Excessive sun exposure is a common cause of brown spots on the scalp due to conditions such as melasma and actinic keratosis, per the Mayo Clinic and the American Academy of Dermatology. While these conditions differ slightly from each other, they both are frequently caused by excessive ultraviolet radiation.
In extreme cases, melanoma is another possibility for brown spots on the scalp. The head is a common place for melanoma to develop, according to the New York Times. Actinic keratosis can also develop into skin cancer over time, warns the Mayo Clinic.
Age spots, or liver spots, also develop on the scalp and in other body areas that receive a great deal of sun exposure. The best way to prevent or limit the spread of these brown spots is to use frequent sunscreen and avoid direct sunlight, per Mayo Clinic recommendation. Regularly use a sunscreen with a 30+ SPF rating, according to the New York Times. Sunscreens with a higher rating than 30 are unnecessary and not worth the added expense.
Melasma and other conditions often look similar to one another, so visit a dermatologist to get a proper diagnosis, as advised by the American Academy of Dermatology. During this doctor's visit, the dermatologist may need to remove a small patch of skin to diagnose the condition properly and rule out potential cancer.