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Have you ever gone through your laundry and noticed unsightly yellow stains in all of your white shirts? If your sweat isn't yellow, what's causing this?

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Sources:

https://www.livestrong.com/article/107724-causes-yellow-perspiration/
https://www.thompsontee.com/blog/the-science-behind-your-pit-stains-and-what-to-do-about-it
https://www.businessinsider.com.au/why-your-white-shirt-yellows-when-you-sweat-2016-4?r=US&IR=T
https://www.nivea.com.au/skin-care-tips/deodorant/underarm-care/what-causes-sweat-stains
https://health.howstuffworks.com/skin-care/problems/medical/chromhidrosis.htm
http://c.ymcdn.com/sites/www.aocd.org/resource/resmgr/jaocd/contents/volume30/30-05.pdf
https://www.gizmodo.com.au/2014/08/how-antiperspirant-works-and-who-it-might-hurt/
https://www.sweathelp.org/home/understanding-hyperhidrosis.html
https://health.howstuffworks.com/wellness/men/sweating-odor/what-is-in-sweat.htm
http://cenblog.org/artful-science/2012/02/27/sweat-stained-artifacts/
It’s maybe the most first world problem of first world problems: pit stains on white clothing.

Maybe you've been there.. Wear any white shirt for long enough and suddenly it's discoloring in embarrassing and irritating ways.

The thing is, your sweat isn’t actually yellow.. It’s colorless—that is, until it reacts with the compounds in antiperspirant. You have two main types of sweat glands, but the culprits here are the apocrine glands..

These don’t kick in until puberty and are found basically anywhere there’s a lot of hair. So, your scalp, underarms… other places… They produce sweat that’s slightly thicker, and secreted onto your skin via hair follicles. To make matters worse, this sweat isn’t just produced during hot weather..

It’s also triggered by periods of anxiety or heightened emotion. Great.. But to make it thicker, the sweat produced by these glands also contains fats and proteins.

And it’s those fats and proteins that are the big issue, along with the bacteria that feast on them. We’ve talked about the connection between bacteria and the odor in farts, and the smell in your armpits is produced in much the same way. Sweat itself is odorless..

It’s the bacteria and the gases they release that make everything smell so wonderful, and that prompted cosmetics companies to create antiperspirants. These products generally contain some version of a compound called aluminum chloride, which works by acting as a sort of gel-like plug for your apocrine sweat glands, so you don’t produce as much sweat. But at the same time, the aluminum reacts with your sweat to produce compounds that react with the fiber in your clothing and mess with the color..

Voila: yellow stains. No one seems to agree about why exactly it turns yellow, but there’s some thought that the reaction with the aluminum causes the proteins in apocrine sweat to really stick to your clothes. In any case, it’s actually the thing making you sweat less that's causing the stains.. That doesn’t seem super helpful.

Fortunately, there are a few things you can do to keep your clothes white. You could try using a deodorant that doesn’t contain aluminum, or else let it dry completely before putting on your shirt. You could also look for a type of detergent that’s boosted with the power of enzymes..

Those detergents are designed to react with and break down proteins so you can get tricky stains, like yellow pit stains, out of your clothes. Like a lot of cleaning hacks, it’s all about knowing the chemistry.. Thanks to Patreon patron Garrett Van Dyk for asking this question, and thanks to all of our patrons, who keep these answers coming.

If you would like to submit a question to be answered, you can go to patreon.com/scishow.. And don’t forget to go to youtube.com/scishow to subscribe! That is wonderful and helps us out, and also you'll get a new episode of SciShow and learn more about how weird and wonderful the world is, including your arm pits.