The Chemistry of Sunscreens: 5 Tips for Application

The Electromagnetic Spectrum and UV Radiation

Sunscreens protect skin from UV rays through chemical and physical mechanisms: inorganic filters (zinc oxide, titanium dioxide) reflect radiation, while organic filters absorb it and convert it to heat. SPF indicates how many times longer protected skin can resist burning compared to unprotected skin.

Ultraviolet (UV) radiation is the leading cause of skin damage in humans, including premature aging and skin cancer.

To protect ourselves, sunscreens are an essential tool to always carry with us. But what lies behind an SPF label? And how do these sunscreens work at the molecular level?

Before delving into the world of sunscreens, it is useful to take a step back and understand what ultraviolet radiation is.

The Sun emits energy in the form of electromagnetic waves, which make up the electromagnetic spectrum. This spectrum includes different types of radiation, distinguished by their wavelength and the amount of energy they carry.

What Is the Relationship Between Frequency and Wavelength?

The frequency and wavelength of electromagnetic radiation are closely related.

• Frequency and wavelength are inversely proportional.
• On the other hand, the energy of electromagnetic radiation is directly proportional to its frequency.
• Consequently, energy is inversely proportional to wavelength.

UV radiation, invisible to the human eye, falls between visible light and X-rays. It is classified into three main categories based on wavelength, measured in nanometers (nm):

• UVA (320–400 nm): They have a longer wavelength and, while less energetic, penetrate deeper into the skin, reaching the dermis. They are the main culprits behind skin aging, wrinkle formation, and, in the long term, contribute to skin cancer.
• UVB (290–320 nm): They have an intermediate wavelength and are the main cause of sunburns and erythema. They are more energetic than UVA and cause direct damage to the DNA of epithelial cells.
• UVC (100–290 nm): They are the most energetic and dangerous radiation. Fortunately, they are completely absorbed by the Earth's ozone layer and do not reach the planet's surface.

Remember: Sun protection is therefore essential to shield our skin from UVA and UVB rays.

The Effects of UV Radiation at the Cellular Level

Exposure to UVA and UVB rays damages skin cells through various mechanisms, which can lead to both short- and long-term consequences:

• DNA damage: UV radiation is absorbed by DNA, causing the formation of abnormal structures. These structures form when two adjacent nitrogenous bases in the DNA bond abnormally, creating a distortion in the double helix (for example, this is precisely the mechanism behind the formation of thymine dimers). This distortion blocks the processes of DNA replication and transcription. If unrepaired, these mutations can lead to uncontrolled cell growth, a fundamental step towards the development of skin cancer.
• Production of free radicals: UV rays can generate reactive oxygen species or ROS (free radicals). These molecules, characterised by the presence of a single unpaired electron, are highly reactive and cause oxidative stress, which damages not only DNA but also lipids and proteins essential for cell structure and function, contributing to premature skin aging.
• Damage to collagen and elastin fibres: UVA rays, penetrating deep into the dermis, damage collagen and elastin fibres. These proteins are crucial for skin firmness and elasticity. Their damage leads to loss of tone and wrinkle formation, a process known as photoaging.

DNA Repair Mechanisms in Humans

Fortunately, our cells are not defenseless against these attacks. They possess sophisticated DNA repair mechanisms that constantly work to correct damage and preserve genome integrity. The main repair mechanism against UV-induced damage in humans is Nucleotide Excision Repair (NER).

NER is a complex and highly regulated process. An enzymatic system recognises the damage, removes a single-stranded DNA segment containing the lesion, and synthesizes a new strand using the "healthy" one as a template. This mechanism is fundamental for preventing mutations that could cause cancer.

Note: It is important to recognise that in humans, the direct repair mechanism (photoreactivation) found in other organisms is not active, making NER our primary ally against UV damage.

The Chemistry of Sun Protection: Mechanisms and Compounds

Sun protection is based on the use of active substances known as UV sunscreen filters, which are designed to prevent UV radiation from penetrating and reaching the deeper layers of the skin.

Although both are chemical compounds, the more accurate classification is between inorganic sunscreen filters and organic ones, commonly known as "physical sunscreens" and "chemical sunscreens" due to their different mechanisms of action.

Physical (or Inorganic) Filters

Physical sunscreens consist of finely ground minerals that act by forming a physical barrier on the skin's surface. The most common compounds are titanium dioxide and zinc oxide. At the molecular level, these are crystalline solids that appear as white powders insoluble in water.

Their primary mechanism is the reflection and scattering of UV rays, acting as a true shield that deflects radiation away from the skin.

However, it has been scientifically proven that these sunscreens also absorb a significant portion of the UV spectrum, especially in the UVA band, just like chemical sunscreens. Their inert and photostable nature makes them particularly suitable for sensitive skin and hypoallergenic formulations.

Chemical (or Organic) Filters

Chemical sunscreens are organic compounds that function by absorbing the energy of UV radiation. At the molecular level, they are complex molecules containing chromophores — chemical structures or functional groups capable of absorbing electromagnetic radiation at specific wavelengths. To deepen your understanding of these concepts, a solid foundation in chemistry is essential.

When a UV photon strikes the chromophore, the electromagnetic energy is absorbed, exciting the chromophore's electrons to a higher energy level.

To return to their ground state (lower energy), the molecule releases the excess energy. This release occurs primarily as heat or as radiation at a less harmful wavelength, which is dissipated before it can damage skin cells.

