What is a scientifically sound method to remove armpit sweat stains from fabric?
Hi, if it's a white shirt: spray locally bleach. Then, let the shirt soak in cold or lukewarm water with soda crystals, with some added whitening powder (usually containing bleach or peroxide as bleaching agent). Also add some vinegar. Soda is a powerful stain remover; easily forgotten nowadays, but it's super cheap and any supermarket should have it. Vinegar is added to remove calcium and magnesium from the "hard" water. The "hard" water helps in forming a layer on the clothes. Wash in the washing machine with washing powder for white clothes. You can also add some soda crystals and vinegar to it. And lastly, let is dry out in the sun. The sun is a powerful bleaching agent as well. You can also locally apply a concentrated liquid mixture of soda, bleach and vinegar. Repeat a couple of times, and the stains should gradually go extinct. In case of a colored shirt, do not use bleach; only soda, vinegar and sunlight. Best of luck.