First, the bad news: all radiation generates radicals – to a greater or lesser extent. The good news: all organisms have adapted to this over the course of evolution – with modern humans at the forefront, who are also technically capable of optimising protection.
There are two ways to escape the dreaded free radicals. The first is to take measures to prevent them from reaching the skin. The second is to destroy the radicals. A third, less discussed option is to tolerate a certain amount of radicals. However, this does not correspond to the widespread black-and-white view of our time.
We are also fixated on solving all problems with a single measure. In this case, similar to a pill for an illness, it is a cosmetic product that can be quickly applied to the skin – be it a cream, gel or lotion.
Conversion into heat
Substances that absorb radiation and convert it into heat are referred to as filters in cosmetics. An abbreviation preceding the name indicates the type of radiation, e.g. UV filter if they absorb ultraviolet light. The heat generated is released into the environment. A simple example from the kitchen at home illustrates how this works: water effectively absorbs radiation from the microwave and boils as a result.
A UV filter is more complicated because it has to absorb high-energy UV radiation. This requires a molecule with an extensive π-electron system to absorb the photons (light quanta) of the radiation and convert them into heat. Among the first representatives of this type were compounds of cinnamon and salicylic acid. However, they are not as effective as today's modern filters. This means that they only partially prevent the formation of radicals and must be used in high concentrations to achieve a significant sun protection factor (SPF).
This is also referred to as light quantum yield, which is around 80% in the case of 4-methoxycinnamic acid 2-ethylhexyl ester (INCI: octyl methoxycinnamate). The remaining 20% of the radiation that is not absorbed still generates free radicals, which antioxidants attempt to neutralise.
Modern filters
Modern UV filters are much more effective and longer-lasting. They work much like a catalyst, enabling a reaction (conversion of radiation into heat) without being consumed, whereas an antioxidant becomes ineffective after reacting with a single radical. It therefore makes little sense to use antioxidants in addition to highly effective filters, as they can at best compete with the filters but are broken down very quickly.
Incidentally, the most effective biochemical filter is the natural melanin produced in the skin, with a quantum yield of almost 100%.
Concentration and bandwidth
However, melanin is only as effective as its concentration in the skin. When considering sun protection, it is therefore important to always remember to keep the concentration of the skin's own protection as high as possible.
Then there is the wavelength of the radiation, to which chemical filters must be tailored. It is desirable to cover as broad a spectrum of radiation as possible, which is usually achieved not by one filter but by a combination of several filters. The sun protection factor is then determined by the concentration of the filters and the matrix (cream, gel, lotion) in which they are embedded.
Gaps in the protective effect
Only over time, with the refinement of measurement technology, did it become apparent that, in addition to ultraviolet radiation, wavelengths in the IR and visible range also generate radicals. As a result, the bandwidths were further expanded, but two points were often overlooked during the refinement process:
- Radiation is also necessary for the formation of protective melanin.
- In physics, as in chemistry, the following applies: it is the dosage that makes a substance poisonous (Paracelsus).
Put simply, the skin needs a minimum amount of radiation for the synthesis of melanin and vitamin D, and not every radical causes the skin to age. For example, the use of light protection filters in ordinary day creams is not very reasonable when indoors.
Boundary conditions
- Anyone who operates a photovoltaic system will notice that, on closer inspection, the system captures more radiation and produces more electricity when it is slightly cloudy or there is high fog with the sun above than under a steel-blue sky. This phenomenon is related to the diffuse refraction of light by ice crystals and small droplets, which practically eliminates the limiting angle of incidence of the sun. This makes it all the more important to protect your skin.
- The conditions are similar at the seaside and on sandy beaches. In these cases, diffuse radiation comes from below or from the side in addition to the incident sunlight. Radiation also intensifies with altitude, i.e. in mountains, and with latitude1; the sun protection factor used must be correspondingly higher.
- Speaking of the sea and water: not all sunscreens meet the criteria for sufficient water resistance.2 As a result, protection in the water quickly diminishes. At the equator, this can lead to a nasty surprise after just a few minutes.
- Sensitivity to radiation can be altered in one direction or the other by medication and diet.
- There is no protection in the form of a preparation against the harmful effects of the intense heat generated by infrared radiation on the skin. Even with the best UV protection, the skin ages faster. With a moderately radiant infrared lamp used for therapy, this is not a concern.
Natural protection
When you read the usual advertising for sun protection and the fight against free radicals, you get the impression that your skin is defenceless unless it is rendered harmless by externally applied antioxidants.
This is, of course, nonsense and greatly underestimates the physiological capabilities of our outer protective shield. Like other organisms, humans protect themselves against free radicals that penetrate from outside and are produced by radiation with pigments such as the natural broad-spectrum filter melanin and UV-B-absorbing urocanic acid3, as well as with the nitrogen-containing components of the natural moisturising factor (NMF) – provided that the skin and barrier are intact. One example is the interception of nitrite resulting from radical nitrogen oxides by amino acids:
This produces alpha hydroxy acids (AHAs) such as lactic acid (from the amino acid alanine: R = CH3), which are also components of NMF. Peptides react in a similar way. These mechanisms naturally reach their limits when the skin is confronted with unusually strong radical formation, as occurs without sun protection in strong sunlight. However, humans are an exception in that they deliberately expose themselves to radiation, while other organisms instinctively seek shade.
Cultural achievements such as daily showers using preparations with a high surfactant content also damage both the barrier and the NMF with their washing-out effect. This damage is not compensated for by a preparation applied afterwards, such as a body lotion.
After exfoliation, a sun hat is a good alternative or supplement to SPF 50+ to protect the sun-exposed areas of the face. To a limited extent, it is possible to enhance skin pigmentation orally, e.g. through carotenoids or active ingredients that stimulate melanin synthesis.
Footnotes
- https://www.bfs.de/DE/themen/opt/uv/uv-index/weltweit/weltweit_node.html
- https://www.cosmeticseurope.eu/files/7914/6407/7400/Guidelines_for_Evaluating_Sun_Product_Water_Resistance_-_2005.pdf
- Urocaninic acid:
 is structurally similar to cinnamic acid:
Dr Hans Lautenschläger |
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