BACKGROUND: There are considerable data to suggest that protection from solar 
ultraviolet (UV) radiation will reduce the risk of acute and chronic skin damage 
in humans. Whereas the sun protection factor (SPF) provides an index of 
protection against erythemally effective solar UV, largely confined to the UVB 
(290-320 nm) and short-wavelength UVA (320-340 nm) region, there is currently no 
agreed-upon method to measure broad-spectrum protection against long-wavelength 
UVA (340-400 nm).
OBJECTIVE: The objective of these studies was to assess the potential of in 
vitro UV substrate spectrophotometry and subsequent calculation of the "critical 
wavelength" value as a measure of broad-spectrum UV protection and as a routine, 
practical procedure for classification of sunscreen products.
METHODS: The spectral absorption of 59 commercially available sunscreen products 
and multiple experimental formulas with one or more UV filters was measured. 
Sunscreen product, 1 mg/cm(2), was applied to a hydrated synthetic collagen 
substrate, preirradiated with a solar simulator, and then subjected to UV 
substrate spectrophotometry. Multiple determinations from 5 independent samples 
per product were used to calculate the critical wavelength value, defined as the 
wavelength at which the integral of the spectral absorbance curve reached 90% of 
the integral from 290 to 400 nm.
RESULTS: We found that a recognized long-wave UVA active ingredient such as 
titanium dioxide, zinc oxide, or avobenzone is a necessary but insufficient 
product requirement for achieving the highest proposed broad-spectrum 
classification, that is, critical wavelength of 370 nm or more. Although SPF and 
critical wavelength are largely independent of each other, UVA absorbance must 
increase commensurate with SPF to maintain the same critical wavelength value. 
Substrate spectrophotometry and the calculation of critical wavelength can 
readily account for sunscreen photostability by UV preirradiation. Finally, 
there is also a strong positive relationship between critical wavelength and a 
currently available in vivo measure of UVA protection.
CONCLUSION: Determination of critical wavelength by means of UV substrate 
spectrophotometry provides a rapid, inexpensive, and reliable measure of 
broad-spectrum protection, which is largely independent of SPF, yet ensures 
long-wavelength UVA protection commensurate with SPF. The procedure provides a 
routine, sensitive means of differentiating and classifying sunscreen products 
and, importantly, obviates the need to subject volunteers to acute exposures of 
high-dose, nonterrestrial UV, the health risks of which are still poorly 
understood.