deciphering the bottle was super helpful.