Sunscreen 101: What are Sunscreens and How Do They Work?

Sunscreen 101, how sunscreens work, and their uv range
Sunscreen 101:  What are Sunscreens and How Do They Work?
As a companion piece to my new weekly Anti-Aging 101 series on Mondays, I've decided to make Tuesdays Sunscreen 101 days. Obviously there is a lot less to talk about with sunscreens in terms of technology, and there aren't the wide array of active ingredients that you'll find with anti-aging products. Which means only a few weeks of "how things work" posts, and then I'll be concentrating fully on Tuesday sunscreen product reviews.

So, most of these posts you've seen before, but I will be updating them a bit. This week I'll go over what sunscreens are and how they work. Next week I'll discuss proper use of sunscreens and how to pick one that works for you. Watch for a new sunscreen review every Tuesday, and perhaps even more often if I have a lot to review.

how do sunscreens work to protect your skin?
So, what's the deal with all the different types of UV radiation? It's all emited by the sun, but that doesn't mean that it affects us. UVC radiation actually never even reaches the earth's surface, it's all filtered out by our atmosphere. So, any UVC exposure we get is all from artificial sources. We're going to chose to ignore the visible spectrum and the infrared (IR) spectrum since we can't protect against it and I'm not even sure if anyone knows much about it's affects on the skin (but, that doesn't mean we should totally ignore it in our real life, that's what sun exposure avoidance is for. I'm just choosing not to talk about it in this post). We're worried about UVA and UVB exposure here.

When sunscreens were first developed they were against UVB rays, since this is what causes the redness we associate with a sun burn. So, what do each of these ranges really do to skin?
• UVB: Skin redness and burning
• UVA: Lowers skin immunity, increasing cancer risk, penetrates to the dermis, contributes to aging via increased skin thickness and increased skin inflammation, tanning of the skin
• Both: Induces DNA damage in skin, increasing cancer risk

What is SPF?
SPF stands for Sun Protection Factor, and yes, a SPF really is multiplied by how long your skin usually takes to burn. So, if you usually take 10 minutes to burn, with well applied SPF 30 that equals 300 minutes. Everything is calculated on delaying the redness of a sunburn.

I recently found another way to think about it on The Skin Cancer Foundation website:
• SPF 15 blocks 93% of all incoming UVB rays
• SPF 30 blocks 97% of all incoming UVB rays
• SPF 50 blocks 99% of all incoming UVB rays

Note that as of my writing this post, there is no accepted international way to test and label products for UVA coverage. In fact, there is no current method of doing this in the US at all (though the FDA has proposed a 4 star rating system, it has not yet been approved). It is up to the consumer to check the label for sunscreens that provide UVA coverage.

How do Sunscreens Work?
There are two main types of sunscreens, physical and chemical.

• Physical: These sunscreens really work by blocking the light from ever reaching your skin. The light is reflected or scattered away from the skin. These ingredients are less likely to irritate skin or create an allergic reaction. However, they tend to create a white cast on the skin, contribute to a product having that "sunscreen" feeling, and they can stain clothing.

• Chemical: These sunscreens absorb UV radiation rather than reflecting it away from skin. To do this, the energy of the light has to go somewhere, or do something. Depending on the ingredient, the light can be used in a chemical reaction, can slightly alter the chemical structure of the sunscreen itself, be given off as heat (I'm not sure that any sunscreens approved in the US actually do this), or be released as a longer UV wavelength. While these chemicals don't feel as heavy as their physical sunscreen counterparts and can cover a wide range of UV wavelengths, they also have their drawbacks.

Chemical sunscreens can be unstable in certain combinations or with prolonged sun exposure (hence we docs tell you to reapply often). They are implicated more often in skin irritation and allergic reactions. And they can even be absorbed into the skin, and picked up in your urine (This is true, I had no idea this was the case until I was revising this post and found it in a cosmetic dermatology text book. I was taught in my general pediatrics training to tell parents to use only physical sunscreens on their infants/toddlers, my guess is that this is the reason. I'd avoid them if you are pregnant as well.)

A Word on Photostability
There's a lot of discussion about photostability if you head to on-line makeup and skin care forums. I'm a long-time member of one such forum, and it amazes me when the members there dismiss certain products because "we all know that those ingredients aren't stable together." I've seen some members contradict their edicts a few months later, with statements about how Octinoxitate should be (and then should not be) used in combination with certain other ingredients to improve photostability. I have no idea where they get this information, because it certainly isn't in the published medical literature for every product.

Remember back in the chemical sunscreen discussion when I explained that with prolonged sun exposure the chemical sunscreen ingredients may breakdown and no longer protect skin. The photostability of a sunscreen product is altered by the combination of sunscreens used, the solvents used in the product and the vehicle used in the product (which is different than the solvent). So, there are a lot of factors.

While we can make some overall statements about photostability based upon combinations of sunscreens (such as the fact that Helioplex is a more stable version of avobenzone), it is virtually impossible to know how photostable a product is unless you are a developer or have tested the product in a lab. I for one would love there to be a rating system for photostability that would appear on the product just like the SPF and a UVA filter rating. Until this happens, I would ignore information about photostability found on-line unless you've obtained it directly from the manufacturer.

Next week I'll discuss different formulations of sunscreen (spray vs gel vs powder), deciding which formulation is right for you and the proper use of sunscreens.


  1. This is an awesome series! Keep it up! I always feel guilty grilling my derm about specifics in my sunscreen and this is a great way to figure out exactly what works for me without bugging him hahaha.

  2. Can you do a review of Neutrogena's new Ultra Sheer Liquid daily sunblock? Comes in SPF 55 and 70.

  3. Anon- It's already upstairs in my pile of sunscreens to try out!


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