Physical Filters: Unveiling More than Sunscreen

Sunbathing is essential for life. We can get vitamin D thanks to the sun, which helps us strengthen our bones and improve our immune system. But in certain circumstances, sun exposure can be harmful, especially when we often sunbathe or for a long time. 

The sun emits energy onto the earth through ultraviolet (UV) rays on all surfaces, including us. Unlike a table, a chair, or a roof, our main protective surface is our beloved skin.

The skin is the largest organ of the body. Although many do not consider it an organ, it has its own blood vessels, specific functions, metabolism, and particular cells that make it comparable to the liver.

We could not live without skin since it allows the internal body to balance, communicate and perceive the surroundings while protecting us and making us look healthy. 

But something terrible happens when the skin is constantly exposed to UV radiation. Photoaging, or ''ageing due to ultraviolet exposure'', causes 90% of skin ageing and cancer today. Being the primary concern of any dermatologist, especially in a world where tanning is gaining ground daily. 

Sunscreen is, so far, the simplest, easiest, most effective, and cheapest option to prevent photoaging and skin cancer. Fortunately, every day, more and more scientists and pharmaceutical specialists are formulating new and better sunscreens to avoid both painful sunburn and the signs of early ageing. 

One of these filters is formulated with inorganic metal oxide. But to not get ahead of ourselves; let's first talk about sunscreens and how they serve our skin. 

Sun protection, our best ally

The sun is a star that represents the largest natural source of radiation, which is classified according to its frequency and wavelength. According to these properties, we find three types of UV radiation: UVC, UVB, and UVA.

UVC rays are the most harmful, but fortunately, they are absorbed by the ozone layer, so they do not touch our skin.

Then follows UVB, which is the leading cause of sunburn (hence the B for burn). Finally, UVA. Although it has less impact, it is so constant and reaches the earth at a higher rate (10-20 times more than UVB) that, over time, it causes ageing (A for ageing).

Now, once these radiations touch our skin, there is a typical skin reddening after a few minutes, which we will call ''solar erythema''. The sun protection factor (SPF) is defined as the time of sun exposure necessary to produce minimal detectable erythema on the skin, i.e., sunburn. 

Many mistakenly think that SPF is a measure of ''which sunscreen is better'', being common that ''the higher the SPF, the more protection''. The truth is that there are many points to consider before making a sunscreen decision that that to rely on a number is very trivial.

But what you need to know is that SPF focuses on protection against UVB rays, not UVA. For example, SPF 25 (SPF 25) protects against 92% of UVB, compared to 30+ (SPF 30+), which protects against over 97.5%. An SPF of at least 30 is a daily must.

Solar radiation transmission has increased with the current ozone layer damage, making it even more critical to wear sunscreen even on cloudy days, so an SPF of at least 30 is a daily must.

Types of sunscreens

According to their formulation, sunscreens can be inorganic (physical) or organic (chemical), depending on whether they contain carbon or not. 

It was thought many years ago that chemical (organic) sunscreens reached the bloodstream to damage our health. Today it is known that, although they are absorbed, studies are still needed to clarify the ''damages'' their absorption could cause. To date, no health loss has been detected. 

Unlike previously believed, physical (inorganic) sunscreens actually do not protect by scattering or reflection but by absorption. 

It was already known that chemical protectors worked by transforming sunlight into heat when applied to the skin. Still, it was always thought that physical protectors worked as a ''screen'' reflecting the light, so they were considered ''safer''. This is slightly true.

These protectors are made from metal oxides, such as zinc oxide and titanium dioxide. Since the 70s, titanium dioxide has been considered to work by reflecting and scattering UV rays rather than absorbing them. It was recently discovered that inorganic metal oxide sunscreens reflect, scatter, and also absorb UV rays. 

Scattering effect

The scattering phenomenon occurs when ultraviolet rays hit a film or screen that deflects their travel path, allowing them to dissipate into the environment. In short, it works as a reflector. 

Absorption

In this case, the sunscreen particles absorb UV radiation. It involves energy incorporation into the sunscreen molecules. The photons are absorbed until they reach the skin and are conducted as heat. 

Inorganic filters work by reflection and absorption, the latter being the most protective, just like organic (chemical) filters. 

The reflection range for zinc oxide and titanium dioxide over the entire UV range is, on average, 4 up to 5% (less than SPF 2), providing minimal protection through this. The rest is provided by the absorbance of the UV photons, so absorption is the principal protection way.

Some sunscreens can produce free radicals and damage collagen, elastin, or skin cells' DNA by absorbing only UV photons, especially those based on oxybenzone and avobenzone. Therefore, the daily use of these filters could be questioned on safety grounds with continued use. However, research is still needed for this.

But then, new skin sunscreen ingredients have been developed, such as Tinosorb S, which offers broad-spectrum protection against UVB and UVA through absorption without any skin risk and with the same light stability as inorganic (physical) filters.

How to find the ideal sunscreen?

Sunscreen is not perfect just because it protects. There are several points to consider when deciding on them. 

Now that you recognize that both physical and chemical sunscreens work by absorption, you should also know that the protection spectrum, the stability against light, and sweat/water resistance are essential. 

• Broad-spectrum: Both physical and chemical-only filters are available on the market, but most filters combine both methods. Sunscreens that protect exclusively against UVB, i.e., against erythema and sunburn, do not offer protection against ageing and UVA-induced cancer risk.
• Light and heat stability: Just as heat changes the structure of an egg while cooking it, some filters change their chemical structure, losing part of their effectiveness when exposed to high temperatures, such as during intense summer. It is recommended to store them in the shade and not buy those on-street displays under the sun.
• Good adhesion and resistance to water, sweat, and friction: This makes it possible to distinguish between water-resistant and waterproof filters, capable of keeping their protection after 40 minutes and 80 minutes in water, respectively.
• Additional key considerations: A suitable sunscreen should be non-irritating, non-sensitizing, non-staining to clothing, and cosmetically acceptable (odourless, colourless, and non-comedogenic).

Continuous and retouched sun protection will always be the key to avoid premature ageing and decreasing skin cancer risk. So protect yourself and your loved ones! Don't miss a day of sunscreen.