What's The Difference Between UVA & UVB Rays, Anyway? An Easy Trick To Tell

With summer just around the corner (!), there's no better time to brush up on sun care. Sure, you may know how to correctly apply your sunscreen (about a shot-glass worth, every two hours), and you may be all-too-familiar with the great mineral vs. chemical debate. You may have even heard the term broad spectrum tossed around, but let's pause: Do you know what the phrase really means? 

According to the World Health Organization1, UVB rays have a shorter wavelength of 280 to 315 nanometers, and they can't penetrate beyond the superficial skin layers (or through windows). That said, UVB damages the outermost layers of the skin, and overexposure is associated with skin inflammation and sunburns. The SPF value on sunscreen labels is tied to UVB light—the number tells you how much UVB a sunscreen can filter out. 

On the other hand, UVA rays have a wavelength of 315 to 400 nanometers; these rays can penetrate more deeply into the dermal layer and cause photoaging by damaging collagen. So when we talk about free radical damage and oxidative stress from UV radiation (which can contribute to fine lines and crepey, fragile skin), much of the focus tends to narrow in on those UVA rays—in fact, research has found that UVA rays are the main contributor to photoaging2.

Their longer wavelength can also penetrate glass, which is why experts recommend wearing a broad-spectrum sunscreen indoors if you're exposed to natural light through your windows. 

If this all sounds a bit difficult to remember (understandable!), here's a quick tip I learned during a virtual workshop with mbg and Burt's Bees: Imagine the "A" in UVA stands for "aging," while the "B" in UVB stands for "burns." (This, of course, isn't the scientific meaning—but a fun way to remember.) That doesn't mean you should choose to guard against one or the other, but it might be helpful to know as you browse the market and decode your sunscreen labels.