The scalp is a common site of rashes for a number of reasons. It is exposed to the environment, and so it can be irritated by toxins. The skin on the scalp is also thin and has numerous sweat and oil glands under it, making it sensitive to irritation and rashes.
Stop using any new haircare products. If you have recently started using a new haircare product, it could be causing the rash.
Wash your hair thoroughly using baby shampoo and water. Baby shampoo is designed to be gentle, so it will not irritate your scalp, but it may help clean the affected area and get rid of any dirt or irritants.
Apply hydrocortisone cream. This is available over the counter and should be massaged into the affected parts of the scalp daily, preferable after a shower. Continue using hydrocortisone cream until the rash goes away.
Take antihistamines. Antihistamines can also help relieve the inflammation that leads to the symptoms of a rash. Follow the instructions on the package for dosing.