Why do freckles come out in the sun?

The freckles that speckle many people's skin are often called "sun kisses" because they tend to appear or darken after exposure to the sun.

But why do freckles come out in the sun?

A freckle develops as a protective mechanism against harmful ultraviolet (UV) radiation, Dr. Jill S. Waibel, a board-certified dermatologist and the medical director of Miami Dermatology and Laser Institute, told Live Science. Sun exposure prompts the skin to produce melanin, the pigment that produces the many hues of human skin and also allows skin to tan.

Melanin scatters UV rays, meaning it sends the rays bouncing in different directions; this prevents them from penetrating the skin and damaging its DNA. However, some patches of skin produce more of this pigment than others, and these melanin-rich spots are referred to as freckles, Waibel said.

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As a result, freckles tend to become more visible during the summer and disappear or lighten during months when the UV radiation is not as strong.

That said, not all freckles respond to the seasonal sun exposure in the same way, Dr. Rebecca Kazin, a board-certified dermatologist and the director of clinical research at Icon Dermatology and Aesthetics, a dermatology practice in North Bethesda, Maryland, told Live Science by email.

There are two main types of freckles: ephelides, which are what people typically think of as freckles, and solar lentigines, also known as liver spots or age spots. Both types are more common in people with fair skin and in those with a genetic predisposition to develop them. However, solar lentigines do not fade in the winter like ephelides do, Kazin said.

Ephelides, the most common freckle type, are usually small and light brown, and they appear on skin that's commonly exposed to the sun: on the face, arms and shoulders, she said. These freckles tend to darken and fade in response to short-term sun exposure, while solar lentigines are associated with accumulated sun damage to the skin and do not fade.

Solar lentigines tend to be larger and darker than ephelides, and they are more common in older people, Kazin said. Also called sun spots, solar lentigines appear when UV radiation damages the DNA in skin and thus changes the behavior of melanin-making cells, according to a 2014 review in the journal Pigment Cell & Melanoma Research. These changes in gene activity boost both the number and production levels of these melanin makers, resulting in patches of skin where melanin pigment accumulates and clumps up.

Solar lentigines are not cancerous and don't require any treatment, but they can look similar to some skin cancers. If you're concerned about a spot or notice that one is undergoing rapid changes, contact a health care provider to get checked out, Kaiser Permanente advises.

Sun kisses — what most people think of when they think of freckles — are harmless, but perhaps a good reminder for those who freckle easily to slather on their daily sunscreen.

This article is for informational purposes only and is not meant to offer medical advice.