To understand how sunscreens work, you must first understand what ultraviolet (UV) radiation is. UV radiation has a shorter wavelength than visible light, meaning it penetrates the skin and can damage DNA, causing mutations that lead to skin cancer. Visible light has a longer wavelength, while X-rays are on the other end of the spectrum with even shorter wavelengths than UV. As you can imagine, X-rays can penetrate through the body and stop when they hit bone; UV rays are just short of that and stop when they hit the first layers of skin.
UV is divided into two categories: UV-A and UV-B. UV-A is to blame for long-term skin damage (sun spots, aging, wrinkles), while UV-B is to blame for sunburn and skin cancer. A good sunscreen is “broad spectrum” meaning it blocks both UV-A and UV-B rays.
Zinc oxide is one ingredient in inorganic sunscreens which blocks both UV-A and UV-B rays. The way it does this is by forming a physical barrier on the skin. This literally causes UV rays to bounce off, reflect, or scatter off of it. In some ways, it is like a layer of clothing. Therefore, the harmful UV rays are not absorbed into the skin! In contrast, the organic sunscreens containing chemicals such as oxybenzone absorb only UV-B rays and release the energy as heat.
In the old days, zinc-based sunscreens were known to form a white paste on the skin (hence their strong barrier function!). However, nanotechnology nowadays has made it possible to make cosmetically pleasing, but just as effective, zinc-based sunscreens that do not leave a white film on the skin.
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