Monday, December 21, 2009

Ultraviolet light levels in the rainforest

I made some measurements of UV-B light in rainforest in North Queensland some years ago. They relate to a theory I have published on the evolution of pygmy stature in humans, which I attribute to low ultraviolet light levels in this environment (under the rainforest canopy).

My theory is cited here:


The data were published in a newsletter put out by the Australasian Society for Human Biology, but are no longer available on the Internet. So I shall give a summary of the results here.

I used an ultraviolet light meter to take readings in rainforest at Kuranda and nearby regions in North Queensland in August 1994. The readings in the open - outside the rainforest - during fine weather ranged from 70 to 245 units. The readings inside the rainforest ranged from 0 to 5.

To take one example, on 5 August 1994, at 11.00 am in rainforest near Kuranda on the Mareeba Road, North Queensland, I obtained a typical reading of 1 unit of UV-B. In the open, outside the rainforest, I obtained a reading of 189 units. Taking these readings inside and outside the rainforest at the same time was fairly easy, as the rainforest was beside the road.

It was clear that the rainforest canopy cut off almost all the ultraviolet light and prevented it from reaching the forest floor. The Aborigines who used to live in these rainforests would have been exposed to low ultraviolet light levels.

There is surprisingly little published on ultraviolet light levels in rainforest. It is well-known that it is gloomy inside a rainforest, but what is not so well-known is that ultraviolet light levels, of the kind that help human skin make vitamin D, are particularly low.



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