It is widely accepted that wearing a form of sunscreen is essential for protecting our skin against the Sun’s harmful UVA and UVB radiation, but how exactly does it work?
Active ingredients in sunscreen have two separate methods of action: chemical and physical. A product which promises sun protection may contain only one kind of active ingredient or a combination of both. For example, ‘mineral sunscreens’ contain only active ingredients which have a physical method of action.
Physical Sunscreens
‘Physical’ (also known as reflector) active ingredients work by reflecting, absorbing and scattering UV radiation away from the skin. These inorganic ingredients typically take the form of nanoparticles, particles which are 1-100 nanometres in diameter.
Because of their small size, a nanoparticle’s radius is smaller than the wavelength of visible light, meaning visible light can pass straight through and these particles (usually) remain transparent on the skin. Meanwhile, the radius of these particles is greater than the wavelength of UV light meaning radiation of this frequency is absorbed and scattered away from the skin at a lower and less harmful frequency.
Physical sunscreens are what are responsible for the infamous ‘white cast’ which, thankfully, is much more avoidable nowadays than it ever was in the past. This is due to advancements in nanotechnology which have allowed the creation of stable particles small enough to remain transparent in visible light whilst still providing UV protection.
However, for people with darker skin tones, physical sunscreens can still leave an undesirable white cast if the size of the nanoparticles used in the blend is too large.
Whilst still providing effective sun protection, sunscreens which leave a white cast are not a suitable option with aesthetics in mind. That being said, there are several physical sunscreens available which don't leave a white cast on dark skin, such as On-The-Defense Sunscreen SPF 30 created by Venus Williams, so there’s no reason to rule out this method of action altogether.
Titanium dioxide (TiO2) nanoparticles are a common ingredient used to provide physical protection in sunscreens. TiO2 is also what is often responsible for the characteristic white colour of many products such as paracetamol tablets and mints. Zinc oxide (ZnO) nanoparticles are another frequently used ingredient.
In the past, physical sunscreens have caused controversy as both zinc and titanium nanoparticles can react in sunlight to generate free radicals - species with unpaired electrons.1 When free radicals are present on the skin, their highly oxidising nature can damage DNA and thus accelerate the aging process.
However, efforts are now taken to modify the surface of particles to prevent nanoparticles participating in the formation of free radicals and many formulations also contain antioxidants.2 For instance, Glossier include a mix of Vitamin E, Vitamin P, Broccoli, and Aloe Leaf extracts in the formulation of invisible shield, all ingredients with the ability to neutralize free radical species that come into contact with the skin.
Chemical Sunscreens
Side note: Use of the word ‘chemical’ here is meant to distinguish these active ingredients from their physical counterparts by emphasising the fact a chemical reaction is taking place. This does not imply that mineral sunscreens do not contain chemicals, they do. TiO2 and ZnO nanoparticles are still chemicals, just with a physical method of action as sunscreens. No cosmetic product can ever be ‘chemical free’.
‘Chemical’ (also known as absorber) active ingredients work by undergoing a chemical reaction in the presence of UV radiation which converts this UV radiation into that of a lower, and less harmful, frequency. This harmless radiation is then dissipated into both the atmosphere and the skin, which may cause a slight warming sensation.
Chemical sunscreens often contain a mixture of different organic (meaning carbon based) active ingredients as, usually, but not always, a particular chemical may only be able to protect against UVA or UVB radiation, rather than both. This is due to chemical structure and strength of bonds within molecules.
Some active ingredients in chemical sunscreens include:
Avobenzene (may appear on ingredients list as Butyl methoxydibenzoylmethane)
Oxybenzone is less prevalent in chemical sunscreens nowadays as concerns were raised over its ability to penetrate the skin
Octocrylene (octocrylene can also be used as a photostabliliser to prevent the degradation of other molecules in UV light)
Octisalate (may also appear on ingredients lists as Ethylhexyl Salicylate)
Octyl methoxycinnamate (may also appear on ingredients lists as octinoxate)
Ecamsule
Note that this is not an exhaustive list. Each chemical can go by several different names so the same ingredient may appear with a different name on an ingredients list depending on who produced the sunscreen/where it was sold.
Often, companies will combine several active ingredients to develop a unique formulation. For instance, La Roche-Posay have combined ecamsule with other UV protecting chemicals to create the patented filtering system Mexoplex®.
Concerns have been raised over the fact that, throughout the day, the performance of these UV protecting chemicals declines as they break down, emphasising the importance of frequent reapplication. Some scientists have suggested that bottles ought to be labelled with a meaningful measure of the breakdown rate of active ingredients so that consumers can have confidence that they will be protected for an adequate amount of time and can be made aware of when to re-apply.3
Additionally, there has been a great deal of worry about the fact many of the ingredients in drugstore sunscreens have been found to pollute the environment, particularly coral reefs, with ‘reef-safe’ being an asset many companies now claim on their bottles. It is unsurprising that the ingredients found in sunscreens have been found among aquatic life with most people wearing sunscreen at the beach.
Whilst researching for this post, I was able to find claims that basically all active ingredients, both physical and chemical, have supposedly been responsible for pollution. That being said, oxybenzone, Octyl methoxycinnamate, and octocrylene appear to be particularly bad offenders, with many claiming that physical sunscreens are better for the environment. To avoid polluting the environment with sunscreen it would seem that the only thorough solution is to just wear loose long sleeved/legged clothing instead and keep out of the sun.
All sunscreens carry risks depending on your skin type, but with the right amount of research and trial and error, you should be able to find one which works for you. Certainly, the inconvenience of having to apply sunscreen frequently is well worth the protection against aging and skin cancer you get in return. Forget anti-aging creams, sunscreens are the best place to start.
References
M D Newman, M Stotland and J I Ellis, JAAD, 2009, 61, 685-692.
G. Wakefield, M. Green, S. Lipscomb and B. Flutter, Mater. Sci. Technol., 2004, 20, 985-988.
N G K Wong, J A Berenbeim, M Hawkridge, E Matthews and C E H Dessent, Phys. Chem. Chem. Phys., 2019, 21, 14311-14321.