Your skin contains a pigment called melanin that is responsible for your natural skin tone. This pigment is sensitive to the effects of ultraviolet (UV) radiation from sunlight, which reacts with the melanin and turns it darker. This type of darkened skin tone is commonly referred to as a 'sun tan' and is completely harmless.
From your description, it seems highly likely that you have a normal sun tan, since it is likely that you arms and shoulders are exposed to direct sunlight. If so, all you have to do is avoid sunlight for a few weeks and your skin will revert to its normal color. You can prevent such a tan by wearing long-sleeved clothes or applying a sunscreen lotion with an appropriate Sun Protection Factor (SPF) for adequate UV protection. However, if you have developed darker patches of skin in areas that are normally shielded from the sun, the cause may not be a sun tan. Your marks may have be symptoms of more serious medical conditions. For example, some women develop dark spots on the skin due to hormonal fluctuations during the menstrual cycle. If you are unsure of the exact cause, it is advised that you visit your doctor or a dermatologist.
Your symptoms could also be caused by hyperpigmentation. This refers to a condition where melanin is present in a higher concentration than normal in certain areas, making the skin appear darker. In this case, you could get good results by using commonly available skin whitening or lightening products. These need to be applied on the affected areas and act by 'bleaching' the skin in a matter of weeks. However, be careful not to overuse the product or let it stray onto the surrounding skin. This can lead to you developing patches lighter than your normal skin tone. You can even try some simple home remedies. Many people believe that milk or milk-based lotions have a lightening and smoothening effect on the skin. You could also apply a paste of baking soda and water after a bath, while your skin is still moist. Allow it to dry for approximately ten minutes and then rinse it off. Lemon juice has also been known to work on smaller areas, since it is mildly acidic in nature. However, you need to be careful not to apply the juice for more than 10 minutes at a time to avoid irritating the skin.
answered by G M