White hair is the result of a reduction in the body's production of melanin, which naturally occurs as the human body ages. According to the Library of Congress, scientists still do not fully understand why this occurs in humans.
Scientific American explains that loss of hair color does not indicate the presence of a health problem. Instead, the pigment known as melanin simply stops entering hair cells with age. Some people even experience this phenomenon very young in life. While genetic factors play a role in the process, as of 2014, researchers are still trying to understand the specifics behind the loss of pigmentation in the hair. The only difference between white and gray hair is simply that white hair has no pigmentation whatsoever, while gray hair still has traces of melanin.