Under the Sun

The article by Stacie Zoe Berg1 raises the interesting possibility that exposure to solar UV-B radiation could induce autoimmune diseases. This certainly sounds plausible. We know that prolonged exposure to UV-B causes basal-cell and squamous-cell cancers in susceptible individuals, particularly fair-skinned people. [Melanoma appears to be caused by both UV-B and the much stronger UV-A radiation.]

But the report states that autoimmune incidence decreases towards lower latitudes, whereas UV-B exposure increases strongly towards lower latitudes. Direct measurements indicate an increase of 200-300% in going from northern to southern states. The increase is about 10% in going from Washington, DC, to Richmond, Va. The increase stems from the lower average zenith angle of the sun as one approaches the equator. Combined with a horizontal layer of stratospheric ozone, this leads to reduced absorption of the incident solar UV-B. [UV-A is not absorbed by ozone.]

Skin cancer occurrence...

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