Sunscreen testing
Sunscreens work. Use them.
While clearing my office the other day I noticed some pretty coloured plots on the wall that hadn’t seen the light of day outside my office. I first produced them way back in 2010. Miraculously, I found the source code that I’d used to generate them, so was able to make the version below.
The plots below show results from an experiment using an inexpensive diode array spectrograph - manufactured by Ocean Optics - to measure the spectral dependence in the effectiveness of a sunscreen at protecting our skin from UV damage from sunlight.
The red curve in the upper panel of the plot shows the spectrum of summer sunlight (measured at 10:30 am on 15 Jan 2010) at Lauder New Zealand (45S) viewed directly though a quartz UV-transmitting microscope slide in front of the entrance port of the instrument, whereas the blue curve shows how it looks when a coating of the sunscreen is applied to the slide. As you can see, the sunscreen blocks some radiation at all wavelengths, but especially in the UV region (wavelengths shorter than 400 nm) where the blue curve flat-lines close to the x- axis. The instrument hadn’t been calibrated, so the y-axis of the spectrum is shown only in counts.
Independently of a calibration, the ratio of the two spectra gives the transmission of the sunscreen, as shown in the lower panel of the plot. According to my notes, the sunscreen was rated at SPF30, where SPF is the ‘Sun Protection Factor’. That means that when the sunscreen is applied at the correct thickness, the transmitted UV radiation reduces by a factor of thirty - so the transmission would be about 3 percent.
In this experiment, the UV transmission is closer to 2 percent, probably because a thicker layer was applied. You’ll see that the sunscreen also blocks much of the violet light and transmits only 70 to 80 percent of the sunlight throughout the remainder of the visible spectrum.
The increasingly noisy data in the UVB region (below wavelength 315 nm) is due to the poor performance of this type of instrument at the shortest wavelengths, as discussed in an earlier post.
Sorry, I didn’t record the name of the sunscreen manufacturer. I think it may have been Coppertone. All I remember for sure is that it was in a brown plastic container that had been sitting on my window sill for years. Despite being well past its use-by date, it still seemed to work OK (though its longer-term stability against photo-degradation by sunlight on exposed skin may have been compromised). If only I’d thought to repeat the experiment a few hours later!
Sunscreens work! Use them. They might save your life.
New Zealand (along with Australia) has the world’s highest rates of skin cancer, with thousands of new melanoma cases each year in a population of 5 million. Around 500 New Zealanders die from skin cancer each year, about 300 from melanoma. Our mortality rates are more than twice that in the UK, and five to ten times larger than the global average.
A Housekeeping Notice. Sometimes my own substack posts find their way to my spam folder! I must stop using naughty words. Avoid losing any substack postings by getting the Substack App (freely available for both Android and iPhone). The versions there include any updates, and back-postings are readily findable.