It won’t be long before the heady musk of coconut-scented zinc oxide suffuses the summer air, announcing the success of vehement public health warnings against the dangers of unprotected tanning. The OG sunscreen mineral, which has been used as a skin shield for thousands of years, ZO isn’t the only UV-sucking ingredient stirred into sunscreens, but it set both a standard and strategy for all of the sun-shielding chemicals that followed in its wake.

In the late 1980s, zinc oxide literally had its time in the sun when the Southern Californian company Zinka made it cool to paint your nose with the chalk-white sunscreen. Tanned babes and brahs, emulating the UV-protected noses of hot lifeguards everywhere, were hooked on the product, which was later sold in Day-Glo colors so that “wacky youths” could “come to the beach as Indians on the warpath.”

While Zinka’s popularity (and, thankfully, its marketing campaign) didn’t last, Zinka remains an excellent illustration of how zinc oxide works. There was nary a burnt SoCal nose in sight because Zinka’s formulation, which contained large, opaque particles of zinc oxide, acted as a physical barrier between skin and sun, blocking harmful UV waves before they can reach our cells.

The metal oxide occurs naturally as the mineral zincite — probably what ancient sunbathers powdered and smeared on their skin — but finding it in the wild is rare; these days, the compound is synthesized in labs as a white powder and blasted into nanoparticles so it’s no longer a chalky white. At the molecular level, it’s especially reflective, so harmful rays can’t penetrate the skin.

Angry sunburns happen when the UVA and UVB wavelengths in sunlight get through the skin’s surface; the visible and long-wave part of UVA infiltrates the deepest parts of the dermis, while UVB and the short-wave parts of UVA burn closer to the top. Because zinc oxide blocks both, it’s considered a “broad-spectrum” protectant.

Without a shield, the skin can’t hold its own for long. Different skin types put up better defenses than others, but soon, fed up with the relentless penetration of UV rays, all skin types angrily signal to the brain to seek some shade by kickstarting the inflammatory process. As the skin’s blood vessels rapidly dilate, ugly redness ensues; the skin pulls tighter as moisture dissipates, causing sensitivity. Tanning begins as cells produce more melanin, a darkly colored compound meant to protect the deeper layers of the skin from UV exposure. When bodies are too slow to get under a damn tree, the cells on the skin’s surface become collateral damage: Exposed to so much UV, they’re at high risk of becoming cancerous, so the body kills them off in swaths before the damage can spread. The result? Intense peeling, often in broad, Goldmember-esque sheets.

Slapping on sunscreen is like being covered in a film of zinc oxide mirrors, like the diamond-skinned vampires in Twilight, only greasier. These days, before the compound is stirred into sunscreens, it’s blasted into nanoparticles that are so minuscule they no longer appear white. It’s usually also mixed together with nanoparticles of titanium dioxide, another white compound, that work in a similar way. Both are immune to degradation by sunlight.

Still, sunscreen is far from foolproof. Swimmers and sweaty individuals alike know the pains of reapplication, and there are growing, albeit largely contested, concerns that the tiny metal oxides, washing into pools and oceans, are harmful for both humans and the environment. Organic chemical ingredients in sunscreens, which also scatter or absorb UV light, are also widely used, though these tend to break down over time — hence, sunblock expiry dates. Zinc oxide and titanium dioxide — basically, powdered rocks — make a much sturdier barricade.

Photos via Getty/Ian Gavan, Getty/Quinn Rooney