Sunscreen Dialog: What is UV, anyway, and why is it bad for you?

Everyone talks about UV light and how it is bad and how I need to wear a sunscreen, but I don’t even know what UV is.

Honestly, seems right. Sometimes it is easier to just enforce the behavior without having to go into the details, you know?

Totally, but I’m interested. What exactly is UV?

Good question. You know the sun?

Um, yes.

What does the sun do?

It fuses hydrogen into helium and makes energy for the earth.

Right. And how does earth get that energy?

From light.. ?

Sure. and what is light?

I’m already regretting asking this.

So physically, “light” as we see it, refers to a specific set of wavelengths electromagnetic radiation. The sun releases a lot of electromagnetic radiation, and at varying frequencies. Some frequencies are lower and longer, like radio waves or microwaves. There are some frequencies of radiation that our eyes are able to interpret as visible, which allows us to see things. There is also radiation with a frequency that is too small for the eye to interpret, like X rays and gamma rays. The shorter the frequency is, the higher the energy of the radiation.

Can you just show me a chart?



As the above chart demonstrates, light, as we see it, is a small portion of the radiation emitted by the sun. There are wavelengths that are too big to see (radio through infrared rays) and wavelengths that are too small to see (ultraviolet through gamma radiation). The smaller the wavelength, the higher the wavelength’s energy.

X Rays and Gamma rays are superlatively strong, capable of blasting atoms apart. Fortunately, the Ozone layer in Earth’s atmosphere protects us from radiation that is smaller than 280 nanometers. Another reason to seek to protect the environment!

For convenience, I will block those wavelengths off the chart.


I see that. What does this have to do with sunscreen?

Nothing yet. I am seeking to provide the necessary context for a good understanding of sunscreen.

Let’s look at the spectrum of visible light through UVB radiation:


A few things to point out, here: First, as the radiation’s wavelength decreases, its energy per photon increases. So, the shorter the wavelength, the fewer wavelengths at that frequency one needs to encounter to cause damage. UV has higher energy than visible light: in fact, it is energetic enough to cause damage to DNA, which triggers immune responses.

But violet light is right next to UV, and they are pretty similar amounts of energy. Shouldn’t I be worried about that too?

Well, yes, actually. There is a lot of research lately about High Energy Visible Light (HEV) which has linked blue & violet light to skin damage, as well. However, it is worth writing that just because they have nearly as much energy as UV doesn’t mean they do exactly the same thing. Indeed, one needs to be exposed to a larger amount of blue / violet light than one does UV light to see a reaction.

I think the point here is that understanding this provides a good framework for understanding why different kinds of radiation impact the skin differently: they have different amounts of energy, so generating a reaction to any individual wavelength will depend on different amounts of radiation. UV has more energy than visible light, so one needs to encounter less of it to have a reaction.

I also want to underscore that UVA and UVB are convenience terms used to group wavelengths that behave similarly. However, they aren’t one thing. Saying a sunscreen “blocks UVB” would mean that it is an effective filter for every frequency between 320 nanometers and 280 nanometers. For most sunscreen filters, this isn't exactly the case.

So we see that UVA and UVB are not individual things, they are groups of individual wavelengths.

It’s important to understand this concept of a spectrum, as it is often used in misleading ways. We often see sunscreen filters presented as UVA filters and UVB filters, and while this is good shorthand, it doesn’t tell the whole story. Most sunscreens don’t block UV as a yes/no: they are most effective at blocking specific wavelengths of radiation.

Okay, so what you are saying is that saying a filter “blocks” UVB doesn’t tell the whole story?

Generally, yes, because most filters don’t absorb all wavelengths in a group as well as each other. Have you ever seen a chart like this?


Yes, what does that mean?

read on.