measurements have been conducted
on organic sunscreens where the SPF is
clearly shown to increase with increasing
application rate or film thickness, with
both linear and exponential dependencies on film thickness being reported.6, 7,
11, 12 However, no measurements of the
effect of film thickness or application
rate on in vivo SPF appear to have been
reported for inorganic sunscreens.
Herzog has shown13 that the step
film model of O’Neil14 provides a basis
for calculating realistic values of in vivo
SPF from values of molar extinction
coefficient, concentration and film
thickness. In Figure 3, values of SPF
calculated using the O’Neil-Herzog13, 14
model are plotted as a function of
applied film thickness for a 20% w/w
zinc oxide sunscreen exhibiting an
in vivo SPF 30 at an application rate of
2 mg/cm2. Film geometry parameters
similar to that used by Herzog13 were
employed in the calculation and were
assumed to be constant for the range of
film thicknesses shown.
As shown in Figure 2, 2 mg/cm2 corresponds to a film thickness of 16.7 μm.
To achieve a film thickness of 2 μm,
it is necessary to increase the mass
loading rate to 2.39 mg/cm2. Using the
O’Neil-Herzog step film model, it is
predicted that the SPF should increase
by 50%, from 30 to 45, when measured
in a 20 μm film as compared with using
20 mg/cm2.
Figure 1. Effect of zinc oxide concentration on sunscreen density
Vol. 126, No. 3/March 2011
www.CosmeticsandToiletries.com Cosmetics & Toiletries® magazine | 167
