Ever wonder what causes freckles? Here's how they form

• Freckles happen when you are born with clusters of melanocytes, which are pigment-producing cells.
• These melanocytes ramp up the production of dark pigment when you are exposed to the UV light from sunshine.
• Freckles are usually harmless, but be sure to see a doctor if they are changing shape.
• This article was medically reviewed by Debra Jaliman, MD, a board-certified dermatologist with a private practice in New York City.
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You've most likely seen freckles on others, and maybe you even have some freckles yourself – but do you know what causes freckles to form?

There are a couple of factors that go into freckle formation, and some people are more likely to get them than others.

What causes freckles?

Ephelides, aka freckles, are a combination of genetics and sunlight. Genes play a role because they determine how many melanocytes you're born with. Melanocytes are skin cells that manufacture the dark pigment melanin. People with darker skin are born with more melanocytes than people with lighter skin.

However, it's those with lighter skin who are more prone to developing freckles. This is because the melanocytes in freckle-y folks are scattered across the body in small, dense clusters — sort of like cities on a map. When those melanocytes are exposed to ultraviolet rays from the sun, they ramp up melanin production, which shows up unevenly on the skin as harmless little flat brown spots, or freckles.

Freckles are usually found on body parts that are exposed to the sun the most. This is why they're so common on the face. However, you may also see freckles on other body parts that are often exposed to the sun such as the chest, hands, arms, and legs, according to Nada Elbuluk, MD, dermatologist at Keck Medicine of USC.

Since freckles are linked to sun exposure, it's common for someone to get freckles during the summer months when they're spending more time outside, and for the freckles to fade in the winter if they live in a climate where they don't get much sun exposure during the winter months.

Freckles don't need to be a cause for concern

Though freckles are usually harmless, be sure to keep an eye on them. "Freckles should not change dramatically in size or shape or appearance, nor should they become symptomatic. If one has a brown spot that is changing, it is best to have this evaluated by a dermatologist to ensure that it is not an abnormal mole, skin cancer or other type of growth," says Elbuluk.

Though the presence of freckles isn't necessarily a sign that you aren't protecting your skin enough, if you notice a significant increase in your number of freckles or the darkening of the freckles, you should consider taking precautions to prevent sun damage.

Greenfield urges everyone to use a broad-spectrum sunscreen of SPF 30 or higher with a high concentration of zinc oxide while outside. Broad-spectrum SPF 30 protects you from 97% of the sun's rays. SPF 50 results in only minimal extra protection – about 98% of the sun's rays. SPF 15, on the other hand, only protects you from sunburns, but not damage that leads to premature aging and skin cancer.

You may also consider wearing protective coverings and seeking shade when the sun is at its strongest, especially if you're prone to burns. These precautions can help protect you from harmful UV radiation which can lead to sunburns, damage to your skin cells' DNA, and increase the risk of skin cancer, eye problems, and premature aging.

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