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Try washing your hair AND the inside of the helmet with white vinegar.
Smelly Scalp Remedies
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Valentina, Makes me wonder if it is some kind of fungal thing. I would try a shampoo with tea tree oil in it, or just mix tea tree essential oil with extra virgin coconut oil and massage into your scalp and hair. (This would be messier, but a stronger solution. ) Let it sit for 10 minutes or so before washing it out. I would do that every day for 10 days. Charcoal is great for odor and toxins, but so very messy to use in your hair... It wouldn't hurt to take charcoal tablets internally (4 tablets 2-3 times a day for a few weeks. ) Don't take charcoal near the time you take any medications as it will absorb them. A probiotic internally, would be a good supplement to the above.
Hope this helps.
~Mama to Many~