an organic sunscreen. In fact, extending the above example to a sunscreen
containing 25% w/w zinc oxide would
predict an SPF of 50+ if the product
were tested at a dosage of 2 μL/cm2.
Figure 2. Effect of density on the thickness of a sunscreen layer deposited at a loading
of 2 mg/cm2
application rate into the relevant test
procedures should be straightforward
since volumetric pipettes of sufficient
accuracy are widely available. Early
laboratory measurements often tested
sunscreens applied by volume.15–20
Such a procedure may actually improve
accuracy as the need to minimize evaporation of the product during application
is avoided. Alternatively, a mass-based
application rate could be used with the
application rate adjusted by the specific
gravity (SG) of the sunscreen (
application rate = SG x 2 mg/cm2).
This analysis may also be extended
to sunscreens containing titanium
dioxide as the UV active. Sunscreens
containing 8% w/w titanium dioxide with SPFs 30+ are currently
available. In comparison to zinc
oxide, titanium dioxide has a somewhat
lower density (4.23 vs. 5.606 g/cm3). By
using the step film model, it is estimated
that the SPF 30+ titanium dioxide
sunscreen containing 8% w/w titanium
dioxide would increase by ~15% if
tested at the same film thickness as an
organic sunscreen.
Figure 3. Variation of SPF with applied film thickness calculated using step film model
Conclusions
The specification of a mass-based
product application rate for the in vivo
and in vitro testing of sunscreens is
fundamentally incorrect and can only
be justified if the densities of all sunscreens are the same. With sunscreens
containing mineral actives, the density
increases with increasing active content,
resulting in the average film thickness
168 | Cosmetics & Toiletries® magazine www.CosmeticsandToiletries.com
Vol. 126, No. 3/March 2011
