Sunscreens: Limitations of Past and Current UVA Protection Metrics in an Era of Rising UVA-Driven Skin Cancers

Abstract

Purpose Current literature indicates a significantly rising incidence of UVA-associated skin cancers, including squamous cell carcinoma (SCC) and malignant melanoma (MM), relative to UVB-associated basal cell carcinoma (BCC) and a shift in the SCC to BCC ratio, challenging the long-held view that BCC predominates. This review examines the extent to which ultraviolet (UV) protection metrics in the United States might have contributed to this ongoing change.

Methods A PubMed search was conducted using the terms “UV protection metrics”, “UVA”, and “UVB”. Results were screened for relevance and articles written in only English. Additional articles were included via citation tracking. The authors' expert clinical opinions were also included. 

Results and Discussion Recent epidemiological data demonstrate a substantial increase in SCC incidence relative to BCC, particularly among older individuals and in regions with high cumulative UV exposure. In the U.S., the BCC to SCC incidence ratio (including subtypes) has shifted from approximately 4:1 in 1992 to nearly 1:1 in 2012 and further to 1:3 by 2024, with SCC subtypes now representing a growing proportion of nonmelanoma skin cancer (NMSC) cases. Similarly, the incidence of melanoma continues to steadily rise.

Sunscreen remains a primary method of UV protection, with early formulations targeting UVB radiation before the role of UVA in carcinogenesis was recognized. The sun protection factor (SPF), adopted by the U.S. Food and Drug Administration (FDA) in the 1980s, labels sunscreens by quantifying protection against UV-induced erythema and is calculated as the ratio of the minimal erythema dose (MED) in protected skin to that in unprotected skin. Because erythemogenic effects are primarily mediated by UVB, SPF largely reflects UVB protection and remains the primary measure of sunscreen efficacy despite advances in broad-spectrum, UVA-filtering formulations. 

Because UVA radiation lacks a clear, easily observable endpoint comparable to UVB-induced sunburn (erythema), a standardized metric for quantifying UVA protection has not been established in the U.S. Instead, UVA damage accumulates subtly, a concept referred to as “the silent UVA,” making sunscreen UVA protection difficult to quantify. While advancements have been made in providing UVA protection through development of broad-spectrum labeling standards by the U.S. FDA, multiple studies have demonstrated suboptimal UVA coverage with modern sunscreens. A study conducted in 2021 evaluating the performance of 32 U.S. commercially available broad-spectrum sunscreens, six showed failure to meet the critical wavelength of ≥ 370 nm needed for broad-spectrum labeling, and approximately 40% of sunscreen products tested had suboptimal UVA protection. 

Conclusion The primary reliance on UVB-derived SPF to quantify UV protection, combined with the lack of a comparable UVA metric, has likely contributed to the rising incidence of UVA-driven cancers and underscores the need for improved UVA-specific metrics and continued research.