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Sunscreen: Physical, Chemical and… DNA?

August 23, 2017



In this ridiculously sweltering sun, we Aussies should really use more sunscreen than we do.


From the poolside to the beach, most of us slap on the stuff in a half-hearted attempt to save ourselves from a painful week of blistered skin. Or perhaps to sate that nagging voice in the back of our heads - probably that of a primary school teacher - reminding us to “slip, slop, slap.” But how does sunscreen work in protecting you from the summer sun, and what’s in it?


Firstly, let’s talk about what we’re trying to protect against.


The ultra violet light rays that cause us damage are part of the broader electromagnetic radiation spectrum, which includes x-rays, infrared, long radio waves, and the visible light spectrum (what we see as colours). Light travels in waves like the rest of the spectrum. A general rule is ‘the shorter the wavelength, the higher the energy’. With this in mind, let’s have a quick run-over the UVA and UVB rays we hear so much about.


UVA rays have a wavelength of 315 to 400 nanometers (or 10-9 meters), while UVB rays have a shorter wavelength of between 290 and 315 nanometers. According to the Australian Cancer Council, both are classified as “human carcinogens”, which cause cancer in us humanoids. Not ideal. There are also UVC rays, which have a wavelength of 290-100nm. Even though they’re more deadly than UVA and UVB rays, UVC rays don’t get talked about much because they’re filtered out by the Earth’s ozone layer, which affects how much and what kind of UV reaches our skin, and so don’t reach us in enough magnitude to do us much harm.


Now that’s out of the way, we can talk about sunscreen! There are two main types of sunscreen: physical and chemical (though most types available are a blend of both). The names ‘physical’ and ‘chemical’ are a bit misleading since really every sunscreen is a chemical simply by nature. Another way to classify them is by whether they contain organic or inorganic compounds. Don’t run away screaming! Just let me explain.


Chemical sunscreens are the ones made up of organic compounds. These are basically molecules that contain a carbon atom somewhere, usually bound to a hydrogen atom. Some organic compounds work to absorb UVA rays, and some absorb UVB. However, to kill two birds with one stone, chemicals like oxybenzone can be used protect against UVA and UVB rays, making it one of the most common chemical components you’ll find in your sunscreen.


Compounds like oxybenzone work because their absorption of UV light corresponds to the wavelengths we want to protect against - those that make up UVA and UVB rays. It’s a pretty cool little hydrocarbon, if you ask me.

What all those tongue-twisting compounds have in common is that they all absorb solar radiation. They also take about 20 minutes after you apply them to be effective, which is why most sunscreen bottles list a half-hour wait time before going out in the sun after application.


A physical sunscreen, as you can probably guess,  is one that creates a physical barrier between your skin and the sun, blocking and deflecting ultraviolet radiation (which is what sometimes makes your skin look white or chalky straight after application). These are those that feature inorganic compounds, that is, there is no carbon in sight. You can probably now see why ‘chemical’ and ‘physical’ classifications make some sense, despite being a bit misleading. You might also hear physical sunscreens being referred to as ‘mineral’ sunscreens, as the two main active ingredients in physical sunscreen are zinc-oxide and titanium-oxide. However, many companies prefer to use zinc-oxide as it’s gentler for those with sensitive skin. You don’t have to wait for a physical sunscreen to be effective like you would a chemical sunscreen, because it acts instantly, much like putting on a piece of protective clothing.


Lots of people worry about nanoparticle (0.000001 of a millimetre) sunscreens, concerned that their bodies might absorb the particles with ill-effects. There are plenty of papers out there on this topic (search: “sunscreen” and “nanoparticles” in Google Scholar, if you dare to be confronted with a whole lot of fun jargon!). Luckily, the Cancer Council is here again to allay your doubts and save the day. In a 2015, they reported that “drawing on the best available evidence… nanoparticles used in sunscreen do not pose a risk.”


Now for a discovery that’s really exciting - something groundbreaking on the unsafe, and ozone-depleted horizon which could totally revolutionise the field - DNA sunscreen. Published in a paper last month by a team from Binghampton University in New York, this sunscreen acts like a second skin – almost like a “sacrificial skin”,  as described by the science editor at the Telegraph, Sarah Knapton. It’s particularly cool because the more it is exposed to UV light, the better it becomes at absorbing it!


This second skin is basically a transparent film of water, ethanol, and the DNA of Salmon (I know it sounds a bit fishy). Not only used in sunblock, it also has huge possibilities in the medical world for treatments of wounds. Additionally, the cosmetic industry may be looking a bit worried as the film acts as a barrier to help the body retain moisture (but this new moisturiser will definitely make your skin as flawless as Cate Blanchett). This would be a sunscreen that you could apply once, and it would last the rest of your life!


I’ll leave you with the typical spiel about being sun-safe you’ve likely heard a million times before - only this time with some rock-hard maths to beef it out a bit: regardless of what type of sunscreen you use, the general recommended amount of sunscreen is 2mg per cm square of skin. Before you get your microscales and rulers out, just make sure you’re putting on a generous coat every two to three hours (or immediately after you’ve dried off with a towel)!.


Until next time - stay sciency, be sun-safe and embrace curiosity!

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