I have been reading a book called Cure For All Diseases by Hulda Regehr Clark, Ph.D.,N.D. Now the book will appear to many to be very left of centre, but she states without much fanfare on pages 126 and 190 that MPB is simply an allergic reaction to nickel:

'Nickel is fat soluble and is stored in your skin fat temporarily when a surge of it enters the body. Your skin oils may be loaded with nickel causing "allergies" in the skin. Male pattern baldness is such an allergy. The sweat tries to excrete it for you. Always wash off your scalp sweat to help with this excretion. The skin oils dissolve nickel from metal jewelry (sometimes leaving your skin with a greenish black color) and transport it into your body.'