it's michelle from lab muffin beauty science chemistry phd skin care nerd
and sunscreen connoisseur
i've had a lot of people ask me to explain the nitty gritty behind how sunscreen ingredients work
and i warned everyone it would be super nerdy and really quite complicated but you all said
it was fine so here it is the fine details the deep dive into how sunscreen ingredients work
if you're more interested in finding a sunscreen that works for you this is probably not the
right video i have a whole playlist of sunscreen videos that i've linked in the description as well
now don't freak out but this is physical chemistry it is upper high school slash
undergraduate level chemistry i'm going to try to explain it very simply so you
should be able to follow it even if you don't remember anything from chemistry
in school i also have a written version of this linked in the video description
so if you prefer to learn by reading or if you zone out and you don't know what's going on
have a look at that as always if you like talking about the science behind beauty
products click the like the subscribe and the nerdification bell so you don't miss any videos
as you all hopefully know sunscreens contain active ingredients that reduce the amount of uv
hitting your skin remember that uv is a type of light it's higher energy and shorter wavelength
than visible light and there are two types of uv that we care about because that's what hits
our skin from the sun uvb which is higher energy shorter wavelength and uva which is
lower energy longer wavelength the shorter the wavelength the higher the energy
there are two main categories of sunscreen ingredients organic and inorganic which is
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 i talk about which ones you should choose in my chemical versus physical
sunscreen videos so if you're more interested in the applications of sunscreen check that one out
i'm going to start by talking about how organic sunscreen ingredients work
overall what they do is absorb uv energy and then convert it to more harmless forms
of energy mostly heat the law of conservation of energy says that you can't create or destroy
energy you can only convert it to 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 to 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 and these electrons depending on which chemical they're in
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 very low energy
it's very stable it's perfectly happy sitting there now if the right type of uv light comes
along the right wavelength the electrons in that sunscreen molecule can absorb that uv
so now it's taken in some energy that sunscreen molecule is very excited so it was down here now
it's up here and it's very energetic excited is actually the technical term for this
things don't like to have a lot of energy so this sunscreen molecule isn't very
stable it wants to get rid of that energy and go back to where it was before
so the electrons in that sunscreen molecule convert that energy to something else
and poop it out the sunscreen goes back to where it was before this is called relaxation again it
is a technical term now that it's back in its lower energy form it's free to absorb more uv
and then convert that to something else and this can happen 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 and 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 and then it releases that energy back out as light that's actually why we have different
colors there are different chemicals inside the fireworks which release different colors of light
now you might be wondering what sorts of energy do sunscreen molecules release then
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 which 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 nothing 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 radiation which turns into heat in your skin visible light or
even lower energy forms of uv the uv can also turn into chemical energy and what that means
is breaking bonds sometimes this is reversible 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 but sometimes it's not reversible so that means you don't get your original molecule
back so your sunscreen slowly decomposes and stops working over time this is what happens when a
sunscreen is photounstable when formulators try to make a sunscreen ingredient more photostable
basically what they're doing is trying to convince that molecule to convert the uv
energy into other forms of energy rather than breaking the irreversible bonds
so with all this talk of heat you might be wondering does your skin heat up the answer
is maybe a tiny bit but the amount of heat is tiny compared to the amount that you're getting
from other things if you're wearing sunscreen and you can feel your skin get hot in the sun
it's probably not the sunscreen in the uv it's probably the infrared light and the visible light
turning into heat in your skin sunlight is only about three to seven percent uv compared to 44%
visible light and 53% infrared your skin also might be irritated by the sunscreen which can
make it go hot but it's almost definitely not the uv you're feeling if you do the conversion
your skin temperature might go up by less than one degree there's probably a bigger
difference between this part of your arm and this part of your arm right now
so organic sunscreen molecules absorb uv but why do some of them absorb uva and some of them absorb
uvb and some of them absorb both let's have a look at their structures the first thing you'll notice
is probably that a lot of them have rings these are pentagons and hexagons they also have this
pattern of having a double bond then a single bond then a double bond sometimes it's in rings
sometimes it's not and this is called conjugation conjugation gives the electrons in the molecule a
special property which is that they can run around a bit this is called delocalization the bigger
your conjugated pattern the more delocalized these electrons are so they can run around more and they
can absorb smaller amounts of energy by default if you have a molecule with no conjugation it can
probably only absorb uvc in other words the energy gap between the lower level and the higher level
