Sunscreen heresy

Why are the SPFs of sunscreens so high these days? Is it a good thing?

Talking of heresy, my second name, Lloyd, is (I think) after the famous Lloyd Geering, who was an old friend of my parents. His wife at the time of my birth may have been a distant relative. A theologian with the Presbyterian church, he achieved notoriety in 1967 when they accused him of heresy. He was born in 1918 and is still going strong, as you can tell from this delightful report just last month.

In the last couple of weeks, I’ve talked about skin damage from UV in sunlight and the use of sunscreens to guard against it. That set me thinking …

At the risk of evoking howls of protests among my medical colleagues and health professionals, I do wonder - as a mere atmospheric researcher - why are the SPFs in modern sunscreens so large? Remember that the SPF is the supposed Sun Protection Factor. If you (correctly) apply an SPF50 sunscreen, the idea is that it should transmit only 1/50th of the sun-burning UV, so the time for skin-damage would be 50 times longer at the same UVI level. Though that interpretation has recently been called into question when it was shown that in sunlight, the SPFs of many sunscreens are less than 50 percent of their advertised values, as estimated in the laboratory.

No sunscreens are required for people with heavily pigmented skin, for whom skin cancer is extremely rare. In my ignorance on medical matters, I would have thought that all human skin should have evolved to cope adequately with the UV levels present in the sunlight received. But maybe there was a trade-off cost that arose as the darker-skinned populations migrated poleward and skin-colours lightened. Lighter skin tones were needed there to maintain healthy levels of vitamin D and folic acid. But the wavelengths for their induction are the same as that that induce skin damage. Perhaps there would be an inevitable cost? Another ‘Faustian’ bargain, as climate-scientist colleague James Hansen may have put it 😊.

Given that UV does indeed cause skin cancer, what is the real damaging dose? And does it depend on how long it takes for that dose to be reached? For the fairest skin types, the first sign of a skin damaging dose, as evidenced by skin reddening (i.e., erythema) the next morning, is generally considered to be about 2 SED, a dose that can be achieved in less than 15 minutes of exposure to UVI=10, or 150 minutes at UVI=1. But does that latter combination really result in the same skin damage? We don’t know, because erythema has been demonstrated only for exposures to UVIs greater than about 3. It’s possible that for UVIs lower than 3, repair mechanisms can keep up well enough to avoid damage. If that were the case, then it would be just the higher UVIs we need to be concerned about, not so much the daily dose. If that’s the case, the issue I raised here would be unimportant.

Whether or not superficial damage like sunburn leads to melanoma, the most dangerous form of skin cancer, is another question entirely. It’s just assumed to be the case. Although UV is almost certainly involved, it may be only after exposure at higher levels that these more dangerous outcomes occur.

Personally, I tend to worry about skin damage only when the UVI is greater than about 3 because such moderate values have always occurred throughout my ancestral homelands. That threshold is commonly used by health professionals too. If we need only to make sure we’re never exposed to UVI greater than 3, the required SPF for sunscreens would be much smaller. Given that UVI=25 is the highest measured anywhere on planet Earth, and that values greater than 20 rarely occur in populated areas, an SPF of around 7 should be sufficient. In the UK, where the UVI never exceeds 9, perhaps an SPF of 3 would suffice, or New Zealand, with a maximum UVI around 15, would an SPF of 5 do? Sunscreens with SPFs around that value were commonly available in New Zealand late last century, as you can see from this old bottle below, where the SPF rating is 6.

A sunscreen manufactured late last century. As tested here.

Nowadays, SPFs of 50 are perhaps the most common in New Zealand, and some are 100 or more.

As far as were aware, there haven’t been huge increases in UV over recent years, or even millennia. In northern Scotland, from whence my at-risk lighter skin may have evolved, UV levels are certainly much lower than here in New Zealand. But by no more than a factor of 5. Not a factor of 50 or more.

Noting that skin cancer does occur there, as well as here (at rates roughly proportional to UV levels), can it be that we now live too long, so accumulated skin damage is higher? Probably not, at least for melanoma, which can arise at an early age. In any case, with our modern lifestyle, the number of hours outdoors is surely less than in pre-industrial times.

Maybe it’s something to do with intermittency of exposure, or diet, or other environmental stressors present in the modern world. These could include factors as diverse as changing hygiene habits at home or changing patterns of international travel.

Is the modern lifestyle choice of a daily morning shower including de-greasing agents detrimental to skin health? In bygone years, perhaps those less frequent baths - usually in the evening rather than the morning - left the skin in a more protected state during the high UV period. It’s sometimes reported - without much evidence it seems - that the SPF of these skin-oils is rather low. Regardless of that, since my own brush with skin cancer, I’ve got into the habit of applying a protective moisturiser (SPF15) after showering, as a first line of protection.

The modern ‘snowbird’ habit of flying to warmer climates for the winter months is potentially more problematic. If your home is at mid- to high-latitudes, those winter excursions could easily more than double your annual UV dose. The initial sudden exposures for unacclimatised skin could be especially damaging. In the normal course of events, the slow increase in UV during spring months would allow some acclimatisation to the skin which could provide some natural protection by the time it’s really needed in summer.

Perhaps the high SPFs we see now are just in recognition of the fact that we never apply enough, or that the real SPF is rather lower than advertised. That’s my pick.

I just hope that the sunscreens themselves aren’t part of the problem. Either something in them, or perhaps (a possibly incorrect) perception of safety with their use that encourages prolonged exposure. The higher the SPF, perhaps the greater that misplaced perception.

Whatever the reason, as I noted in an earlier post, our incidence rates for skin cancer remain stubbornly persistent, and those for melanoma continue to increase, as you can see below, despite all the publicity and health warnings.

Age-Standardised-Rates (ASR) for in-situ melanoma incidence in New Zealand from 2000 to 2021. Adapted from here.

Food for thought. If any of you have more ideas, please share them as (preferably polite) comments.

In the meantime, stay safe by following the advice below from Sunsmart New Zealand. Use your common sense.