You are in the car and lean over to grab something. When you lean back to your seat, you get a whiff of something, and it's not pleasant. It's like a musty, sweaty or sharp smell. Where is it coming from? What just happened? Repeat the motion, same result and now it hits you. It's coming from your hair! Or more likely, from your scalp.
Smelly Scalp Syndrome, (let's call it SSS for short) is caused by bacteria on the scalp. It can be caused by being unhygienic, not washing your hair or scalp thoroughly or often enough or an excessive sweaty head between washings. In fact, a one-day hiatus from washing after a workout for or after a night where you worked up a head sweat, slept on it then didn't wash the next day can create the perfect environment for such odor-causing bacteria.
The first thing you should do is use a scalp cleanser or rinse before you shampoo. You will find these scalp rinses in bottles with nozzles on the top, to ensure you get the product to the scalp where it can do the most good and not sit on the hair where it can't.
Assuming you don't have an oily scalp condition like seborrheic dermatitis or a scaly condition like psoriasis, which is causing you to build up a flaky scalp which traps bacteria, it may just be you are not washing your hair the right way or often enough. If you have a buildup and simply can't get rid of the bacteria, find a shampoo containing zinc pyrithione, an antifungal/antibacterial agent. But use sparingly (once about a month) because it is harsh and if used too often can cause blowback of the condition.
If you start noticing flakes and itchiness, consult a dermatologist who can evaluate you for a medical condition and prescribe a course of treatment.
Homemade Fix for Smelly Scalp
You can make a home-made scalp cleanser easily using a half cup of Apple Cider Vinegar (ACV), a few drops of tea tree oil and a few drops of shampoo.
Mix these ingredients into plastic squeeze bottle then and fill to the top with water.
Give it a shake then squeeze the mixture onto your scalp and rub in. Leave this on for a few minutes then shampoo. You may feel it tingling, which means it is doing its work.
For persistent conditions, leave on for 30 minutes before rinsing.