- Slip versus Slop: A Head-to-Head Comparison of UV-Protective Clothing to Sunscreen

Ultraviolet protection factor (UPF) is primarily a measure of UVB protection whereas critical wavelength (CW) is a measure of the degree of broad-spectrum protection. (A) UPF is a mathematical function designed to recapitulate sun protection factor (SPF) from a laboratory measurement of transmittance and is weighted toward the UVB portion of the spectrum. Left: Plots of the erythemal effectiveness function (Eλ) and the solar spectral irradiance (Sλ) over the UVR spectral range. Right: The product of Eλ and Sλ has a peak that lies predominantly (75%) within the UVB range (280–315 nm). Very little of the UPF function comes from wavelengths longer than 360 nm, where the UVR intensity is highest but the erythemal effectiveness is near zero [48]. (B) Visual representation of the CW as the wavelength below which 90% of the total absorbance (area under the curve) in the UVR region is contained. Left: Hypothetical sunscreen that primarily blocks UVB radiation has a critical wavelength below the 370 nm threshold required by the FDA to be labeled broad-spectrum. Right: Hypothetical sunscreen with improved UVA blocking performance meets the broad-spectrum criterion. A sunscreen could meet the CW criterion of 370 nm without comprehensively blocking UVA radiation.