Sunscreens have been a staple in skincare routines for centuries yet the science behind these invisible barriers have gone unacknowledged. Sunscreen provides protection against skin cancers, notable forms of skin cancer include squamous cell carcinoma, basal cell carcinoma, and melanoma. The endless list of damages caused by repeated exposure to strong ultraviolet (UV) radiation includes photoaging and photocarcinogenesis. Photoaging can cause early signs of sagging and wrinkling while photocarcinogenesis damages cells, modifies DNA and is often the prognosis of cancer.

Melanin is the only natural pigment considered to be a sunscreen ingredient as it absorbs UV light. The melanin in skin, produced by melanocytes, is nature’s sunscreen which limits the penetration into the skin tissues by thickening the epidermis. The cells which are affected by radiation will be coloured and closer to the surface. As the cells die, the pigmentation will no longer be visible. Melanin percentage in skin varies among people and can not protect individuals from pro-longed exposure however it minimizes the risk of sunburns.

Sunburns, also referred to as “erythema” is a photochemical reaction. Sunburns lead to increased blood flow to the skin’s capillaries which contributes to the redness visible when an individual suffers from a sunburn.

Ultraviolet light is segregated into UVA (320-400 nm) and UVB (290-320 nm) based on the wavelength. The shorter the wavelength, the greater frequency and energy possessed by the wave. UVC, which has the shortest wavelength, does not reach Earth’s atmosphere.

Figure 1:Wavelength regions of UVA (green), UVB (blue) and UVC (violet) radiation in the standard solar spectrum (black line) in comparison to wavelength at Earth’s surface (red line). Source: National Renewable Energy Laboratory (NREL)

There are two categories of sunscreen. Physical blockers, also known as inorganic sunscreens, which reflect UV rays away. They contain one of two active ingredients, zinc oxide (absorbs UVA and UVB) or titanium dioxide (absorbs UVB). Th white cast on the user’s face can be attributed to the scatter of visible light caused by large particles.

Contrary, chemical blockers contain chemicals that absorb the sun's UV rays. These primarily include avobenzone, octisalate, octocrylene, and oxybenzone which are all aromatic compounds conjugated with a carbonyl group. Only atoms on the surface absorb UV thus they are designed to have smaller particles with larger surface areas for efficient absorbing.

When the UV ray’s energy is absorbed, the sunscreen molecule’s electrons have excess energy and are in an “excited” stage. The molecule is now unstable and will release this excess energy as heat or as a light with a longer wavelength (usually either infrared waves or visible light rays). Bringing the system back to the ground (lowest energy level) is done through photochemical processes, such as intramolecular vibrational redistribution (IVR). This process of eradicating its excess energy is referred to as “relaxation”. Bonds break when energy is released. The fastest of these relaxation mechanisms occur within femtoseconds (10−15  x sec) which is the standard timescale of nuclear motion and the breaking of chemical bonds. In cases with longer timeframes, such as fluorescence, the excited state will persist for long enough that there is a higher chance for detrimental photochemistry to occur where this excess energy can propel damage to DNA.

A molecule's photochemistry is dependent on the arrangement of its atoms and its electronic and vibrational energy structure (scientifically referred to as “vibronic structure herein”). The conjugation structures allow the molecules to have delocalised electrons. Pi-bonds (π-bonds) are formed by the overlap of two adjacent p-orbitals. This overlap is called conjugation and allows for the de-localization of electrons in a process known as “resonance”. The chemicals have "band gap" structures with closely packed energy levels instead of discrete molecular  order.

UV-absorbing compounds include groups which release electrons, such as amine (-NH₂) or methoxyl (-OCH₃), that contribute electron density to aromatic ring systems. These groups are positioned at ortho or para positions on benzene rings to optimize electronic properties.

UVA absorbers utilize ortho-substitution as adjacent electron-releasing groups can form stabilizing intramolecular hydrogen bonds. This creates rigid molecular conformations which effectively reduces the energy gap and shifts absorption towards longer UVA wavelengths.

The intermolecular hydrogen bonding allows for organized crystalline structures that increases thermal stability and melting points. The hydrogen bonding with polar solvents improves water solubility and causes solvatochromic effects—wavelength shifts in UV absorption spectra depending on the solvent environment.

Over time, the sunscreen decomposes and becomes “photounstable”. This refers to the breakdown process, as the chemicals undergo irreversible bond-breaking, after they have been exposed to UV radiation for an extended period of time. This is why sunscreens require frequent re-application.

The standard recipe for sunscreens does not contain toxic chemicals however oxybenzone has raised concerns due to its reputation as a hormone disrupter. A hormone disruptor is a chemical that has the ability to cross cell membranes and interfere with the body's natural hormone production. However, according to a study conducted by the Journal of the American Academy of Dermatology, only direct ingestion of oxybenzone can cause hormonal chaos.

Chemistry has diffused into our daily lives, with millions of molecular interactions designed to convert harmful UV energy into harmless heat occurring on our bare skin every second, understanding textbook concepts allows innovations to morph into essentials for public health!