I used to paint in the nude (though until now I've always kept this to myself, figuring people would really wonder about me). It seemed easier to clean up my skin than clothes, and I enjoyed the freedom. I paint with oils, however, and have a warning for oil painters.
There are some diseases that have no known cause, such as the lymphomas. It is believed chemical pollutants may be the cause of these types of blood cancers. I know this from extensive research after being diagnosed with non-Hodgkin's lymphoma.
It has really made me conscious of things that can be absorbed through the skin, such as mineral spirits, cadmium (I was always careful with this one) and other pigments. I now paint fully covered, even on the hottest days of summer.
More About Paint Health and Safety:
When you're using a tube of paint, check the label which should give you information on what's in that particular tube. The Art and Creative Materials Institute standards logos often appear on a label too.