Underwire Bras

• Some underwire bras can cause perspiration underneath the breasts if they are not the proper size. A bra that doesn't fit properly either because it is too tight or too loose can cause problems. For instance, an underwire bra that is too tight will constrict the breasts, preventing the skin from breathing. This can cause the underwire to rub up against the chest, underneath the breast, and a rash can develop. However, a bra that is too loose also can cause a rash, because the material will leave a pocket for sweat. Underwire bras are wonderful for support and coverage, but they are effective only if they fit properly. Getting a professional bra fitting can ensure that you are wearing the right type of bra and the right size.
  
  

Heavy Breasts

• Large, heavy breasts also can cause a rash to develop underneath, because the weight of the breasts can put pressure on the chest wall and all the surrounding tissues. Women who have large breasts also naturally may perspire more in this area, as there is more skin and fat there, leaving less room for the skin to breathe. Wearing a supportive bra or losing weight may help some women, depending on their actual size.
  
  

Sweating

• Excessive sweating can cause the breasts to develop a rash. The sweat gets trapped underneath the breasts and causes irritation. This is more likely to happen during a workout or on a hot summer day. Sweat can contribute to the growth of bacteria and yeast, with a rash as a result. To help combat the problem, sprinkle some baking soda and baby powder underneath the breasts right after a shower. However, be sure to dry off first, because the water will absorb the powders. During the summer months, wear a supportive cotton bra without underwire, and breathable clothing.
  
  

Allergies to Fabrics

• A rash also can develop under or even around the breasts due to allergic reactions to some fabrics. Acrylic and polyester cause rashes and itching for some people. Others even may be sensitive to pesticides or other chemicals used during the clothing manufacturing process. Avoid wearing clothes that are made with blended fabrics, so you can determine to which fabrics you are allergic. Wear 100 percent organic cotton clothing and bras.
  
  

Breast Cancer

• In rare cases, a rash under or around the breasts can be a symptom of breast cancer. Inflammatory breast cancer is a rare but deadly disease that spreads very quickly. The breasts can appear to have a spreading rash and other symptoms such as pain, fatigue or the sensation of heat. The rash can be underneath one or both breasts. It can appear underneath or in other areas of the breast. Seek medical attention right away if you suspect the rash is caused by breast cancer.