Determinants of real-world sunscreen effectiveness: from ultraviolet radiation photobiology to human behaviour

Sunscreen is often presented as a straightforward intervention for reducing sun-induced skin damage. In practice, however, its effectiveness is determined by a broad network of interacting factors rather than labelled Sun Protection Factor (SPF) alone. Real-world protection depends not only on filter chemistry and test performance, but also on wavelength coverage, formulation, durability, regulatory standards, user behaviour and public understanding. At the same time, sunscreen has become an increasingly contested topic, shaped by debates regarding systemic absorption, environmental impact, misinformation and the limitations of uniform public-health messaging. This review examines the determinants of sunscreen effectiveness in the real world. First, the photobiological basis of photoprotection is considered, including the differing contributions of UVB, UVA, and, in selected contexts, visible light to skin damage. Second, the ways in which sunscreen efficacy is measured and regulated are reviewed, with particular attention to the limitations of SPF as a sole marker of protection. Third, current evidence relating to human and environmental implications of sunscreen use is assessed. Finally, behavioural, social and communicative factors that influence real-world use are explored, together with emerging personalised and technological approaches to photoprotection. Overall, the evidence suggests that sunscreen effectiveness is best understood not as a fixed property of the product alone, but as the outcome of an interaction between photobiology, formulation science, regulation and human behaviour. This broader framework helps explain why scientifically efficacious products may deliver suboptimal protection in practice, and why future improvements in photoprotection will depend not only on innovation in formulations, but also on clearer, more risk-stratified guidance that supports consistent use in everyday life.