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UVB has a shorter wavelength and more energy than UVA, whereas UVA has a shorter wavelength and less energy. The energy increases with decreasing wavelength.
There are two main categories of sunscreen ingredients: organic and inorganic, which are sometimes called chemical and physical. Even though those aren’t really good descriptors.
Organic sunscreens are the ones with complicated names, and they contain lots of carbon atoms joined together. Inorganic sunscreens, which are sometimes called mineral sunscreens, are zinc oxide and titanium dioxide.
ORGANIC SUNSCREENS
Organic sunscreens absorb UV light and then transform it into mostly heat-based sources of energy that are safer. You cannot generate or destroy energy, according to the rule of conservation of energy. It is only capable of transformation into other forms.
So that’s exactly what these sunscreen molecules do. They’re like little factories that take in the UV energy. And convert it into something that isn’t going to make your skin massively age or give you cancer.
The way that they do this is with electrons. Chemicals all have electrons inside them. These electrons depend on which chemical they’re in. It can absorb different types of energy and convert it into other forms.
So you can imagine that your sunscreen molecule is sitting on your skin. It’s low energy, very stable, and perfectly happy sitting there. Now, if the right type of UV light comes along at the right wavelength, the electrons in that sunscreen molecule can absorb that UV.
Consequently, that sunscreen molecule is quite interesting now that it has absorbed some energy. I dislike being very energetic. The molecule in sunscreen is therefore not particularly stable. It desires to expel that energy and return it to its original position.
The electrons in that sunscreen molecule convert that energy into something else and release it. The sunscreen goes back to where it was before. It's called relaxation now that it’s back in its lower energy form and is free to absorb more UV. Then convert that to something else, and it happens again and again.
This is actually a similar process to why black things get hot in the sun. Black dye is really good at absorbing visible light, so it absorbs that light. The electrons in it get excited, and then it releases that energy back out as heat.
Fireworks also work this way, so the electrons and fireworks absorb the heat energy from the explosives. Then it releases that energy back out as light.
That’s why we have different colors. There are different chemicals inside the fireworks that release different colors of light. Now you might be wondering what sorts of energy sunscreen molecules release.
It depends on which sunscreen chemical you’re talking about, but it is usually a combination of different types of energy. It can be vibrations that turn into heat. As those vibrations knock into other molecules and those vibrations get passed on, heat is really just vibrations.
It can be lower-energy forms of light, so remember, you can’t create energy out of anything. So, that sunscreen molecule might convert part of the UV into a lower-energy form of light and the rest of it into heat.
Sunscreens can release infrared, which can heat your skin. Visible light or even lower-energy forms of UV. UV can also turn into chemical energy, and what that means is breaking bonds.
This is reversible sometimes, so the sunscreen molecule can keep working. So, the bond breaks and unbreaks as it converts the UV energy into chemical energy, just like we talked about before.
However, occasionally it's irreversible, so you don't get your original molecule back. As a result, your sunscreen gradually loses its effectiveness. When sunscreen is photo-unstable, this occurs.
when manufacturers attempt to increase the photostability of a sunscreen component. Instead of destroying irreversible bonds, convert UV energy into different types of energy.
INORGANIC SUNSCREENSThe inorganic sunscreens are zinc oxide and titanium dioxide. Back in the day, the cosmetics industry thought they worked just by scattering UV. It was written into the 1978 FDA legislation. Then taken out when they revised it in 1999. That's probably why this is so widespread.
But scientists have known that it works by mostly absorbing UV for quite some time. Scientists keep pointing it out in peer-reviewed papers, but it’s just one of those myths that won’t die. The way that zinc oxide and titanium dioxide work is very much like an organic sunscreen.
So, they have electrons that can absorb UV and get excited into a higher state. When those electrons relax back down, that UV energy converts to more harmless forms. heat, light, or low-energy UV or infrared
When UV light enters, the direction of the particle changes, causing scattering. Typically, it bounces forward rather than backward. So, everything continues, but from a new perspective. Because inorganic sunscreens come in a variety of particle sizes, they are rather complex.
Less than 200 nanometers is the typical size of the particles in sunscreens. This is due to scattering's relationship to diameter. It absorbs less and scatters more light as it becomes bigger.
Longer wavelengths also get scattered more than shorter wavelengths. So, even though the particles are mostly absorbing UV, they can also scatter visible light.
Zinc oxide absorbs both UVA and UVB; Titanium dioxide mostly absorbs UVB and scatters UVA. But it does depend on the particular particle size of the sunscreen.