matches the uvc range this is completely useless for us because uvc doesn't reach us on earth
if it's a smaller molecule with conjugation then this gap gets a bit smaller and it goes
into the uvb range if the conjugated pattern gets bigger then this gap gets even smaller and we get
into the uva range so you'll notice that the uvb filters tend to be a bit smaller than the
uva filters if you have rings you also get more delocalization which is why you see all these
rings in sunscreen chemicals if you make this gap even smaller then it's going to absorb visible
light and this is actually how you get dyes if you look at dye molecules these absorb some of the
visible light and the rest of the color bounces into your eye and so you see it as a color so this
is acid violet 43 which is what's currently in my hair it absorbs yellow and so you see it as purple
so apart from conjugation other things on the sunscreen molecule can also change the amount
of delocalization oxygen and nitrogen atoms can add extra electrons so this extends how far the
electrons can run if atoms on the molecule can interact with other atoms and form rings this
also increases the area where these electrons can run so for example let's look at avobenzone
avobenzone has two forms and these can turn into each other if you look at the conjugation pattern
this one has a much smaller one you can see that it breaks in the middle where we have two single
bonds next to each other but in this version we have a much bigger pattern it pretty much takes
up the whole molecule so this version which has less delocalization absorbs a shorter wavelength
this one which has a bigger delocalization pattern absorbs a longer wavelength
this whole idea where the wavelength of uv that a particular sunscreen molecule absorbs is linked
to the amount of delocalization also explains a lot of the things that people keep on saying
about the importance of the formula for example if you have an acidic filter at a high pH it gets
a negative charge this means that there's more electrons more delocalization so it will absorb
a longer wavelength if you have a basic filter these usually have nitrogens on their structure
if you put them at a low pH they'll actually get a positive charge and this shortens the
amount of space that the electrons have to run around and so they'll absorb a shorter
wavelength solvents can also interact with either the lower state or the higher state and this will
lower their energy if the solvent interacts with the lower state that's called the ground state
that lower state will decrease in energy and so that gap gets bigger that means that the
sunscreen molecule will absorb higher energy uv a shorter wavelength if it
interacts with the top state the excited state then that will get lower and so
now you get a smaller gap it will absorb lower energy longer wavelength for octinoxate its
main uv absorbance can actually change by 20 nanometers just by changing the solvent
so let's ungracefully switch for a moment to these graphs that you might have been seeing around
each line shows the uv absorbance for a specific sunscreen ingredient i'm going to get rid of
everything except avobenzone for a moment the way we read this graph is the horizontal axis is
the wavelength as we go from left to right the wavelength gets longer and so we're going from
higher energy uvb to lower energy uva the vertical line is absorbance and so the higher
the line is the more of that wavelength it's absorbing so avobenzone absorbs some uvb it
absorbs a bit more uva it reaches a maximum at 357 nanometers so it's absorbing the most there
then it drops down again as we keep on going through the uva region
that's why we call it a uva filter it does absorb a bit of uvb but it mostly absorbs uva
a broad spectrum ingredient means that it absorbs more wavelengths and so you'll see
a fatter chunkier curve tinosorb s's curve for example looks like this so it absorbs both uva
and uvb it is a broad spectrum filter now you might be wondering if a sunscreen molecule
absorbs uv that matches the energy gap and we only have one sunscreen ingredient why is it
absorbing all these wavelengths shouldn't it just absorb one single wavelength shouldn't we get a uv
dot rather than a uv curve why is it absorbing 357 and 360 and 350 nanometers why isn't it just
absorbing one single wavelength that's because in real life molecules move around and in a single
drop of sunscreen you have billions and billions and billions and billions of sunscreen molecules
so there'll be billions that look like this and like this and like this and like this
and all of these will have slightly different delocalization and so they all have slightly
different energy gaps and so all of them will absorb slightly different wavelengths so the
energy gap diagram actually looks a bit more like this there are lots of different energy
gaps possible and some of these are more likely to happen than others so there's more
of the 357 nanometer transition there's less of the 350 less of the 340 and so on and this
happens with every sunscreen ingredient which is why we get lots of different curves rather
than lots of dots all of these molecules absorb lots of different wavelengths in different amounts
now some of these curves are a bit more complicated so tinosorb s for example it
has a double hum it's because it actually has two overlapping humps tinosorb s and a lot of
the newer sunscreen filters as well as some of the older ones have more complicated structures so if
you look at their delocalization systems they look a bit more 2d rather than 1d they look more like
a shape rather than just a line so there's more than one possible
electronic transition that can happen there's one in the uva and one in the uvb region both
of these transitions are again a bit fuzzy and so again you get two curves rather than two dots
now if we look at the curves of different sunscreen filters you'll see that some of these
are higher and some of these are lower the higher the line the more it absorbs in that particular
wavelength so if it's high in the uvb it absorbs uvb more strongly if it's higher in the uva it
absorbs uva more strongly these are all comparing the same mass concentration of the sunscreen
