Slip, Slap and Spot

The latest research shows that the incidence of malignant melanoma in Maori has nearly doubled. Only 12% of that rise is thought to be due to the depleted ozone layer. It is speculated that some of the rise is due to the lightening of Maori skin tones by intermarriage. A similar rise has been reported in dark-skinned people in the US.

The MacDoctor has a different theory.

I suspect that a large proportion (but not all) of the rise in melanoma incidence has been due to the intensive “slip, slap, slop” campaign and similar programs run in the US.

Some of you are thinking that the swine flu has gone to my brain and is causing delirium. Not so (what is this brain thing you speak of?) It is my belief that the inappropriate use of the “slip, slap, slop” message to people with dark skin colour has resulted in a paradoxical increase in melanoma. The problem has been two-fold.

The first part of the problem is that darker-skinned people tend to use lower SPF products than lighter-skinned people. I have seen kids from our youth group all diligently putting on sunscreen. The pale-skinned kids put on SPF 30, the dark-skinned kids mostly put on SPF 4 (some 8). All the kids spent the same amount of time in the sun. Guess which ones burnt?

The second part occurs when the darker-skinned kids diligently put on higher SPF factor. The reduction in UV penetration reduces the concentration of melanin in their melanocytes (pigment cells), lightening their skin. Coupled with decreasing outdoor exposure, this means that Maoris are becoming paler, without the addition of intermarriage. Occasional, incidental sunburn therefore has greater consequences, as far less UV radiation is absorbed by the melanocytes, making sunburn much worse. This effect is more notable in people of dark skin as the amount of radiation absorbed by melanocytes is linear (twice as much melanin absorbs twice as much UV radiation). Therefore a 20% reduction in melanin in a dark skin (high initial concentration of melanin) is far more significant than a 20% reduction in melanin in a light skin (because the starting concentration in light skin is so much lower).

This means that the UV dosage absorbed by the exposed skin of a unprotected, dark skinned person – who otherwise regularly uses sunscreen – is significantly greater than the extra dose of the unprotected skin of a light-skinned person (who also otherwise regularly uses sunscreen). It is therefore unsurprising that the net result of the use of SPF 30 cream in dark-skinned people is greater incidental (occasional) exposure to UV radiation and therefore an increasing incidence of melanoma. There is quite a lot of evidence that suggests that it is not long term low level exposure to UV that is a problem, but incidences of excessive exposure (otherwise known as sunburn). It is perfectly possible that dark-skinned Maoris would be better off simply spending less time in the sun, rather than slapping on sunscreen.

All of us would benefit from shorter periods in the sun (except in winter!), but it seems strange that the people who may derive the most benefit are the people with the darkest skin. Of course the idea that all the Maoris would be willing to sit in the shade while their SPF-50-smeared white mates are playing football seems a tad unlikely, doesn’t it?

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