I see a lot of patients presenting with pigmentation on the chest, back and armpits. This is in most cases caused by yeast infection. There is slight flaking of skin with dark and pale colours intermixed. This is a disease of warm and moist weather. It can sometime clear in cooler months but recurs again in summer months. It can affect many members of the same time.
Most of time its not itchy but some patients feel itching a major problem however its a very minority of patients. It is caused my Melasezia furfur, a yeast. This yeast grow more in summer months when moisture in the environment.
Exposure to sunlight makes involved area to become lighter and normal skin becomes darker. Hence two tone colour of skin.
It can be easily treated with topical and oral anti fungal treatments however identification of condition is tricky.
Get yourself checked early by proper qualified dermatologist. We recommend you visiting our clinic to be seen By Dr Ghazanfar Ali who is the best skin specialist in the city at the moment providing care to most embassies in Islamabad.