 Once the last time you cleaned your hairbrush, if your hair looks dirty and greasy even when it's clean, it's probably time. Those bristles you're brushing your hair with every day are full of product buildup oil and dirt, making them the perfect breeding ground for bacteria, yeast, and dust mites, and a possible reason for your itchy scalp and dandruff. You should remove the hair from your brush after each use, but if you use styling products like hairspray and gel, you should deep clean it once a week. Get a bowl of warm water, a gentle shampoo, and baking soda, and soak for a few minutes. Rinse with cool water and let it air dry. And if it's really bad, you can go in with a toothbrush.