in vivo SPF 30 when measured using
the accepted protocol. The values of
concentration and density assumed in
the analysis are shown in Table 1.
Table 1. Properties of Sunscreens
Sunscreen
Inorganic (B)
Ingredient
Active
Excipients
Water
Active (ZnO)
Excipients
Water
Concentration (wt %) Density (g/cm3) SPF
12 1.0 30
qs to 100 1.0
50 1.0
20 5.606 30
qs to 100 1.0
50 1.0
The effect of the UV active concentration on product density is shown in
Figure 1. As the concentration of the
zinc oxide active increases, the density
of the mineral sunscreen increases due
to the relatively high density of zinc
oxide. In the organic sunscreen, there is
no variation in density with concentration since the active and excipient are
assumed to have densities equal to 1.
Figure 2 compares the corresponding film thicknesses for the zinc oxide
and organic sunscreens for an application rate of 2 mg/cm2. To calculate the
film thickness, the application rate (mg/
cm2) was divided by the density (g/cm3).
Therefore, if the application rate is 2 mg/
cm2 and the density is 1 g/cm3, the film
thickness is 0.0002 cm or 20 microns,
which is the film thickness for organic
sunscreens.
On the other hand, the higher density of the zinc oxide active causes the
film thickness to decrease with increasing active concentration. For example,
for a concentration of 25% zinc oxide,
the film thickness of the zinc oxide
sunscreen is only ~80% of that for the
organic sunscreen. In other words, the
application rate of 2 mg/cm2 divided by
1.26 g/cm3, yielding a film thickness of
0.00016 cm or 16 microns.
For valid SPF testing, it is essential
that the applied film thicknesses are the
same for all products being tested. The
results shown in Figure 2 demonstrate
that the standard SPF test protocol of
applying the sunscreen at a rate of 2 mg/
cm2 results in the film thickness of zinc
oxide sunscreens being significantly
reduced, in comparison to sunscreens
containing organic actives. And since
the SPF of a sunscreen decreases with
decreasing film thickness, the measured
in vivo SPFs of zinc oxide sunscreens are
reduced relative to the values that would
be measured at the same film thickness
as the organic sunscreen.
Film Thickness and SPF
The dependence of SPF on film
thickness is not well-understood. Theoretically, the maximum increase of SPF
with increasing film thickness will be
given by Beer’s Law only if the extinction
coefficient does not vary with wavelength
and the absorber concentration and
film thickness are spatially uniform—
conditions that are never achieved in
reality. Both in vivo and in vitro SPF
166 | Cosmetics & Toiletries® magazine www.CosmeticsandToiletries.com
Vol. 126, No. 3/March 2011
