Both UVA and UVB rays can cause sunburn and tanning, which damage the DNA in your skin cells and increase your risk for skin cancer. UVA rays also cause the unwelcome signs of premature aging, including dark spots and wrinkles. Those UVA rays account for up to 95 percent of the UV radiation reaching the Earth’s surface and are quite good at finding you. They can bounce off reflective surfaces like water and, most relevant during the workday, they can penetrate window glass. If you’re sitting next to a window on a sunny day, those UVA rays hitting your skin are doing damage.