I want to talk to you about sunlight. And I want to specifically talk about what your skin does in response to sunlight, beyond just causing harm.
Let’s start with UV light. So UV light is a spectrum of light that comes from the sun and other sources. It ranges between 200 and 400 nm. if you’re looking at a chart on electromagnetic spectrum (feel free to google it if you’re not driving) then you’ll notice that UV light sits right above visible light in terms of its energy. It makes perfect sense because visible light — like what you need to see things — is in the range of 400 to 700 nm. So, UV light is invisible to the human eye, but it has profound effects on the skin.
There are three sets of UV light. UV C is the shortest wavelength of UV light and is between 200 and 280, UVB is between 280 and 320, and UVA 320 to 400. UV C is more energetic than UVB, which is more energetic than UVC. The energetics of UV also determines how deeply UV light penetrates the skin. For example, UVC barely penetrates the skin, while UVB penetrates slightly deeper, and UVA, the deepest.
Exposure to UV light on the skin has both short term and long term effects and different spectra of UV light have different effects on the skin.
You’re all familiar with the short term effects of UV light on your skin—think of a sunburn. Your skin gets red, swollen, painful, and may even blister. This is followed by characteristic peeling and, if you’re able, tanning.
Long term effects of UV light on your skin are manifold but mainly include aging (think of wrinkles, discoloration, thinning of the skin) and skin cancer formation.
I want to spend a little bit of time on this skin cancer piece because there’s a ton of misinformation out there on some social media channels. Some influencers tell you not to wear sunscreen and to get a ton of sun for better health. While there may be some benefits to sunlight—principally in the form of vitamin D synthesis—long term sun exposure leads to skin cancer. this is a very clearly documented phenomenon. UV exposure leads to DNA damage, DNA damages lead to formation of mutations in genes that either turn on growth pathways that become uncontrolled or turn of tumor suppressor genes that would ordinarily prevent cancer, which ultimately lead to the formation of skin cancer including basal cell carcinoma, squamous cell carcinoma, and of course melanoma. Your skin does have some repair mechanisms and your skin is also invested with a really robust immune system to kill off skin cancers, but long term sunlight and accumulation of these DNA changes and mutations overwhelms the system. Aging does too because as you age, your body’s mechanisms that cope with early skin cancers fail and so age is also a risk factor in getting skin cancer. That’s why the majority of cancers develop in older people, though there are obvious exceptions.
So, please, don’t listen to the instagram influencer who says that sunlight doesn’t cause skin cancer but that sunscreens do—that’s just plain false and you’re putting yourself in danger if you follow that advice.
The last thing I want to talk about is the effect of UV light on your immune system. Sunburn shows the profound effect that UV light has on your skin’s immune function—because a sunburn is basically an exaggerated inflammatory response. Remember, inflammation is simply redness, swelling, pain, and heat—all of which constitute a sunburn. So, UV light has a pro inflammatory effect, meaning it stimulates an immune response. But, it also has an anti-inflammatory / immunosuppressive effect. We know this for a fact because we use UV light to actually treat some skin disorders like psoriasis. If you’re a patient of mine and are listening to this, you might have even been offered narrowband UVB phototherapy for treating your psoriasis, eczema, or itchy skin. That’s because it has an anti-inflammatory effect.
You may be asking yourself—how the heck can this be? On the one hand UV light is pro inflammatory and yet on the other it is anti inflammatory? Great question and truthfully we don’t exactly know why. I don’t want to overwhelm you with details but the immune system that all humans have has two parts. Our current understanding of this paradox is that UV light stimulates the immune system due to activation of the older (also called innate) immune system, whereas the immunosuppressive effect of UV is due to inhibition of the adaptive immune system.
Here’s what I find even more interesting—UV light effects on the skin are not limited to areas directly exposed to UV light. in other words, UV light on your skin has an effect on your whole body. Think about how cool that is — your skin is a light sensing organ that can talk to other parts of your body. This is in part due to the fact that your skin has its own immune system and can release chemical messages like IL-10 which turn on regulatory T cells which are immunosuppressive and can work on your whole body. The corollary to that is that some diseases like lupus are exquisitely sensitive to UV light and a little exposure to sunlight for example on the face of someone with lupus can cause a whole body flare of lupus including activation of lupus kidney disease.
So, think of your skin just like any other sense organ that influences the health of your body as a whole. And that’s what I think is super cool about the skin. Frankly, I’ve known that because there are a plethora of skin diseases that are just windows into the inner workings of the body. In residency, I was a huge fan of a book called skin signs of internal disease written by one of my mentors Jeff Callen who practices in Louisville KY.
We’ll end on this note: long term exposure to UV light has an overall immunosuppressive effect on your skin’s immune system which is the second hit with respect to skin cancer risk because your skin’s immunity normally kills off early skin cancers. When impaired by UV light, you have a greater risk of getting skin cancer.
