What could be the reason for a black spot appearing below the chest, and what should I do about it?

Black discoloration below the chest may be caused by various factors such as dirt accumulation, friction-induced pigmentation from clothing, pigmented nevi, infected sebaceous cysts, or seborrheic keratosis. Depending on the specific situation, improvements can be made through daily care, medication, or surgical treatment. Detailed analysis is as follows:

1. Dirt Accumulation

The skin under the chest has many folds where sweat and dead skin cells can mix and accumulate, forming dark deposits. Clean the area daily with lukewarm water and a gentle body wash, gently massaging the folds during washing, and keep the area dry afterward.

2. Friction-Induced Pigmentation from Clothing

Wearing tight, rough clothing over a long period can repeatedly rub against the skin under the chest, stimulating melanocytes and leading to hyperpigmentation. Switch to loose, soft clothing to reduce local friction, and apply a moisturizing lotion after cleansing to nourish the skin.

3. Pigmented Nevus (Mole)

Formed by clusters of melanocytes in the skin, these appear as black or dark brown raised lesions and are usually asymptomatic. If there's no change in appearance, no special treatment is needed—avoid scratching or irritation. However, if the mole increases in size or develops ulceration, seek medical attention promptly for possible excision.

4. Infected Sebaceous Cyst

When sebaceous gland ducts become blocked, secretions accumulate and form a cyst, which may turn dark upon secondary infection. Under medical guidance, topical antibiotics such as mupirocin ointment or fusidic acid cream may be used, along with oral cephalexin capsules. Once the infection is controlled, surgical removal of the cyst may be performed.

5. Seborrheic Keratosis

This condition results from abnormal proliferation of keratinocytes due to skin aging, presenting as black or brown patches. As directed by a physician, topical treatments like tretinoin cream or fluorouracil ointment may be applied, along with oral vitamin E supplements. Alternatively, cryotherapy can be used to remove the lesion.

Maintain good hygiene under the chest and wear comfortable clothing to minimize irritation. Monitor any changes in the dark spots—if they grow larger or become painful, seek medical help promptly. Establish healthy lifestyle habits, maintain a balanced diet, and support overall skin health.