UV damage & skin colour

Two steps forward, but one step back?

A table in my recent post about the time for skin damage by UV for different skin included the clipping below, which implies that the sensitivity of white skin to UV damage is between 5 and 7.5 times greater than for black skin.

The numbers there are based on the UV doses required to produce the first signs of skin reddening (called ‘erythema’) 24 hours after exposure. There’s a wide range in those in the literature - especially for darker skin types where it’s probably much harder to detect erythema. The comparison between skin types therefore needs to be taken with as grain of salt.

But that sort of skin damage may have nothing to do with the more serious issue of skin cancer -particularly melanoma.

My colleague David Whiteman in Brisbane drew my attention to a couple of relevant papers that I hadn’t seen before about UV effects on different skin type. The work was carried out by a group at Kings College, London, that’s led by another colleague, Antony Young.

The first paper, published in 2016, includes the figure below that compare the incidence rate of melanoma by ethnicity in the USA.

It shows that the risk for white males is about 20 times greater than for black males (and about 15 times greater for white females than black females). A much larger difference than implied by differences in skin-sensitivities shown in my table. Of course, differences in exposure between skin types may also be important.

The same paper then goes on to describe experiments involving DNA damage rather than skin reddening as an end point. As well as being much more relevant for skin cancer, it’s measurement is also much more objective. The idea is that the melanin in darker skin blocks the UV before damage can occur. So far, so good.

But the paper then makes the statement “DNA photo-damage initiates non-melanoma skin cancer and is reduced by a factor of about 3 in pigmented skin compared with white skin”, and concludes that “regular use of a low sun protection factor sunscreen or other photo-protection strategies (equivalent to melanin photo-protection – dose reduction factor of 3–6), preferably from an early age, would be very beneficial.”

It all seems rather odd. That ratio between blacks and whites is much smaller than implied by the different skin sensitivities for erythema. Yet the ratio in incidence rates, shown by the plot above, is much larger.

A later paper from the same group, published in 2018, muddies the water even more. Even these [slightly edited] sentences in the abstract of the paper seem a bit contradictory.

Melanin in black skin protected against DNA damage by 8.0-fold in the overall epidermis and by 59.0-, 16.5-, and 5.0-fold in the basal, middle, and upper epidermis, respectively. Protection was related to the distribution of melanin, which was most concentrated in the basal layer of black skin. These results may explain, at least in part, the considerable skin color differences in keratincyte cancer (KC) incidence. These data suggest that a DNA protection factor of at least 60 is necessary in sunscreens to reduce white skin KC incidence to a level that is comparable with that of black skin.

There seems to be a mismatch between that and the ‘factor of 8’ or ‘factor of 60’ mentioned in the abstract. Though, with different combinations of those three skin layer factors, either of those factors (8, or 60) could just be possible.

Using DNA damage as a marker does seem a step in the right direction. But there must be something else that I haven’t quite grasped because the numbers don’t add up (I’m only an atmospheric physicist). Any ideas that help resolve this for me would be greatly appreciated because it may affect the advice given in our smartphone apps.

I understand there will be a session on this at a meeting I’ll be attending later this year. I wait with bated breath.