After learning that breast deodorant exists (with kitschy names like Bust Dust and Fresh Breasts no less), my next question was: but how much do breasts really sweat? Anecdotally, I can attest that breasts -- diminutive in size though they may be in my case -- can easily soak a sports bra on a summer day. Between-the-boobs sweat also happens when the mercury rises whether I'm exercising or not; oh, and don't even get me started about the boob sweat havoc that can be wreaked beneath the unbreathable folds of a poly-blend blouse. And for bustier women, under-boob perspiration (which apparently some folks refer to as "swoob") can be particularly particularly vexing.
The simplest answer to how much breasts sweat is that it depends since everybody sweats a little bit differently depending on factors like weather, sex (men sweat more), body size (bigger people = more sweat glands due to basic surface area geometry) and fitness level (NB: it's commonly thought that fitter folks sweat more during exercise, but as with many physiological phenomena, it ain't necessarily so) -- and the same goes for breast sweat. It's going to depend based on a host of environmental and biological factors.
Anatomically, female breasts are thought to be modified sweat glands. That doesn't mean our mammaries don't sweat, of course. They're covered in sweat glands like the rest of our bodies, and Henry Gray of "Gray's Anatomy" fame estimates there are around 155 sweat glands per square centimeter on the chest, or roughly the same sweat gland concentration as the stomach and forearms.
Aside from it awkwardly seeping through clothes on swampy day or in cases of hyperhidrosis, big boobs are probably the most common culprit of day-to-day breast sweat discomfort. The combination of trapped heat, sweat and friction underneath larger breasts can be very real annoyance for women, hence applying Bust Dust or good old fashioned baby powder to keep things rubbing smoothly. Allowing under-boob sweat to sit in that area can actually promote rashes and yeast infections, which generally can be prevented by wiping them dry after exercise and swabbing with isopropyl alcohol.
For smaller-chested ladies like myself who don't encounter under-boob sweat very often, this lesson in How Larger-Than-A-Cup Boobs Work also is a reminder that immediately laughing off products like anti-chafing powders for breasts ignores the blessedly wide variety of female figures out there, as Melissa McEwan tweeted:
One sweaty side note: while digging around for information on female chest and breast sweat, an odd thing happened that reflects how the female body so often is perceived as a delicate flower that doesn't produce noxious farts, excessive sweat and nipple hairs (yes, ladies, we've all got those). I searched "sweatiest part of the female body," and in a benevolently sexist way Google suggested that perhaps I meant "sweetest" (which is so much grosser than "sweatiest" when you think about it). Curiosity piqued, I switched up my search to the "sweatiest place on the male body," and guess what? No suggested change.