Dandruff, seborrheic dermatitis, psoriasis or just a dry scalp?

Flakes are flakes, until you look closely. The white specks on your collar could be ordinary dandruff, the inflamed version called seborrheic dermatitis, scalp psoriasis, or simply skin that’s dried out over a long winter. Each one looks a little different, behaves a little differently and responds to different things, so it’s worth learning the clues before you buy a third bottle.

The four suspects at a glance

The short version:

• Dandruff: loose white-to-yellowish flakes, scalp only, no redness. Mild itch at most.
• Seborrheic dermatitis: red or discoloured patches with greasy, yellowish scale; can also affect the eyebrows, sides of the nose, ears and chest.
• Scalp psoriasis: thick, raised, well-defined plaques with silvery-white, dry-looking scale; often crosses the hairline; can itch intensely.
• Dry scalp: rough, tight, itchy skin with fine scale, worse in winter and after hot showers; no greasy feel.

None of these is contagious, and none is a sign you’re unclean.

Dandruff: flakes without redness

Dandruff is the most common of the four. A 2015 review in the Journal of Clinical and Investigative Dermatology puts it at about half of adults worldwide. The defining feature is flaking of the scalp without visible inflammation: the skin under the flakes looks normal, not red or swollen.

The flakes are white to yellowish, and the review notes they can look either oily or dry. Itch ranges from none to mild. Dandruff usually starts at puberty, peaks around 20 and eases after 50, and it tends to flare in winter. The cause isn’t dryness or dirt: it’s a reaction to by-products of a yeast, Malassezia, that lives on nearly everyone’s scalp and feeds on sebum (the skin’s natural oil). That’s why antifungal shampoos help and oils generally don’t.

Seborrheic dermatitis: the same thing, plus inflammation

Seborrheic dermatitis (sometimes spelled seborrhoeic) is what dermatologists consider the more severe end of the same spectrum. It’s a type of eczema that shows up in the oily areas of the body. The review describes well-defined red plaques with greasy-looking yellowish scale. On darker skin, the American Academy of Dermatology notes the patches may look pink, purple or lighter than the surrounding skin rather than red.

Location is the biggest giveaway. Dandruff stays on the scalp. Seborrheic dermatitis reaches the eyebrows, the creases beside the nose, the eyelids, behind and inside the ears, and the upper chest, usually on both sides symmetrically. Itch is more common, and the scale can build into honey-coloured crusts stuck to the hair.

How common is it? Estimates vary: the 2015 review gives 1–3% of adults, while DermNet cites 3–12% of the population. Either way it’s far rarer than dandruff. It peaks in the first months of life (as cradle cap, which usually clears on its own by 6 to 12 months), again in adolescence and adulthood, and it’s especially common after 50. It flares with winter, stress and lack of sleep, and it’s more frequent in men and in people with Parkinson’s disease or a weakened immune system.

Scalp psoriasis: thick plaques and silvery scale

Psoriasis is a different disease altogether. It’s an immune-driven condition in which skin cells are produced far too fast and pile up into plaques. About 2% of people have psoriasis, and DermNet notes that roughly 80% of them get it on the scalp at some point.

What it looks like, according to DermNet and the American Academy of Dermatology:

• Red, thickened, sharply outlined patches or plaques, from barely noticeable to thick and inflamed.
• A silvery-white, dry-looking scale, sometimes with a sheen. The AAD singles this out as the feature that separates psoriasis from dandruff.
• Plaques that extend beyond the hairline onto the forehead, the back of the neck or around the ears. Dandruff and seborrheic dermatitis rarely form such distinct borders.
• Itching that can be mild or intense enough to disrupt daily life; scratching can crack and bleed the skin.
• In severe cases, temporary hair loss where the scale is thick, which regrows once the plaques clear.

Clues outside the scalp help too. Psoriasis often appears on the elbows, knees, palms and soles, and can affect the nails. It commonly runs in families, and about one person in ten with psoriasis also has arthritis. Like the other conditions, it flares with stress, cold and dry air.

To make things more confusing, DermNet notes that seborrheic dermatitis and scalp psoriasis can coexist, an overlap called sebopsoriasis. This is exactly the kind of case where a trained eye matters.

A dry scalp: when the problem really is moisture

Skin on the scalp is still skin, and it can dry out like the skin on your shins. DermNet describes dry skin (xerosis) as skin that lacks moisture in its outer horny layer, giving it a dull, rough, scaly look, with itch and, when it’s bad, fine cracks. The usual culprits are low humidity, indoor heating, hot water, and harsh soaps and detergents, plus age: nearly everyone over 60 has some dry skin.

Translate that to the scalp and you get a picture that’s familiar in a Canadian January: tight, itchy skin, fine flakes, and no greasiness. It tends to get worse with very hot showers, frequent washing with strong shampoos, and a furnace running full-time, and better with lukewarm water, a gentler cleanser, less frequent washing and a humidifier in the bedroom.

Here’s the honest complication. The 2015 review found that dandruff-prone scalps themselves have a subtly damaged skin barrier that reacts more to irritants, so “dry” and “dandruff” aren’t mutually exclusive, and dandruff flakes can look dry. The practical test is what helps: if gentle care fixes it, dryness was the main issue; if only an antifungal shampoo fixes it, it was dandruff.

Other look-alikes worth knowing

A few less common conditions can also masquerade as dandruff; the NHS lists them:

• Scalp ringworm (tinea capitis), a fungal infection seen mostly in children, with scaly patches, patchy hair loss and sometimes tiny black dots where hairs have broken. It’s contagious and needs prescription treatment.
• Contact dermatitis, a reaction to something applied to the hair, such as hair dye, sprays, gels or mousses, giving red, irritated skin that may blister or crack.
• Atopic eczema, which can involve the scalp along with the usual dry, very itchy patches elsewhere.
• Cradle cap in babies: greasy yellowish crusts on the scalp, eyebrows and sometimes the diaper area, usually harmless and self-limiting.

When to see a doctor

Ordinary dandruff can be managed at home with an anti-dandruff shampoo; if you’re comparing options, our hair care product list is a place to start. The NHS says to see a doctor if:

• You still have symptoms after using an anti-dandruff shampoo for at least four weeks.
• Your dandruff is severe or your scalp is very itchy.
• Your scalp is red or swollen.
• You have flaky, itchy patches on your face or other parts of your body.

Add to that list patchy hair loss, oozing or crusting, and the thick silvery plaques described above. The American Academy of Dermatology also points out that several scalp conditions can occur together, so an obvious-looking self-diagnosis can still be incomplete. Your doctor, nurse practitioner or pharmacist can usually sort it out by looking; occasionally a skin scraping or biopsy is needed.

Look at the skin under the flakes, not just the flakes. Normal-looking scalp with loose white flakes is dandruff. Red, greasy, yellow and spreading to the face is seborrheic dermatitis. Thick, sharply bordered plaques with silvery dry scale that cross the hairline point to psoriasis. Tight, rough and worst in winter is probably dryness. And whenever the picture is mixed, or a month of shampoo hasn’t helped, let a trained eye take a look.