Rational determination of real-life UV exposure across latitudes: An extended Diffey-based model integrating behavioural factors

Abstract

Objectives

In this study, we quantify both the UVB and UVA doses emitted by the sun at ground level and the realistic effective UVB and UVA doses received (ERD) by an individual across latitudes from 0° to 60° North and for every month of the year. These outputs provide a quantitative basis to derive protection indices and to support informed discussion on photoprotection. Our aim is not to prescribe SPF guidance for consumers or dermatologists, but to offer a robust scientific framework that clarifies how real-life exposure, behavioural factors, effective UVB and UVA-weighted doses and solar geometry collectively shape the necessary UV protection.

Methods

Daily UV doses were computed for the 21st day of each month to capture seasonal extremes. These doses, expressed as Standard Erythemal Doses (UVB) and UVA-weighted doses, were calculated for each latitude based on Diffey's spectral irradiance framework. Two corrective factors were applied: (i) effective outdoor exposure time and (ii) body orientation. These adjustments allowed conversion of ambient daily doses into effective UVB and UVA-received doses (ERD). Finally, a third factor, representing realistic sunscreen application density (0.8 mg/cm²), was introduced to estimate the necessary level of UV protection.

Results

Across all latitudes and seasons, UV-weighted doses requirements follow predictable patterns driven by solar geometry. At most temperate latitudes, daily SED values range from approximately 5 to 60, while extreme values of 65–70 SED occur in tropical regions (10–30° N). The greatest differences between latitudes arise from seasonality. UVA-PF requirements exhibit similar constraints.

When converted into effective received dose and then expressed in SPF terms, values above 60 provide minimal additional UVB benefit under realistic conditions.

Conclusions

By converting ambient irradiance into effective received dose, this model provides a realistic framework for daily UV exposure. It complements Diffey's approach rather than replacing it, supports rational discussion on SPF and UVA-PF adequacy without prescribing individual recommendations and offers a quantitative basis for realistic product design, regulatory discussion and consumer guidance.