On the shoulders, they are caused by sun burn. During pregnancy, or with contraception, the face can develop areas of pigmentation, this is melasma. In certain patients, the skin can tan unevenly, forming lighter-coloured spots, often on the arms. Others suffer from vitiligo, a depigmentation disorder where very white spots form on the skin.
Vitiligo is a genetic disorder, but it is greatly intensified by sun exposure. Melasma is hormonal, but it is also triggered by sun exposure. I also explain that actinic lentigo is a warning signal telling them that their “sun capital” (maximum sun exposure limit) has been used up. Any additional exposure will make them more visible.
On exposed parts of the skin, a high factor sunscreen is essential. For melasma, I recommend daily sun protection from the first sunny days in spring, to prevent the spots from getting any darker. For actinic lentigo, all available sun protection resources need to be used, including clothing, a hat and sunscreen.