Sunscreens are essential cosmetic products designed to protect the skin from the harmful effects of ultraviolet (UV) radiation, which is a major contributor to photoaging, skin cancer, and other forms of skin damage. The primary active ingredients in sunscreens are UV filters, which are broadly classified into two categories: organic (chemical) filters and inorganic (mineral) filters.
Organic UV filters work by absorbing high-energy UV photons and converting them into heat, which is then released from the skin. A wide range of organic filters is available, each with a specific absorption spectrum covering different parts of the UVB (290-320 nm) and UVA (320-400 nm) ranges.
Some organic filters primarily absorb UVB radiation, such as p-aminobenzoic acid (PABA) and its esters, the cinnamates (e.g., octinoxate), and salicylates.
Others offer broader protection, absorbing both UVB and some shorter wavelengths of UVA, examples include octocrylene and benzophenones (e.g., oxybenzone, avobenzone).
Ecamsule (Mexoryl™ XL/SL) is an organic filter that provides good coverage across the UVB spectrum and a significant portion of the UVA range.
Avobenzone (Parsol 1789) is a widely used organic filter that effectively absorbs UVA radiation.
Inorganic UV filters, consisting of microfine particles of mineral oxides like titanium dioxide and zinc oxide, protect the skin by reflecting and scattering UV radiation away from the surface. These filters are generally considered to be broad-spectrum, offering protection against both UVB and UVA wavelengths.
Titanium dioxide effectively blocks UVB and shorter UVA wavelengths.
Zinc oxide provides even broader protection, covering almost the entire UVA spectrum in addition to UVB.
The concentration and particle size of inorganic filters influence their efficacy and cosmetic appearance. Older formulations with larger particles could appear white and opaque on the skin. However, modern technologies using microfine and nanoparticle forms of titanium dioxide and zinc oxide offer high levels of UV protection with improved transparency and cosmetic elegance.
Achieving effective and well-balanced UV protection requires the careful combination of different organic and/or inorganic filters in sunscreen formulations. Regulatory guidelines in various regions specify minimum Sun Protection Factor (SPF) values for UVB protection and also set criteria for UVA protection, often requiring a UVA Protection Factor (UVAPF) that is at least one-third of the SPF value and a critical wavelength above a certain threshold. Broad-spectrum protection, covering both UVB and UVA, is increasingly recognised as important for reducing the risk of skin cancer and premature skin ageing.
Photostability is a critical factor in the performance of sunscreen formulations. Some UV filters, particularly certain organic filters like avobenzone, can degrade upon exposure to UV radiation, leading to a reduction in their protective efficacy over time. To address this, formulators often include stabilising compounds in the formulation that help to maintain the integrity and effectiveness of the UV filters during sun exposure. For example, combining avobenzone with certain UVB filters like octocrylene or specific benzylidene camphor and diphenyl cyanoecrylate derivatives can enhance its photostability.
Sunscreen formulations can be aqueous-based (hydrous) or anhydrous. Aqueous formulas often utilise emulsifiers or stabilising agents. Anhydrous formulations, commonly found in solid sticks and some roll-ons, may include ingredients like cyclosiloxanes for transient solvent delivery and solidification agents.
Regulatory frameworks govern the ingredients and labeling of sunscreens. In the United States, sunscreens are regulated as over-the-counter (OTC) drugs by the Food and Drug Administration (FDA) . Regulations specify permissible UV filter ingredients, their maximum concentrations, and labeling requirements, including SPF and broad-spectrum claims . Similar regulations exist in other regions, such as Europe and Australia, with variations in permitted filters and testing protocols.
Beyond UV filters, sunscreen formulations often contain other ingredients to enhance their cosmetic properties and skin benefits. These may include emollients, moisturisers, antioxidants, and preservatives. Antioxidants can provide additional protection against free radicals generated by UV exposure that are not directly blocked by the filters. In conclusion, sunscreen formulations rely on a combination of organic and/or inorganic UV filters to provide broad-spectrum protection against harmful UV radiation. The choice of filters, their concentrations, and the overall formulation are critical for achieving the desired SPF and UVAPF, ensuring photostability, and maintaining cosmetic acceptability. Regulatory requirements play a significant role in guiding the development and labeling of sunscreen products to ensure consumer safety and efficacy.