Skin rashes are a common condition. Usually they stem from something pretty harmless, like a reaction to heat, medicine, a plant like poison ivy, or a new detergent you’ve come into contact with.

Rashes can show up on any part of your body, from your head to your feet. They can even hide in the cracks and crevices of your skin. Sometimes they itch, crust, or bleed.

Less often, bumps or redness on your skin can be a sign of skin cancer. Because cancer can be very serious — even life-threatening — it’s important to know the difference between a rash caused by irritation and one caused by skin cancer. See a dermatologist for any rash that’s new, changing, or that doesn’t go away.

Because it can be hard to tell a noncancerous skin growth from a cancerous one, look for any new or changing rashes or moles and report them to your doctor.

Actinic keratosis

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Actinic keratoses are crusty or scaly dark or skin-colored bumps that appear on areas of sun-exposed skin — including your face, scalp, shoulders, neck, and the backs of your arms and hands. If you have several of them together, they can resemble a rash.

They’re caused by damage from the sun’s ultraviolet (UV) radiation. If you don’t get actinic keratosis treated, it can turn into skin cancer. Treatments include cryosurgery (freezing them off), laser surgery, or scraping off the bumps. You can learn more about actinic keratosis here.

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Actinic cheilitis

Actinic cheilitis looks like scaly bumps and sores on your lower lip. Your lip might also be swollen and red.

It’s caused by long-term sun exposure, which is why it often affects people with fair skin who live in sunny climates like the tropics. Actinic cheilitis can turn into squamous cell cancer if you don’t have the bumps removed.

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Cutaneous horns

Just as the name suggests, cutaneous horns are hard growths on the skin that look like an animal’s horns. They’re made from keratin, the protein that forms skin, hair, and nails.

The horns are concerning because about half the time they grow out of precancerous or cancerous skin sores. Larger, painful horns are more likely to be cancerous. You’ll usually just have one cutaneous horn, but they can sometimes grow in clusters.

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Moles (nevi)

Moles are flat or raised areas of skin. They’re usually brown or black, but they can also be tan, pink, red, or skin-colored. Moles are individual growths, but most adults have between 10 and 40 of them, and they can appear close together on the skin. Moles are often benign, but they can be signs of melanoma — the most serious type of skin cancer.

Check each mole you have for the ABCDEs of melanoma:

  • Asymmetry — one side of the mole looks different than the other side.
  • Border — the border is irregular or fuzzy.
  • Color — the mole is more than one color.
  • Diameter — the mole is larger than 6 millimeters across (about the width of a pencil eraser).
  • Evolving — the mole’s size, shape, or color has changed.

Report any of these changes to your dermatologist. You can learn more about spotting cancerous moles here.

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Seborrheic keratosis

These brown, white, or black bumpy growths form on parts of your body like your stomach, chest, back, face, and neck. They can be tiny, or they can measure more than an inch across. Although seborrheic keratosis sometimes looks like skin cancer, it’s actually harmless.

However, because these growths can get irritated when they rub against your clothes or jewelry, you may choose to have them removed. You can find more information about seborrheic keratosis here.

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Basal cell carcinoma

Basal cell carcinoma is a type of skin cancer that appears as red, pink, or shiny growths on the skin. Like other skin cancers, it’s caused by prolonged exposure to the sun.

While basal cell carcinoma rarely spreads, it can leave permanent scars on your skin if you don’t treat it. More information about basal cell carcinoma is available here.

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Merkel cell carcinoma

This rare skin cancer looks like a reddish, purple, or blue-colored bump that grows quickly. You’ll often see it on your face, head, or neck. Like other skin cancers, it’s caused by long-term sun exposure.

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Basal cell nevus syndrome

This rare inherited condition, which is also known as Gorlin syndrome, increases your risk of developing basal cell cancer, as well as other types of tumors. The disease can cause clusters of basal cell carcinoma, especially on areas like your face, chest, and back. You can learn more about basal cell nevus syndrome here.

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Mycosis fungoides

Mycosis fungoides is a form of T-cell lymphoma — a type of blood cancer that involves infection-fighting white blood cells called T-cells. When these cells turn cancerous, they form a red, scaly rash on the skin. The rash can change over time, and it may itch, peel, and hurt.

The difference between this and other types of skin cancer is that it can show up on areas of the skin that have not been exposed to the sun — like the lower belly, upper thighs, and breasts.

Yes, skin cancer can be itchy. For example, basal cell skin cancer can appear as a crusty sore that itches. The deadliest form of skin cancer — melanoma — can take the form of itchy moles. See your doctor for any itchy, crusty, scabbed, or bleeding sore that’s not healing.

You won’t have to worry as much about whether a rash is cancer if you take steps to protect your skin:

  • Stay indoors during the hours when the sun’s UV rays are strongest, from 10 a.m. to 4 p.m.
  • If you do go outside, apply a broad-spectrum (UVA/UVB) SPF15 or higher sunscreen to all exposed areas — including your lips and eyelids. Reapply after you swim or sweat.
  • In addition to sunscreen, wear sun-protective clothing. Don’t forget to wear a broad-brimmed hat and wraparound UV-protective sunglasses.
  • Stay out of tanning beds.

Check your own skin for any new or changing spots once a month. And see your dermatologist for an annual whole-body check.