What Parts of Sunlight Hurt Us?

After a rainstorm, when the sun begins to shine, have you ever looked to the horizon hoping to spot a rainbow? Sunlight, which usually looks transparent to us, is broken apart and we can see individual components of its light spectrum. The result? That beautiful arc in the sky with its gradation of colors, from red at the top to violet at the bottom. And that’s only the wavelengths our eyes can see - a fraction of what’s coming at us!

Sunlight contains many more parts than what we can see in rainbows. So, if there are so many different parts of sunlight, how do we know which parts help us and which are dangerous to our skin?

What Are The Parts Of Sunlight?

The gradation of colors in a rainbow shows us that sunlight is a spectrum. The part of the spectrum we see in a rainbow is called visible light - wavelengths from 400 nanometers (violet) to 700 nanometers (red). Nanometers are extremely small - there’s one billion in a meter! To give you a sense of how small the wavelengths are, a single human hair is about 75,000nm wide, or over 100 times longer than red light.

The type of sunlight that is most dangerous to our health is light less than 400nm, which includes ultraviolet (UV) light (290-400nm). The energy content of a light wave varies with wavelength. Shorter waves carry more energy than longer waves. Higher energy (shorter) waves can be more damaging to our skin than lower energy (longer) waves.

What Does UV Light Do To Our Skin?

UV light is the part of light that causes skin cancer and skin aging (1).  Visible light and longer wavelengths (infrared 700 nm and up) are not known to cause cancer (2). There is some evidence suggesting that infrared light may play a small role in skin aging, however we'll go into this in depth in a separate article to come (3).  UV light has a lot more energy than visible light - enough energy to damage proteins and DNA in our cells.

 A general framework we find helpful is (4):

What Is UVA vs. UVB Light?

Within UV light, there are two general subtypes that make it to the Earth’s surface: UVB light, (290-320nm) with more energy, and UVA light (320-400nm) and less energy than UVB but more than visible light.  Shorter UVB can only penetrate the superficial part of our skin,  but its high energy means it does a lot of damage. UVB plays a key role in developing skin cancer, sunburn, and also contributes to skin aging (4).

In contrast, longer UVA can penetrate into deeper layers of our skin, but has less energy than UVB. The type of damage UVA does leads to photoaging (wrinkles, sunspots and leathery skin). UVA is also responsible for tanning and plays a lesser role than UVB in cancer, but it still contributes (4). Just how these types of UV radiation damage skin will be the topic of our next article.

So the next time you look at a rainbow, remember that beneath the bottom violet color is UV light we can’t see, but it's bombarding our skin every day. That's the part of sunlight we most need to protect ourselves against.

What do you think? Does this help you better understand what UV radiation is? How do you stay protected? Put your questions in the comments; I'd love to discuss!

1. Gonzaga ER, Role of UV light in photodamage, skin aging and skin cancer : importance of photoprotection. Am J Clin Dermatol. 2009;10(Suppl 1) 19-24
2. de Gruijl FR. Action spectrum for photocarcinogenesis. Recent Results Cancer Res. 1995; (139) 21-30
3. Cho S, et al. Effects of infrared radiation and heat on human skin aging in vivo. J Investig Dermatol Symp Proc. 2009;14(1) 15-9
4. Skin Cancer Foundation. UVA& UVB,  https://www.skincancer.org/prevention/uva-and-uvb