Modern formulations often combine several chemical sunscreens to cover a broader spectrum of UV wavelengths and ensure more complete protection.

What Is the Sun Protection Factor (SPF)?

The Sun Protection Factor (SPF) is a crucial indicator found on sunscreen labels. The SPF measures the product's protection against UVB rays, the main culprits behind sunburns.

For example, an SPF 30 does not indicate that the product blocks 30% of UV rays, but rather that it increases by 30 times the time required for protected skin to burn compared to unprotected skin. This means that if your skin takes 10 minutes to burn without protection, with SPF 30 it would take approximately 300 minutes (5 hours).

It is important to emphasise that SPF does not offer total protection and that effectiveness depends on correct and generous application.

• SPF 15 blocks approximately 93% of UVB rays
• SPF 30 blocks approximately 97% of UVB rays
• SPF 50 blocks approximately 98% of UVB rays

As you can see, the difference in protection between SPF 30 and SPF 50 is minimal in percentage terms. Generally, for optimal protection, it is recommended to reapply the product at least every two hours, or even more frequently after swimming or sweating, regardless of the SPF value.

The label "broad spectrum" also guarantees that the product protects against UVA rays, which are responsible for skin aging and skin cancer risk.

The Importance of Knowing Your Skin Phototype

The phototype is a scientific skin classification that indicates its reaction to sun exposure, based on the amount of melanin produced. Melanin is the pigment that protects the skin from UV rays. Depending on the phototype (ranging from I to VI), the skin has different sensitivity and a greater or lesser tendency to burn.

• Light phototypes (I–II): They have very fair skin, red or blonde hair, and light eyes. They tend to burn easily and tan with difficulty. They have a higher risk of sun damage and skin cancer. For these phototypes, using sunscreens with high SPF and careful protection is essential.
• Intermediate phototypes (III–IV): They have darker skin, hair, and eyes. They tend to burn less easily and tan gradually. The risk of sun damage is lower compared to light phototypes, but not negligible. Consistent sun protection is essential for them as well.
• Dark phototypes (V–VI): They have very dark skin, hair, and eyes. They burn very rarely and tan easily. Although the greater amount of melanin offers natural protection, they are not immune to cellular damage caused by UV rays. Using sun protection remains essential to prevent long-term damage and skin cancer.

Important: Regardless of phototype, using adequate sun protection is a habit that should be adopted by everyone to maintain healthy skin and reduce the risk of disease.

5 Tips for Correctly Applying Sunscreen

Effective sun protection depends not only on the quality of the product but also on its correct application.

Here are 5 essential tips to ensure maximum protection:

• Be Generous with the Amount: To achieve the SPF level stated on the label, you need to apply an adequate amount of product. In general, think of using the equivalent of a shot glass to cover the entire body.
• Apply Sunscreen in Advance: It is essential to apply sunscreen at least 15–30 minutes before going outdoors. This gives the sunscreen filters time to "bind to the skin" and form an effective protective barrier.
• Reapply Frequently: Sunscreen tends to degrade over time and be removed by sweat or water. Reapply the product approximately every two hours, or more often if swimming or sweating heavily.
• Don't Forget the "Hidden" Areas: Many people focus on arms and legs, forgetting vulnerable areas like the ears, neck, tops of the feet, and scalp.
• Use Sunscreen in Combination with Other Methods: Sunscreens are only one part of a more comprehensive protection strategy. AIRC recommends wearing protective clothing, seeking shade, and using sunglasses to protect the eyes.

Conclusion

Behind sun protection lies a concentrate of science — a combination of chemistry, physics, and biology working to defend our body from the harmful effects of the sun. Understanding how sunscreens work, what damage UV rays can cause, and how our skin naturally tries to repair itself allows us to make more informed decisions for our health and enjoy our time outdoors with greater peace of mind.

Remember that the correct use of broad-spectrum sunscreen not only prevents annoying sunburns but is a fundamental action to reduce the risk of long-term skin diseases and preserve skin health.

Make sun protection an essential part of your daily routine, regardless of your phototype and the season.

Happy Summer from Up to Ten!

This article is for informational purposes only and is not intended to replace the advice or consultation of a physician.

FAQ

What is the difference between physical and chemical sunscreens?

Physical (inorganic) filters like zinc oxide and titanium dioxide reflect and scatter UV rays by forming a barrier on the skin. Chemical (organic) filters absorb UV radiation and convert it to heat through chromophores. Modern formulations often combine both for complete protection.

How often should I reapply sunscreen?

Sunscreen should be reapplied every 2 hours, or more frequently if swimming or sweating. The SPF stated on the label is based on generous application (about 2 mg/cm2 of skin), and protection decreases over time due to degradation, sweat, and friction.

Does SPF 50 protect much more than SPF 30?

No, the difference is minimal: SPF 30 blocks 97% of UVB rays, SPF 50 blocks 98%. The bigger difference lies in proper application and frequent reapplication. An SPF 30 applied generously and reapplied every 2 hours protects better than an SPF 50 applied sparingly.

Do people with dark skin need to use sunscreen?

Yes. Even though melanin provides natural protection, dark phototypes (V-VI) are not immune to UV-induced cellular damage. The risk of skin cancer is lower but not zero, and photoaging affects all skin types. Sun protection is important for everyone.