Yellow sweat stains in the armpit on white t-shirts and shirts are as common as they’re frustrating. Fun fact though: It’s actually not sweat that is the main culprit. Sweat itself is colorless, what causes the yellow stain is when deodorant with aluminum-based ingredients react to your body's natural oils. Sometimes it can even get worse with laundry detergent. Once you get those stains, they are usually really hard to get rid of.
No single treatment is best at removing sweat stains, because of the big differences between deodorant brands and detergents. Therefore examine the stain to get an idea of how to remove it.
If the stain is white, hard, and/or brittle, a stain remover will probably help you get rid of the stain. Treat the stain as soon as you notice it. Just rinse it under cold water for a while and then gently treat it with a stain remover and laundry it as normal*. For colored garments, make sure to use color safe, non-chlorine/bleach-free stain remover.
If the stains are yellow, they can be tough to remove. A trick for removing them is to make a solvent out of three parts baking soda and one part water. Let the solvent work on the stain for up to one hour and rinse the garment or wash it as normal*. This procedure might have to be repeated for the best result.
While it seems like bleach could be the best option for treating this type of stains on white clothes, it is not. Bleach can actually react with the stain and make it even worse.
As you can tell, these stains are very tricky and the best thing to do if this is a common issue for you is to try and prevent these stains from happening in the first place. To do so, make sure to:
* For ASKET garments, find more detailed wash instructions here.