Itchy scalp: the usual suspects and how to tell them apart

An itchy scalp is hard to ignore and easy to misread. Most of the time it’s something ordinary: dandruff, a scalp dried out by winter, or shampoo that didn’t get rinsed out. Sometimes it’s an allergy to a hair product, a skin condition such as psoriasis or eczema, or, especially in a house with school-aged kids, head lice. Each leaves different clues. Here are the usual suspects and what each one looks like.

Dandruff and seborrheic dermatitis

The American Academy of Dermatology (AAD) puts dandruff first on its list of reasons a scalp itches: a scalp that feels dry and itchy, with flakes on the hair or shoulders. A 2015 review in the Journal of Clinical and Investigative Dermatology explains that dandruff is a reaction to a yeast that feeds on scalp oil, not a sign of dryness or dirt, and that the itch in plain dandruff ranges from none to mild.

When the itch is stronger and the scalp is red, you’re probably looking at seborrheic dermatitis, the inflamed version of the same condition. The clues: greasy, yellowish scale, redness (or pink, purple or paler patches on darker skin), and patches beyond the scalp on the eyebrows, sides of the nose, ears or chest.

Next step: an anti-dandruff shampoo, lathered on the scalp and left a few minutes before rinsing. The NHS suggests giving it up to four weeks; if you’re not better by then, or your scalp is red or swollen, see your doctor or pharmacist. You can compare options in our hair care product list.

Dryness

Scalp skin dries out like any other skin. DermNet describes dry skin as skin lacking moisture in its outer layer: rough, dull, scaly and itchy, sometimes with fine cracks. The triggers are familiar in a Canadian winter: low humidity, indoor heating, hot water and harsh cleansers.

The clues: a tight, itchy scalp that’s worse after hot showers and in cold months, with fine flakes and no greasy feel. Next step: lukewarm water, a gentler shampoo, washing a little less often, and a humidifier in the bedroom. If that doesn’t help within a few weeks, it may be dandruff after all.

Product residue

The AAD says an itchy, dry, flaky scalp can simply mean you need to rinse shampoo out more thoroughly, because product left on the scalp irritates it. Conditioner, styling products and leave-ins can do the same, especially with heavy use between washes.

The clues: itch that started when you changed products or routines, worse at the crown or wherever product concentrates, without much redness. Next step: rinse longer, keep conditioner on the lengths rather than the roots, and apply shampoo to the scalp so it can clear built-up product, dead skin and oil.

Contact allergy: hair dye and other products

If your scalp itches and there’s a rash, the AAD says you may have allergic contact dermatitis, an immune reaction to an ingredient that touched your skin. It’s common in people who dye their hair, and the usual culprit is para-phenylenediamine (PPD), an ingredient in permanent dyes, especially dark shades.

DermNet explains the pattern. PPD becomes a sensitizer while it’s partly oxidized during dyeing; the first exposure may cause nothing, but later ones react faster and harder, hours to days after contact. A mild reaction shows up as an itchy, dry rash on the upper eyelids or the rims of the ears. A severe one brings marked redness, swelling and blistering of the eyelids, scalp, face and neck. Once you’re sensitized, it’s likely lifelong. Darker dyes contain more PPD, and it also turns up in black temporary tattoos. Dye packages carry a warning to do a self patch test first, though DermNet notes a negative test doesn’t guarantee you won’t react.

You can also react to a shampoo, conditioner or styling product; the giveaway, per the AAD, is a rash on the scalp and on any other skin the product touched, such as the eyelids, ears or neck. Next step: stop the product. If you can’t identify it, a doctor can arrange patch testing.

Psoriasis, eczema and the rarer causes

Psoriasis is an immune condition in which skin cells pile up into plaques, and the AAD says about half of people with plaque psoriasis get a flare on the scalp at some point. The clues: reddish patches, dandruff-like flaking, a silvery-white scale, a dry scalp, and itch ranging from mild to intense. Plaques often extend past the hairline to the neck or behind the ears.

Atopic dermatitis, the common form of eczema, can also settle on the scalp with redness, scale, itch and sometimes a burning feeling, usually in someone who has eczema elsewhere. Next step: both are treatable, but both need a proper diagnosis, so see your doctor. Some over-the-counter medicated shampoos help mild scalp psoriasis; prescription options exist for the rest.

The AAD’s list also has rarer causes, each needing a doctor rather than a product:

• Scalp ringworm, a fungal infection that causes an intensely itchy red rash, sometimes with hair loss, and needs prescription treatment.
• Scabies, a mite that can burrow into the scalp and cause itch severe enough to keep you awake.
• Hives, red raised itchy bumps that come and go within hours.
• Nerve problems: an intensely itchy scalp with no rash at all can be neuropathy, linked to conditions such as shingles or diabetes, or to scarring from hair loss.
• Skin cancer on the scalp, which can itch and is worth ruling out if a spot persists.

Head lice

The Canadian Paediatric Society (CPS) says an itchy scalp is one of the first signs of head lice, but it’s possible to have lice with no symptoms at all, and most itchy scalps are not lice. Lice are tiny insects that live on the scalp and lay eggs (nits) glued to the hair close to the skin. They spread easily by hair-to-hair contact and by sharing hats, combs, brushes and headphones, which is why they’re so common in school-aged children. They don’t jump or fly, and having them says nothing about how clean you are.

The clues: adult lice are about the size of a sesame seed and move fast, so you may notice movement rather than the insect. Nits are whitish-grey, tan or yellow ovals about the size of a grain of sand, stuck to the hair near the scalp, and the CPS notes they can look like dandruff. Check under good light at the back of the neck and behind the ears, and check again weekly for two weeks if you’ve been exposed. Next step: the CPS says not to treat anyone unless you find live lice. Canada has approved both insecticide rinses and non-insecticide products; a pharmacist can walk you through the options and the age limits for each.

Reading the clues

Put together, a short checklist covers most cases:

• Flakes, no redness: dandruff. Flakes with redness and greasy yellow scale: seborrheic dermatitis.
• Tight, fine flakes, worse in winter and after hot water: dryness.
• Started with a new product, no rash: residue.
• Rash on the scalp and eyelids, ears or neck after dyeing or a new product: contact allergy.
• Thick, sharply edged plaques with silvery scale: psoriasis.
• Kids, school, sand-grain ovals glued to hairs: lice.

See your doctor or pharmacist if four weeks of the right basic care hasn’t helped, if the scalp is red, swollen, weeping or painful, if hair is falling out in patches, if a rash follows a product, or if the itch is intense with nothing to see. An itchy scalp is almost always fixable once you know which suspect you’re dealing with, and the clues above usually narrow it down.