How does sunscreen work?

For most people, it’s good enough to know that sunscreen works by squeezing it out of the bottle and rubbing it onto our skin.

But what is in the stuff that keeps us from getting the kind of nasty sunburn that can lead to even worse things — like skin cancer? Here’s a look inside the bottle.

women applying sunscreen

In short, most sunscreen products work by absorbing, reflecting, or scattering sunlight through the use of chemicals that interact with the skin to protect it from ultraviolet (UV) rays.
Active physical ingredients in sunscreen

The physical compounds in sunscreen work to reflect, scatter, and absorb both UVA and UVB rays.

The broad-spectrum active ingredients titanium dioxide and zinc oxide leave a white residue on the skin following application when used in a larger particle form. However, when these active ingredients are converted into nanoparticles (smaller, lighter molecules) they are transparent — appearing to vanish on the skin — and do not leave a residue, according to Daniel M Siegel, MD, FAAD, president of the American Academy of Dermatology.

Not only are they less obtrusive, but the fact that the pieces of titanium dioxide and zinc oxide are so tiny actually gives the sunscreen enhanced ability.

“One of the main benefits of nanoparticles in sunscreens is that the small molecules can provide more protection and more even coverage on the skin’s surface than larger particles,” says Dr Siegel.

“Considerable research on the use of nanoparticles on healthy, undamaged skin has shown that the stratum corneum — the outermost layer of the skin — is an effective barrier to preventing the entry of nanoparticles into the deeper layers of the skin.”

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The AAD adds that titanium dioxide and zinc oxide have a long history of safe use in sunscreens, and offer good options for broad-spectrum UV protection.

Active chemical ingredients in sunscreen

Broad-spectrum sunscreens often contain a number of chemical ingredients that absorb UVA and UVB radiation. Many sunscreens contain UVA-absorbing avobenzone or a benzophenone (such as dioxybenzone, oxybenzone or sulisobenzone), in addition to UVB-absorbing chemical ingredients (some of which also contribute to UVA protection).

Oxybenzone is one of the few FDA-approved ingredients that provides effective broad-spectrum protection from UV radiation, and has been approved for use since 1978.

“Available peer-reviewed scientific literature and regulatory assessments from national and international bodies do not support a link between oxybenzone in sunscreen and hormonal alterations, or other significant health issues in humans,” says Dr Siegel.

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The FDA has approved oxybenzone in sunscreen for use on children older than six months, and dermatologists continue to encourage protecting children by playing in the shade, wearing protective clothing and applying broad-spectrum sunscreen.”

Combined for greater sun protection

Organic sunscreens — not organic in the no pesticides/free-range sense, but meaning a compound with a carbon base — actually absorb the UV rays, according to research published in The Lancet, and consist of a range of complex organic molecules which blend together to give photoprotective qualities.

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As explained in a 2013 medical journal article, “Chemical sunscreens absorb high-energy UV rays, and physical blockers reflect or scatter light. Multiple organic compounds are usually incorporated into chemical sunscreening agents to achieve protection against a range of the UV spectrum. Inorganic particulates may scatter the microparticles in the upper layers of skin, thereby increasing the optical pathway of photons, leading to absorption of more photons and enhancing the sun protection factor (SPF), resulting in high efficiency of the compound.”

Some downsides to the chemicals

uva uvb rays skinAccording to The Skin Cancer Foundation, the UVB absorber PABA (para-aminobenzoic acid) was patented back in 1943. For years, it was a very common ingredient in sunscreens — until it was found to cause allergies in some users, and raised fears of a link to cancer development.

Those concerns, plus that PABA could cause yellow stains on clothes, has caused many brands to replace that ingredient in their products. While PABA is still FDA-approved for use in sunscreen, starting in the mid-1980s, its popularity has declined sharply.

In rare cases, chemical ingredients cause skin reactions, including acne, burning, blisters, dryness, itching, rash, redness, stinging, swelling, and tightening of the skin. Such reactions are most commonly associated with PABA-based sunscreens and those containing benzophenones.

Some sunscreens also contain alcohol, fragrances, or preservatives, so avoid those if you have skin allergies.

Don’t overdo

While many studies have shown that sunscreen protects against acute UV skin damage and non-melanoma skin cancer, whether or not sunscreen will stop the development of melanoma — the most serious type of skin cancer — has not yet been conclusively proven.

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In a published work from 2007, Dr Stephan Lautenschlager, Outpatient Clinic of Dermatology at the Triemli Hospital in Zurich makes one final point: “Sunscreens should not be abused in an attempt to increase time in the sun to a maximum.”

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