How Does Sunscreen Work? SPF and UV Light Explained

Kids dread it, parents carry it religiously in the summer and many have suffered the consequences of overlooking this crucial ingredient for enjoying hours in the sun. The white goop known as sunscreen is a summer must – but how does it work, and why does our skin need it?

Sunlight contains a type of radiation known as ultraviolet, or UV, which not only turns skin darker but can also damages DNA in those cells. What sunscreen does is protect the skin by reflecting or absorbing these rays in lieu of skin itself.

The two types of ultraviolent light that can damage the skin, UVA and UVB, have different wavelengths, according to SkinCancer.org. UVA has a longer wavelength and it is mainly responsible for turning skin darker – that’s why tanning booths primarily use UVA light. UVB is the culprit in sunburn, as it tends to damage the outer layers of skin, and it also plays a role in the development of skin cancer.

The CDC recommends using sunscreen that is at least SPF 15. SPF stands for Sun Protection Factor, and it's a measure of how well it blocks UV rays. The higher the number, the more protection it offers. Theoretically, the SPF represents how much more time it will take for your skin to start burning if you’re wearing sunscreen.

For instance, if it takes 10 minutes for skin to begin burning, using SPF 30 means it will take 30 times longer to start reddening, or about five hours. It's important to recognize that no SPF will block all ultraviolet rays, and because it can often rub off or come off with sweat or water, reapplying sunscreen regularly is key to preventing sunburn on bright summer days.

Sunscreen is made of both inorganic (non-carbon-based) and organic (carbon-based) materials. Inorganic ones include zinc oxide (the stuff that makes sunscreen white) and titanium oxide, and they physically block UV rays from scorching your skin. Organic materials act chemically, absorbing the rays of ultraviolet so the skin doesn't have to.

To decide which sunscreen is best for you, check out the Environmental Working Group's guide, Sunscreen 101, that explains which sunscreen best suits which purposes. Don’t forget that physical blockades can shield your skin too – not just sunscreen. Hats and clothes help block UV rays, as do sunglasses.