they're all one percent in ethanol so this is very roughly reflected in the actives in an actual
sunscreen product but you do have to remember that these curves don't completely reflect what happens
in a sunscreen product these are for the sunscreen filters dissolved in ethanol when you actually put
a sunscreen on your skin the ethanol evaporates pretty quickly the sunscreen also dries down
into a film rather than floating around in liquid which is what's happening in these curves and so
that also changes the optical properties of the film and remember there are also lots of things
like pH and solvent that could change those energy gaps and so that means that the curve might shift
to the left might shift to the right might shift up and down a bit it gets even more complicated
if you put more than one filter into a single sunscreen if you have a uvb filter and a uva
filter then having them both in there means that you can get a broad spectrum curve so that means
for the overall sunscreen the final curve will look very different from these it'll be sort of
like you add them together when i say add together i don't mean you can just sum them up the final
curve can be a bit tricky to predict the filters might interact synergistically so the curve ends
up higher than you'd expect they might change each other's delocalization and so the curve
might shift left and right there's lots and lots of possibilities that you can't really predict
again these curves are for one percent in a solution not whatever percentage in your actual
sunscreen product if you have a higher percentage the curve will go up because you have more
sunscreen molecules on your skin the sunscreen molecules can absorb more uv and so you have
higher absorbance another thing to notice is that it doesn't matter how many filters you have
to build that final curve what matters is just that final curve i've seen some skin care science
influencers say that you need more filters for redundancy but that's not how uv absorbance works
it's like saying that a 2 meter ladder made of steel aluminium and wood
will get you higher than a two meter ladder just made of steel
they both get you two meters it doesn't matter what materials they're made of
now to the inorganic sunscreen zinc oxide and titanium dioxide back in the day the cosmetics
industry thought that they worked just by scattering uv it was actually written into
the 1978 fda legislation and then taken out when they revised it in 1999 so that's probably why
this is so widespread but scientists have known that it works by mostly absorbing uv
for quite a long time and 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
then when those electrons relax back down that uv energy converts to more harmless forms
so heat or light or lower energy uv or infrared but zinc oxide and titanium dioxide are inorganic
that means they don't have carbon atoms in their structure so they don't have the same ability to
make conjugated systems instead what they have is called a bandgap structure so here is what zinc
oxide looks like there are a lot of atoms packed closely together which means lots of electrons and
so those electrons can interact with each other so we get lots and lots of different energy levels
for lots of different electrons so instead of a low energy level and a high energy level you have
a low energy band made up of lots of levels and a high energy band made up of lots of levels the
smallest gap between them from the highest low energy level to the lowest high energy level is
the minimum absorbance so the smallest energy gap that can be absorbed so that's the minimum
energy or longest wavelength they can absorb and they can absorb pretty much every uv energy level
higher than this so every shorter wavelength of uv and that's because of that band structure the fact
there's so many different levels there's lots of different energy transitions that can happen
so that's why the absorbance curves look like they suddenly go off a cliff
that cliff matches the shortest energy gap now this absorbance explains about 95% of how they
work the other approximately 5% is scattering scattering is when the uv comes in and the
particle changes the direction that it's going in it doesn't actually usually bounce backward
it usually bounces forwards so it keeps on going but at a different angle so a lot of
the time uv light actually gets scattered towards your skin which is actually okay
because it means that it takes a longer path it's not taking the shortest path anymore
and that means there's a higher chance of that uv hitting another particle and getting absorbed
now inorganic sunscreens are pretty complicated because you can get different particle sizes
in sunscreens the particle size is usually really small less than 200 nanometers that's because
scattering is proportional to diameter the bigger it is the more it scatters the less it absorbs
it's not only uv that gets scattered it's also visible light and so the larger the particle
the more scattering you get the more visible light gets scattered into your eye from the sunscreen
and you see it as a white cast there's also the fact that only atoms at the surface of the
particle can absorb uv if you have a big particle and you break it down into smaller particles you
get more surface exposed and so you get greater uv absorbance longer wavelengths also get scattered
more than shorter wavelengths so even though the particles are mostly absorbing the uv
they can also at the same time be scattering visible light zinc oxide absorbs both uva and uvb
titanium dioxide mostly absorbs uvb and mostly scatters uva
but it does depend on the particular particle size of the sunscreen
i'm going to leave it there if you got to this point congratulations well done
i am very proud here is your prize the cutest thing i've seen in a long time
these two penguins both had their partners pass away but
every night they meet up to watch the city lights and comfort each other for hours
if you have any questions drop them down below and i'll try my best to answer them if you like the
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and check out my blog for more nerdy beauty content see you next time to nerd out more