There are several potential causes of these light spots on your back. You will need to see your primary care doctor
, or your dermatologist
, in order to have a formal diagnosis, however.
One possibility is that this is a fungal infection. There is a type of fungus called tinea versicolor (commonly called "sun fungus") that very commonly infects the skin of the back and chest. The symptoms are irregular patches of light colored skin, which sometimes may appear a bit dry or itch, although often they are completely asymptomatic. Fortunately, tinea versicolor is very easy to treat with several different anti fungal medications, which your doctor can prescribe for you if they diagnose this condition.
Another possibility is that this is some sort of skin allergy or eczema
. This would be especially the case if the spots are reddened or irritated or if they itch quite a bit. The treatment for allergies and eczema are, fortunately, also very effective and straight forward. However, they are quite different than the treatments for fungal infections of the skin, which is one of the primary reasons that it is important to have your doctor take a look at this and make a formal diagnosis.