What's the REAL SPF of that sunscreen?
.... why it can differ from lab tests and for different sun elevation angles.
Sorry about the double posting on the same day last week. That was the cost of being away from my computer for too long. I trust you were all fully edified, yet not too overloaded.
The real SPF of your sunscreen is probably a lot less than you think, because you probably don’t apply nearly enough of it, as I’ve mentioned previously. But there’s more to it than that.
STOP PRESS. A related scandal and PR disaster broke out just yesterday in Australia when ABC News reported that several popular sunscreen failed to meet their advertised SPF rating. Only 4 of 20 tested met it. For most, the measured SPF was around half that advertised. The worst was WAY below the mark with a measured SPF of only 4 for a supposed SPF50 sunscreen! It’s worth noting that the company involved disputes that finding. I was relieved to see that the sunscreen I’ve been using doesn’t seem too bad. It’s SPF was found to be 41 instead of the advertised 50.
Rather than using real sunlight, the tests were carried out in a laboratory using a lamp with its output designed to simulate the solar spectrum. In view of the huge divergence, which is much larger than previously measured shortfalls, I wonder how well their lamp really simulates sunlight? See below ….
Coincidentally, a reader had recently asked: why is the SPF measured in the laboratory different from that in sunlight? Good question.
So, here we go …
The transmission spectrum of sunscreens typically have some dependence on wavelength, as illustrated below for some examples where the ‘absorbance’ of three different sunscreen materials has been measured. In this example, the layers must be very thin: much less than the recommended applications for sunscreens. The transmission is just one minus the absorbance, so an absorbance of 0.1 implies a transmission of 0.9 (or 90 percent). The absorbance of zinc oxide for all wavelengths less than 380 nm is close to 0.5 in this case, corresponding to a transmission of 0.5 and an SPF of (only) 2.
The wavelength dependence of these materials can lead to SPF differences depending on the shape of the spectrum of light involved. There will inevitably be differences between the spectrum of sunlight and the spectrum of the solar simulator lamp used in laboratory assessments of sunscreen performance. Therefore, for sunscreens with a wavelength dependence in their absorption (especially for the blue and red curves above), the SPF calculated from those laboratory lamps will be different from its SPF in sunlight.
Even in sunlight, the effective SPF will have some dependence on ozone amount and sun elevation angle, which both affect the shape of the spectrum, as illustrated below.
The upper panel there shows summer and winter spectra (red and blue lines) at Lauder New Zealand (45S), along with the weighting function for skin damage (erythema), shown by the dashed grey line. Noting that the y-axis has a logarithmic scale, you can see that in the UVA region the summer values are higher than in winter by a factor of about 5. But at 300 nm, it’s higher in summer by more than a factor of 100!!
The curves in the lower panel of that figure show the corresponding spectra of skin-damaging radiation, which is obtained by multiplying the point for each wavelength on the red and blue curves above with the corresponding point in the action spectrum for erythema. The first thing to note is that the summer spectrum, with its higher sun elevation angles, is much more damaging to the skin, by about a factor of 10.
More relevant to this discussion: you can see that for the summer spectrum, the shorter wavelengths become relatively more important than in winter. For the summer spectrum, the most damaging wavelengths are near 305 nm, whereas for winter, wavelengths longer than 310 nm are most damaging.
In both cases, but especially the latter, the contribution from UVA wavelengths can’t be ignored. To illustrate that more clearly, the plot below shows the cumulative percentage contribution to the skin-damage as a function of wavelength for the summer and winter spectra shown above. For the summer spectrum, about 75 percent of the damage is from the UVB (i.e., wavelengths less than 315 nm), and the remaining 25 percent is from the UVA (wavelengths longer than 315 nm). For the winter spectrum, the relative UVA contribution is even larger. In that case, it dominates, with more than 60 percent of the skin-damaging radiation is from UVA wavelengths. It therefore follows that to be effective, sunscreens need to block a substantial proportion of UVA radiation as well as essentially all of the UVB radiation in sunlight.
For example, let’s assume we have a sunscreen that blocks everything below 360 nm, and nothing above that wavelength. From the graph, that implies that about 97 percent of the damaging radiation in summer will be blocked, leaving only 3 percent transmitted. The effective SPF for that summer spectrum would therefore be about 30. But for the winter spectrum, only 90 percent would be blocked, meaning that the SPF would be only 10. To achieve an SPF better than 30 for both, the sunscreen would need to block all radiation at wavelengths less than 380 nm.
The sad truth is that, for most sunscreens, the effective SPF will vary throughout the day. The good news is that they’ll probably be best at times when the need is greatest (i.e., near midday).
Referring back to that very first plot, you can see that even zinc oxide may not suffice. Remember, though, that the application thickness would be much greater than for the data shown there. Interestingly, the worst-performing sunscreen in the tests reported yesterday was of this type. But a drop-off in absorbance at longer wavelength like that couldn’t account for its observed (disputed) failure, unless the spectrum from the solar simulator was wildly different from reality.
There’s a nice write-up of the sunscreen-testing issues here, written a few years ago by my colleague, Antony Young.
Finally, I must apologise to longer-term subscribers of UV & You, who may have noted some duplication from a post last April. I remembered that only after writing most of this one. ☹ If you read both, please alert me to any contradictions. 😊