How do sunscreens actually work to protect our skin from UV radiation, and what’s the difference between UVA and UVB protection?

-Karthik Gurumurthy

I’ve always been fascinated by how sunscreen actually works to protect our skin. When you think about it, it’s pretty incredible that a thin layer of cream can shield us from something as powerful as the sun.

Natural sunlight contains ultraviolet (UV) photons that we can’t see because they fall outside the visible spectrum. These photons have shorter wavelengths and higher energy than visible light. As the saying goes, “what you can’t see will hurt you” – these high-energy photons can generate free radicals and directly damage our DNA when they strike our skin.

The UV rays we’re exposed to come in two main varieties: UVB and UVA. The shorter wavelength UVB rays don’t penetrate deeply but cause significant DNA damage and are primarily responsible for sunburn and skin cancer. UVA rays, with their longer wavelengths, penetrate deeper into the skin where they produce free radicals, leading to premature aging and immunological problems.

I think of sunscreen as essentially a thin bulletproof vest for my skin. It works by stopping these UV photons before they can reach the skin and inflict damage. Modern sunscreens contain organic molecules that absorb UV radiation as well as inorganic pigments that absorb, scatter, and reflect UV. For maximum protection, a sunscreen needs sufficient quantities of these protective agents distributed evenly over all the peaks and valleys of our skin’s surface.

The SPF (sun protection factor) label on sunscreens actually stands for “sunburn protection factor” and primarily measures protection against UVB rays. I like to think of it in practical terms – an SPF 10 sunscreen allows about 10 out of every 100 harmful photons to reach my skin, while SPF 20 allows only 5 out of every 100.

What’s important to understand is that a high SPF doesn’t necessarily mean broad spectrum protection. Since sunburn is primarily a UVB effect, a sunscreen can deliver high SPF while still allowing significant UVA photons to reach the skin. For true broad-spectrum protection, products need to block a significant portion of UVA photons too, which is why ingredients like zinc oxide, avobenzone, or titanium dioxide are so important.

This also explains why tanning beds aren’t a safe option – they typically emit less UVB but significantly more UVA than natural sunlight. While this leads to less sunburn and more tanning in the short term, those UVA rays take their toll on skin health over time.