How Does Sunlight Affect Your Skin?
Quantum physics defies the imagination. According to the concept of wave-particle duality, every quantum entity has the properties of both particles and waves. In 1905, Einstein showed that light, which had previously been thought of as just an electromagnetic wave, also exhibits particulate behaviour. Gilbert Lewis called these light particles photons in a 1926 letter to the journal Nature, and the name stuck.
Though particles, photons can also be described in terms of characteristics like a wavelength, that are typically used to describe waves and as in the case of waves, the wavelength of photons is also inversely proportional to their energy: the shorter the wavelength the higher the energy and vice versa. Sunlight consists of photons with wavelengths that span the UV, visible light, and infrared segments of the electromagnetic spectrum.
Visible light is largely un-harmful to us and its wavelengths span the 380 - 750nm range. UV radiation however, has shorter wavelengths and more energetic photons that can cause irreversible damage to skin cells. Of greatest importance to us are UVA and UVB rays - UVC rays are blocked from reaching the atmosphere by the ozone layer - that have wavelengths in the 320-400 nm and 280-320 nm ranges, respectively.
When UV photons hit our skin, they are absorbed by various molecules within it. Absorbing the energy of a photon leaves a molecule in an excited, unstable state and to regain stability, the molecule must release the energy that it has absorbed. Physics tells us that energy cannot be created or destroyed, but can be converted from one form to another. Excited molecules often release their energy as heat, but can also release it as chemical energy, i.e. they can take part in chemical reactions in the skin that have biological consequences.
As an example - a very important one in fact - let’s see how UV radiation affects DNA. DNA is a large molecule that quickly releases the energy absorbed by a UV photon as heat. Less than 0.1% of the time however, an excited DNA molecule causes chemical reactions that result in damage. Many chemical reactions can occur, but one of the most common ones is the fusion of two base pairs. This can lead to mistakes when DNA is copied during cell division and these mistakes can change how DNA encodes proteins resulting in the formation of abnormal proteins. Mutations that occur in areas that code DNA repair enzymes or tumour suppressing proteins can subsequently lead to cancer. The link between UV radiation and skin cancer is well researched and the International Agency for Research on Cancer has declared UV radiation to be a Type I Carcinogen.
0.1% might not seem like a lot but let’s look at it from a different perspective. More than 4 x 1016 photons hit our pupil, assuming it has a diameter of only 2mm, every second. 8% of these photons are UV photons and 0.1% of these 8% have the potential to cause damage. This means that every second, you have 3.2 x 1012 photons hitting a mere 2mm radius that have the potential to do some serious damage. While this number may vary and may even be an overestimate, since the intensity of sunlight tends to vary and not all UV photons are absorbed by our DNA, these numbers still help to illustrate that 0.1% of a lot is still a lot.
UV radiation also affects the skin in other ways. It causes tanning as UVA rays induce the production of higher levels of melanin pigment in the skin as the skin tries to protect itself. Through complex pathways, UV radiation also generates free radicals, including reactive oxygen species in our skin. While our skin does have a natural antioxidant network to help neutralise these, excessive sun exposure overloads the skin’s defences so free radicals are able to cause cellular damage. They also damage DNA and collagen and can interrupt cell signalling pathways and gene expression.
Several pathways can lead to the generation of free radicals. One example involves the pigment melanin, which is there to protect us from UV radiation. While melanin normally releases absorbed energy as heat, occasionally a higher energy melanin molecule can bump into, and excite, a more sensitive molecule as it dissipates its energy. When this happens, the excited molecule can excite an oxygen atom turning stable oxygen into a free radical or reactive oxygen species that can damage other cellular structures including DNA. What makes reactive oxygen species so dangerous is their long lifespan as free radicals and their capacity to set off a chain of skin damaging events.
UVA rays penetrate deeper into the skin than UVB and cause the destruction of collagen. As collagen degrades, our skin loses elasticity and smoothness, leading to wrinkles. UVA rays are widely thought of as the ageing ray, while UVB rays are thought of as the burning ray.
The end result of all these reactions is damage that accumulates over the years from repeated exposure - and this happens even if your skin is dark, albeit to a relatively lesser degree. However, that doesn’t mean that people of colour can breathe easy since regardless of the amount of melanin we have in our skin, excessive sun exposure will still eventually lead to skin ageing and skin cancer.
Given all of the ways that UV can significantly damage your skin, protecting yourself from the sun is essential and for when we can’t completely stay out of the sun, we have sunscreen. Sunscreens contain UV filters, which are molecules designed to help reduce that amount of UV that reaches the skin’s surface. A film of UV filter forms a protective barrier that absorbs (or in very small amounts in the case of mineral sunscreens, reflects) UV before it can reach the skin and wreak havoc.
There are two main types of sunscreens which you might know as chemical sunscreens (sunscreens with carbon-based filters like octyl methoxycinnamate, avobenzone and octyl salicylate) and physical sunscreens (that contain zinc oxide and titanium dioxide). The technically more correct names for these however, are organic and inorganic sunscreens respectively, since the former are carbon-based, while the latter are not.
Sunscreen molecules work similarly to melanin: by absorbing the energy of a photon and converting it to heat, which is a more harmless form of energy. Similar to how electrons associated with molecules in your skin can absorb UV photons, sunscreen molecules can too. This excites the sunscreen molecule and in order to re-stabilise, it too must release the absorbed energy one way or another in a process called relaxation. In order to become stable again, electrons release energy in one of three forms, or a combination of them:
Heat. A molecule that has absorbed a photon has more energy and begins to vibrate faster and since heat is just the energy with which molecules vibrate, this results in the release of heat (or in other words, an increase in temperature) .
Lower energy forms of visible light, infrared radiation or even lower energy UV along with heat.
Chemical energy, i.e., the breaking of chemical bonds. Sometimes this is reversible so sunscreen molecules can keep doing their job. Other times however, it is not and the sunscreen loses efficacy overtime.
So long as no chemical bonds are broken, thus changing the nature of the compound in question, once an electron is back in its lower energy form, it's ready to absorb more UV photons in the same spectrum and repeat the process of absorbing and emitting, again and again. This is a similar process to why things heat up in the sun - electrons absorb photons (that may or may not be from the UV spectrum), get excited and then release the energy as heat. Interestingly, fluorescence works similarly - the only difference is that fluorescent substances tend to release absorbed energy as light instead of heat.
All electrons can’t absorb all wavelengths and their absorption and emission spectrums depend on the structure of the chemical they’re in. Sunscreens utilise molecules that absorb photons in the UV spectrum and different sunscreen molecules absorb different parts of the spectrum to different extents. This is why all sunscreens are not broad spectrum and most good sunscreens utilise a combination of filters instead of just one to give you high protection over a broad range of wavelengths.
I hope you found this informative! If you have topics that you’d like for us to cover in future blogs, drop them in the comments or message us on WhatsApp and we’ll try our best to cover them. Don't forget to check out our Ultimate Sunscreen, formulated with photo-stable UV filters that give you long-lasting, broad spectrum protection. Until next time!