Dark freckles appearing on the surface of the skin might occur for a number of reasons. The usual culprits include lifetime overexposure to the sun, a genetic predisposition to freckles, and a combination of both factors. Occasionally, diseases can also cause dark-colored spots to appear on the skin. One example is melanoma, a serious form of skin cancer, and another is xeroderma pigmentosum, a relatively rare disorder in which patients are unusually sensitive to all ultraviolet sources of light. In nearly all cases, dark freckles or spots are benign in nature and not a serious health threat.
The main scientific cause of dark freckles and increased skin pigmentation is the presence of epidermal cells called melanocytes. As skin is exposed to ultraviolet light, melanocytes attempt to protect the inner skin layers by producing the pigment, melanin, which better absorbs ultraviolet rays and helps to prevent further damage of the deep epidermis. People who are able to produce a lot of melanin usually have darker skin, less freckles, and don’t burn as easily. In other people, the melanin occurs in small clusters, resulting in more freckles than people with darker skin. These freckles can range in color from light tan to dark brown, or even reddish tones, in some cases.
Sunburn freckles and simple freckles are the two most common types of freckles. Simple freckles typically appear during childhood and are small and fairly light in color; they might be passed down genetically or occur from normal sun exposure, especially in people with fair skin. Sunburn freckles are darker, larger, and may have irregularly-shaped borders. These types of skin spots are usually caused by frequent instances of sunburn and appear most often on the arms, back, and shoulders. Both types of freckles appear most often during sunny summer weather and may fade away during the winter months.
The plural term lentigines refers to dark freckles that develop over time and do not fade away. While darker than normal-colored freckles, lentigines are normal and usually not indicative of a health threat. As they near middle age, adults who have experienced severe sunburns and overexposure to ultraviolet rays throughout their lifetime might develop lentigines as well as seborrheic keratoses. These keratoses are raised, rough patches of skin that might range from light brown to black in color. Like other skin discolorations, seborrheic keratoses are typically benign and pose no serious health risks.