Adapting SPF Testing Methods for Mineral Sunscreen Density

Mar 1, 2011 | Contact Author | By: Paul G. McCormick, University of Western Australia
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Title: Adapting SPF Testing Methods for Mineral Sunscreen Density
sunscreen regulationsx SPFx film thicknessx mineral sunscreensx densityx
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Keywords: 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.

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PG McCormick, Adapting SPF Testing Methods for Mineral Sunscreen Density, Cosm & Toil 164 (2011)

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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.

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.

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Table 1. Properties of Sunscreens

Table 1. Properties of Sunscreens

The values of concentration and density assumed in the analysis are shown in Table 1.

Figure 1. Effect of zinc oxide concentration on sunscreen density

Figure 1. Effect of zinc oxide concentration on sunscreen density

The effect of the UV active concentration on product density is shown in Figure 1.

Figure 2. Effect of density on the thickness of a sunscreen layer deposited at a loading of 2 mg/cm2

Figure 2. Effect of density on the thickness of a sunscreen layer deposited at a loading of 2 mg/cm2

Figure 2 compares the corresponding film thicknesses for the zinc oxide and organic sunscreens for an application rate of 2 mg/cm2.

Figure 3. Variation of SPF with applied film thickness calculated using step film model

Figure 3. Variation of SPF with applied film thickness calculated using step film model

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.

Beer's Law

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)

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.

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