Adapting SPF Testing Methods
for Mineral Sunscreen Density
Paul G. McCormick
KEY WORDS: sunscreen regulations, SPF, film thickness, mineral
sunscreens, density
ABSTRACT: A sunscreen layer’s thickness is critical to its SPF.
However, current regulations specify a mass application
rate for testing, rather than a volumetric application
rate. This significantly underrates the SPF values of
mineral sunscreens due to their higher densities since,
compared with their relative organic counterparts,
thinner films are being tested, as will be shown here.
BEER’S LAW
It is accepted that typical sunscreen application is based on volume,
not mass. For example, the Australian
Cancer Council recommends that adults
apply about a half teaspoon of sunscreen
to the face, neck and ears; a teaspoon to
each arm and leg; and a teaspoon each
to the front and back of the torso.1 However, when the efficacy of sunscreens
is evaluated, testing is carried based
on a mass, not volumetric, application
standard. For example, in vivo SPF and
in vitro UVA test specifications require
that the sunscreen be applied by mass
of the layer per unit area.2–5 Since the
attenuation of incident solar radiation
through a sunscreen layer at a particular
wavelength depends directly on the
thickness of the layer (see Beer’s Law),
not the mass applied per unit area,
this procedure is incorrect and has no
scientific basis.
The reason for adopting a testing
standard based on mass rather than
volume is historical. Until recently,
nearly all sunscreens have employed
only organic UV absorbers to attenuate
UV radiation. Such sunscreens invariably exhibit product densities close
to 1 g/cm3. With a density of 1, the
application rate of 2 mg/cm2, specified
for in vivo SPF testing by the various
regulatory bodies,2–4 corresponds to a
2 μL/cm2 volumetric application rate
and a uniform 20 micron film thickness.
In the past, many studies6–9 as well as
the US Food and Drug Administration (FDA)10 have used 2 μL/cm2 and
2 mg/cm2 interchangeably, even referring to thicknesses in units of mg/cm2.
Beer’s Law characterizes the
absorption of light passing through
a sunscreen layer in terms of
the properties of the sunscreen.
Mathematically, Beer’s Law states
that the transmittance of light
through the layer (τ) is determined
by the absorption coefficient of the
sunscreen (α) and layer thickness
(t), as illustrated by the following
equation:
τ = I1/I0 = exp(-αt)
In vivo SPF values
currently reported for
inorganic sunscreens may
significantly underrate the
true level of protection.
Here, I0 is the intensity of light
incident on the layer and I1 is the
intensity of light that passes through
the layer. When expressed this way,
the transmittance is always on a
scale from 0 to 1. In a sunscreen, the
absorption coefficient depends on the
concentration of UV actives and the
wavelength of light.
Recently, however, transparent
mineral sun care products have increasingly entered the market, which due the
high density of their inorganic actives,
exhibit densities greater than 1 g/cm3.
For example, in Australia there is a
rapidly growing market for zinc oxide
sunscreens containing 20–25% w/w zinc
oxide—and a sunscreen containing 20%
zinc oxide has a density approximately
21% greater than a typical organic
sunscreen.
The present paper therefore demonstrates that the use of a mass-based
application rate for in vivo SPF testing
significantly underestimates the in vivo
SPF of sunscreens containing high loads
of inorganic UV actives, particularly
zinc oxide. Further, this supports the
notion of a volumetric application rate
for in vivo SPF testing, or alternatively,
a mass-based application rate adjusted
to account for the specific gravity (SG)
of the sunscreen.
Analysis
In the present study, a comparative
analysis was conducted of a sunscreen
containing organic UV filters and
a mineral sunscreen containing the
inorganic UV filter zinc oxide. Both
sunscreens are assumed to be emulsions
containing 50% water and to exhibit an
164 | Cosmetics & Toiletries® magazine www.CosmeticsandToiletries.com
Vol. 126, No. 3/March 2011
