I get these when I tan during summer. It is caused when you are subjected to a lot of sunlight. A special fungus then grows on your skin, causing the loss of pigment. My dermatologist gave me Spectazole Cream (Econazole Nitrate 1%). I put it on for a month, and they went away and never came back. But, the doctor warned that they will come back in summer again, so just save the cream. You need a perscription for this. There is no way to prevent them, except wear a shirt and don't go outside, which is unlikely. So, just ask for this cream.