First, it's important to know what the root (pun intended) causes are. And it turns out that there are a lot of factors here. Sebaceous glands—also known as oil glands—within the

hair follicles produce natural oils and waxes," explains Sarah Potempa, celebrity hairstylist and founder of The Beachwaver Co. "The natural oil produced by the glands, called sebum, is how your body naturally protects, moistens, and softens the hair. As the oil builds up, it moves from the scalp down to the ends of the hair. Heavy hair products build up and mix with the natural oils produced. If you work out often, the natural oils mix with the sweat, too!"

There are also a couple of lifestyle factors that might cause a greasy scalp. "An excess of saturated fats in your diet comes out through the hair follicle as the fats speed up oil production," says trichologist Helen Reavey, founder and creative director of Act+Acre. Soft water can also be the cause since it's harder to rinse out your hair products and you might leave them in without releasing. Hormones or certain medications can also cause more oil production. Really hot water also makes the scalp more oily. And Reavey adds that harsh products can strip the scalp and make it think it needs to protect itself, so it creates more oil as a barrier.

There are also certain hair types that are more prone to a greasy scalp. "It often happens to kids and teenagers as they go through puberty. Those with fine and straight hair are most susceptible to oily hair and scalp," says Gretchen Friese, a certified trichologist with BosleyMD.