In reply to Richard Carter: A polarization filter eliminates reflected light, which is polarized, as you turn it. So any light that reflects is filtered out. In this way, you can take pictures through a window or a surface of water without getting the reflections.
It is also used in landscape photography, as it eliminates light which is reflected off ice crystals in the sky, making the image clear of reflected sunlight.
I do not know about the effects on sweaty skin, I suppose it would have a certain effect there, too.
L.