Often clogged sebum and sweat glands are the causes of acne on chest. A large number of small whitehead pimples can occur on your skin as the result of such problem.
What we wear can become a cause of acne on chest. Red pimples can appear on chest as the result violation of organism thermoregulation. It means you wear clothes that fit too tight. Your body might lack ventilation especially when your clothes are made of synthetic materials and are too tight. Such pimples because of improper thermoregulation may also appear under your breast.
Hormonal imbalance is one of the causes of pimples on chest. Hormonal disorders change sebum production and local immunity. In case hormonal changes are caused by natural causes (for example lactation or pregnancy), such rashes will disappear on their own almost completely as soon as hormonal balance gets normal again.
What else causes pimples on chest? Reproductive system diseases and disorders may cause acne on chest. Some girls experience acne before the periods. Later such rashes are naturally gone by themselves. It can also be regarded as hormone fluctuations.