Polycarbonate, which is what most safety eyewear is made from, inherently blocks UV rays up to 380 nanometres (nm). Eyewear that protects up to 380 nm covers most of the UVB spectrum which is beneficial, however, it still allows some of the UVA rays through. These rays are particularly damaging over the longer term.
UV400 refers to lenses that block out nearly 100% of harmful ultraviolet (UV) light rays, including both UVA and UVB rays, up to a wavelength of 400 nm. This provides the highest level of protection for your eyes.