There are several reasons why white spots or sun spots become noticeable on the body once the tanning process begins.
Areas of the skin which do not tan could be the result of genetics; the melanocytes in a certain area may simply not be efficient at producing melanin.
Tanners can also develop white spots that do not develop colour, which are caused by the presence of a common fungus living on the skin’s surface. While the fungus is harmless, it absorbs UV light which would normally penetrate the skin. This fungus is Not a result of UV tanning; it merely becomes noticeable once a tan develops. It can be remedied by prescription medication or topical lotions created specifically to address this problem.
Certain medications can cause this type of problem when combined with UV exposure. For example birth control pills can cause blotches and uneven skin pigmentation.