Sweat happens. We should be thankful for sweat because it is a physiological function that regulates our core body temperature. Sweating is like an internal air conditioning system. When moisture from sweat reaches the skin surface during a vigorous workout, we glisten with perspiration. The evaporation of this wetness to the outside cools down the temperature of our insides. Without sweating, we could easily overheat and our organs might be damaged in the process. Thank you, perspiration for keeping us healthy! https://pubmed.ncbi.nlm.nih.gov/31608304/

There is underarm sweat but there is also under breast sweat. Both armpits and under boobs are examples of intertriginous skin sites, aka, skin folds, and they are similar. However, there are unique aspects about sweaty breasts. Keep reading.
We all know how uncomfortable sweating too much, feeling damp and drippy, can be. Add to that sweat stains. Ugh!  These tell-tale sweat marks are the worst when they occur in the wrong place at the wrong time (hello, underarm stains on your shirt when you are on stage in front of a crowd.). Dreaded excess moisture under and around the boobs is particularly annoying because it gets complicated by rubbing from shapewear, bras or sports bras and can lead to skin fold irritation such as chafing and can even cause other problems like a yeast infection in the skin fold known as intertrigo. https://www.verywellhealth.com/intertrigo-yeast-infection-in-skin-folds-1069272. 

The combination of sweat and friction is a recipe for a rash and odor. 

Why Do I Sweat In My Sports Bra?

Why Does My Bra Area Sometimes Smell?

How To Treat A Sweaty Bra Area*

• Avoid tight fitting clothing. Especially on very humid days or in your garden, yoga studio or work environment, allow air flow to the chest by wearing light colored and loose- fitting clothing. Tighter fit means more ability to trap sweat. Choose easy breezy tops. Also look for fabric that is a loose weave and not clingy to your body.
• Get the right bra for you. This means finding a bra that fits correctly and is made of a breathable fabric. Also, get rid of underwire bras. To keep skin cool, avoid extra padding. Push up bras? They may lift the breasts so the skin is not rubbing against skin, but the materials used in the push up mechanism may be irritating or add to warming the temperature of the skin, leading to more sweat. Not clear which is better, so test out options. Better fit and less rubbing means happier boobs all around.
• Or... go braless? If this is an option, especially when sleeping or reclining, remove the bra. Why? the more airflow and less clothing you have on, the less chafing will occur when you sweat. Alternatively, when you’re active, just the opposite! My opinion is that wearing a bra is highly recommended to reduce skin friction when you are in motion.
• Choose clothing fabric correctly. Notice the difference between moisture wicking fabric and breathable fabric.

• Antiperspirant. Applying a sweat blocking product such as a clinical strength antiperspirant is a must for people who suffer from excessive sweating or have the condition known as hyperhidrosis. Antiperspirants work by the presence of aluminum salts in the ingredients which physically plug the sweat duct opening within the skin. This is often the path taken by those with underarm sweating problems. However, is it safe to use an aluminum containing antiperspirant spray for the breasts? There is some controversy about this and the verdict: not clear. While studies do not show an absolute cause and effect relationship between the use of aluminum containing products and health risks, there are enough doubts about the effects of aluminum, that many of us choose to stay away from aluminum salts on the body as a precaution. Suggestion: all things in moderation. If you know you are very prone to heavy sweating and it causes distress and you predict an event when you might be sweating a lot, use an antiperspirant occasionally but not as a daily application. Another downside, antiperspirants may cause skin to be too dry and lead to dermatitis or inflamed and itchy skin, especially for those with a history atopic dermatitis, eczema or sensitive skin. 
• Deodorant. Stick or or roll on or cream deodorants that do not include aluminum (read the label) rely on perfumes to mask odor. This might be a good choice if smell prevention is your goal more than wetness prevention. Generally speaking these natural deodorants have fewer harsh chemicals than the antiperspirant counterparts. But beware that baking soda, perfumes, charcoal and other ingredients may still cause irritation in some people. Start by doing a “test site” of the deodorant on one small area for a few days before using a deodorant more widely on the whole chest/breast area.  Cleansing or odor preventing pads can be a clean alternative. Surface Deep Pads are a glycolic based, pH balancing product to swipe under the boobs to prevent odor without harsh chemicals, without aluminum and without perfumes.
• Bra Liners. I have seen patients visit me with paper towels placed in their cleavage or tucked into their bras to absorb sweat. That is one option… but a better one is to use bra liners.  These are typically cotton pads that fit into the bra and serve as a sweat sponge. They can be put through the laundry along with the bra itself and worn again.
• Powders. Cornstarch, Baby Powder. Talc Free Powder. While powders, especially those containing talc, have been the focus of scrutiny in the past few years, a sprinkle of an absorbent product can be just the right approach to minimize boob sweat. An all natural approach is to use cornstarch powder. This absorbs well and feels cool on the skin (but may be messy on the floor of your bathroom). My concern with using this, although my research does not confirm my hypothesis... is that cornstarch is high in sugar and can invite yeast/candida and potentially result in a yeast rash. So if you have a medical condition such as diabetes, hyperglycemia or other predilection to yeast infections, you may wish to choose a powder that also contains anti fungal /anti yeast component such as Zeasorb Powder ingredients.https://zeasorb.com/.
• Air It Out. Use a hairdryer on a cool setting or a fan to dry your under boob area with air after a shower or bath. Start the day cool and dry before putting on a bra and clothing which will trap any residual dampness.
• Argan oil. Some swear by using an oil such as argan oil to the skin before putting on a sports bra. The argan oil can help bolster the skin’s natural barrier function. As a protector of the skin surface it may slow the transfer of sweat from the skin to the clothes. Another benefit, the argan oil provides an antidote to friction by creating a chafe- free barrier between your skin and your bra.
• Body wash. Boob sweat can lead to pimples, rashes and bumpy, uneven skin. A good remedy is to use a body wash that contains certain ingredients which can help prevent these skin changes. At the same time these same cleanser constituents may also help fight against under boob body odor. Seek out a glycolic acid body wash that is not too harsh and can be used on the delicate breast skin, such as Surface Deep Skin Wash Surface Deep Skin Wash. Benzoyl Peroxide (PBO)  body washes Panoxyl Wash are also a great choice. With BPO washes, be aware that if the cleanser residue gets on dark fabric it can cause bleaching or white spots on towels or clothing.
• Embrace sweat and love your body and all it does for you!

*Please Note: All of the suggested remedies listed above should be evaluated and approved safe to use by your OB GYN physician if you are pregnant or breastfeeding before using. See your dermatologist if your sweating or body odor is affecting your quality of life or is persistent or getting worse.