Purifying or clarifying shampoos can do your hair and scalp good if you know when and why to use them, and how to choose one.

1. You have product build-up or hard water.

Dirt, oil and dead skin cells can build-up on your scalp, while mineral oils, silicones and waxes from products build-up on your hair. In addition, hard-water minerals, like iron, calcium and copper, stick around and can shift or fade your haircolor. And if you’re a swimmer, chlorine, bromide and iodine from pools or salt from sea water can dry your hair and also affect hair color. Even if you have a very oily scalp, weekly clarifying is good for you. You’ll soon realize that your regular shampoo doesn’t stop working, like some claim—it just has too hard of a job.

2. You wear extensions and shampoo less frequently.

Women who wear protective hairstyles (twists, braids and weaves that protect ends) or various forms of extensions tend to shampoo less often. This can cause build-up on your scalp and hair, flaking and even scalp disorders. Stylists have hundreds of stories about women with extensions who try to shampoo monthly. Even if you use a dry shampoo and wet your hair just 1-2 times a week, a clarifying shampoo is a must for you.

3. You need/want a special hair treatment.

If you regularly deep condition, get moisturizing or protein treatments, or use hair masks or scalp treatments, using a clarifying treatment first helps moisture, protein, vitamins and essential oils penetrate.

4. You plan to color your hair.

Permanent haircolor has to open the cuticle to penetrate, while semi-permanent products coat the surface of the hair. In either case, just as with any hair treatment, the cleaner and more free of build-up your hair is, the better. One caveat: don’t clarify the day of a coloring service, do it the day before, or your scalp may be sensitive to coloring products, ammonia and lighteners. Think of your hair as a canvas you are about to start painting. That’s why clarifying is what colorists call “prepping the canvas,” and it always creates better results.

5. You need a scalp treatment.

Whether you have a very oily scalp, minimal flakes or dry patches, clarifying can help make your scalp healthier. A scalp clarifying treatment and massage is great for men wear fades or a bald look. Follow up the treatment with a little oil. And today, don’t forget those hipster beards.

Like anything else, moderation and education are key when using clarifying shampoos and treatments. If they rely on serious surfactants, avoid over-use, particularly if you have haircolor. Check out the ingredients label for products that rely on gentle and/or natural ingredients, and put back some natural oils and vitamins, so your hair is not only super-clean, but shiny and manageable.