Mechanism of Action of Sunscreen
Sunscreens protect skin by absorbing ultraviolet (UV) radiation through two distinct mechanisms: organic (chemical) filters that absorb UV photons via aromatic chromophore moieties, and inorganic (physical) filters that primarily absorb—not reflect—UV radiation through semiconductor band gap-mediated absorption. 1
Primary Mechanisms of UV Protection
Organic (Chemical) Sunscreen Filters
- Organic UV filters contain aromatic chromophore moieties that absorb UV light in the UV region while remaining transparent in the visible spectrum. 1
- These chemical compounds work by absorbing UV photons and converting them to less harmful energy forms, preventing UV radiation from penetrating the skin and causing damage such as thymine dimerization, which can lead to mutations and skin cancers. 1
- Common organic filters include avobenzone (Parsol 1789), oxybenzone, and benzophenones, each with specific UV absorption spectra. 2, 3, 4
Inorganic (Physical) Sunscreen Filters
- Zinc oxide and titanium dioxide—the only FDA-recognized GRASE (Generally Recognized As Safe and Effective) inorganic filters—protect primarily through absorption, not reflection or scattering as commonly believed. 1
- Research demonstrates that these metal oxides provide only 4-5% UV reflection (less than SPF 2), with the remainder of protection achieved through semiconductor band gap-mediated absorption of UV photons. 5
- Zinc oxide has peak absorption in the UVA region, while titanium dioxide absorbs predominantly in the UVB region. 1
- These filters are typically formulated with particle sizes between 50-150 nm to optimize absorption while minimizing visible "white cast." 1
Film Formation and Photoprotection
Critical Role of Film Formation
- The effectiveness of sunscreen depends critically on forming a stable photoprotective film on the skin's surface at the recommended thickness of 2 mg/cm². 1
- Film formation ensures optimal dispersibility and adhesion of UV-absorbing materials to the skin surface, which is the primary determinant of Sun Protection Factor (SPF) value in real-world application. 1
- Because human skin is porous and uneven, sunscreen films distribute non-uniformly, making proper application technique essential for adequate protection. 1
Formulation Design
- Sunscreens are formulated as oil-in-water (O/W) or water-in-oil (W/O) emulsions containing dispersed UV filters. 1
- Modern formulations incorporate hydrocarbon film formers, carbon-rich polymers, and emollient dispersions to stabilize UV filter distribution and prevent aggregation. 1
- Film thickness directly correlates with UV protection through a logarithmic relationship: doubling film thickness results in approximately ten-fold decrease in UV light transmission. 1
Clinical Outcomes and Cancer Prevention
Proven Benefits
- The International Agency for Research on Cancer has concluded that topical sunscreen use probably prevents squamous cell carcinoma of the skin. 1, 6
- Clinical trials demonstrate sunscreens effectively reduce actinic keratoses (precursors to squamous cell carcinoma) and squamous cell carcinoma itself. 1, 7
- In high-risk children, sunscreens reduce moles, which are the strongest risk factor for melanoma development. 1, 7
Mechanism of Cancer Prevention
- By absorbing UV radiation, sunscreens prevent endogenous photocrosslinking such as thymine dimerization in DNA, which can lead to mutations in protein-coding regions and contribute to malignancies. 1
- Protection against both UVB (primarily responsible for sunburn and skin cancers) and UVA (implicated in photoaging and deeper skin damage) is essential for comprehensive photoprotection. 8, 9
Common Pitfalls and Clinical Considerations
Application Errors
- Most users apply sunscreen at less than the recommended 2 mg/cm² thickness, significantly reducing actual SPF protection below the labeled value. 1
- Sunscreen must be applied 30 minutes before sun exposure and reapplied after swimming, sweating, or towel drying to maintain protective film integrity. 1
- Approximately 30 ml (one ounce, or a handful) is required to adequately cover the entire body. 7
Complementary Protection Strategy
- Sunscreen should serve as a complementary measure, not the primary defense—sun avoidance, seeking shade, and wearing protective clothing constitute the first line of protection against UV radiation. 1, 6, 7, 10
- This recommendation exists because many individuals use sunscreen to justify prolonged sun exposure, potentially acquiring equal or greater UV radiation than they would with shorter exposure and no sunscreen. 1