Seven teaspoons (35 millilitres) – that’s the recommended amount of sunscreen to provide the correct thickness for an average maximally exposed sunbaking body, with application to be repeated early after the initial sun exposure period. If you use sunscreen frequently, you’ll be aware that this wipes out a standard 110-milliltre tube very quickly (and don’t forget to take your teaspoon to the beach).
According to the relationship known as Beer’s law, absorbance (which takes into account light absorption, reflection and scattering) is proportional to thickness. For a closer look at the Beer’s law prediction for sunscreen, check out the box.
Through thick and thin
According to Beer’s law, reducing the applied thickness from the level now used to test sunscreens internationally (2 mg/cm2) to around what consumers actually use (1.25 mg/cm2), the product labelled SPF would decrease from 50 to around 10. However, in practice this result doesn’t follow.
A multicentre study (bit.ly/2jPTway) on the influence of applied quantity of sunscreen on SPF for a variety of sunscreen preparations showed that, at the quantities consumers are found to apply:
- effective sun protection factor (SPF) was about a third of the label’s declared value
- the experimental relation between SPF and thickness is pretty close to linear, not exponential.
There goes the theory.
Why the discrepancy?
Unless you use something like opaque zinc cream, there really isn’t any way of telling whether you have done a good job of uniformly covering the exposed skin or of measuring deterioration of the screen with activity. (In contrast, the coverage by clothing is unambiguous. Even non-UV-rated clothing is generally good enough protection under most circumstances.)
Importantly, the skin surface isn’t smooth – it has valleys and ridges whose prominence and depth vary with the age of the skin. Just how sunscreens behave when spread on this wrinkled landscape is a subject of tantalising research by DSM Nutritional Products (bit.ly/2FvndU0). According to this research, we need to move from chemistry of absorbance to the physics of spreading and fixing the sunscreen film. We need rheology (the physics of flow) with its yield stress and the (negative) Marangoni effect (the positive effect of which is seen when alcoholic drinks rise up the sides of a glass).
Thus, sunscreens can be expected to initially cover the skin very unevenly, like a first coat paint job on a rough surface. To allow for this, thicknesses were set very (unrealistically) high for testing. (High applied thickness was also selected for the international sunscreen tests because anything less did not allow for reliable results.)
Quality of sunscreens and absorption by the skin (with one exception – see bit.ly/2DlaMmt) no longer appear to be a problem with modern ingredients, so this is probably not a factor.
Sun strength and skin type
SPF is the ratio of how long you can stay out in the sun with and without sunscreen. So how long can you stay in the sun without it?
This depends on strength of the sun and skin type. For compliance to the standard, the strength of the sun is set at the maximum found in the strongest exposed global regions, at midday, and for the worst possible behaviour in the sun (lying flat, motionless and exposed all the time) (bit.ly/2DlaMmt). (See the box for more about the effects of UV on skin.)
The standard erythemal dose (SED) is a unit of UV exposure. A very fair-skinned person can receive sunburn and skin damage from only two SEDs. A full day under the extreme conditions yields 70 SEDs of UV exposure. So for this sensitive person, an SPF of 35 will, given all the safety factors, provide more than enough adequate protection for a day in the sun, provided the screen is applied and renewed as per instructions. Non-conforming application may lead consumers to rely on a level of protection that just won’t be provided. As Hamlet might have said, ‘There’s the rub’.
Getting the blend right
How do we work the realities of sunscreen application and sun exposure into useful advice for the consumer? Should the labels on sunscreen display an SPF that is less than the test-accredited SPF? Should we stop using the ‘SPF’ label, and instead identify sunscreens as ‘low’ (SPF 4–7), ‘medium’ (SPF 8–14), ‘high’ (SPF 15–24) and ‘ultra high’ (SPF > 25), as done in the EU?
The use of opaque sunscreens such as zinc cream has diminished (except for children). Incorporating a pale pigment that fades in the sun would allow monitoring of proper application, and, to please the aesthetes, it should disappear fairly quickly. (Such a product appeared on the market but has since disappeared.)
So slip, slap and be conscientious with the slop. To read more, get a copy of Sun, skin and health (ed T. Slevin, CSIRO Publishing, 2014).
My thanks to John Staton of Eurofins Dermatest Pty Ltd for many discussions, and to Professor Brian Diffey for correspondence and the graph of UV exposure effects.
I acknowledge the input from industry members of Australian Standards committee CS/42 [1976-1985], which I chaired. Ten years of ‘consensus building’ then needed lobbying of the government of the day to finally make this standard mandatory (a world first), and then to convince the supermarket chains to sell large packs of sunscreen at a low price; that is, not as a cosmetic (very cooperative).