I was facing almost same issue with some addition of dust accumulated in couple of month's use. Inside of helmet becomes somewhat muddy due to perspiration.
I bought the helmet in about 4,000 Rs. about a year and half ago.
So instead of purchasing a new one, What I did, I filled a bucket with water, Added some washing powder (the same which we use to wash clothes) in a bucket. Removed visors and put the helmet in bucket with some weight on top of it so that the helmet remains fully inside the water.
After half an hour I took it out, gently rubbed inside of the helmet with a piece of cloth. Rinse fresh water multiple times and made sure that washing powder is completely drained.
Put helmet in shade to dry. And it was good to go. Nothing was damaged including the inside fine cloth or foamy type material.
Just two precautions :
1. Do not rub hard.
2. Do not put under direct sunlight to dry.
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