Ultraviolet A radiation. The sun produces a range of ultraviolet (UV) radiation, of which UVA and UVB affect our skin. UVA rays have wavelengths of 320 to 400 nanometers; UVB rays have wavelengths of 290 to 320 nanometers. UVB radiation causes sunburn, while UVA radiation does not produce any visible short-term evidence of skin damage. Nonetheless, UVA radiation creates serious cumulative changes in skin that may be far greater than the sunburn caused by UVB radiation. Research has shown that unprotected exposure to UVA rays can, within one week, create distinct injury.
Research also indicates UVA exposure is even more damaging than we once knew: it seems that when UVA radiation hits skin, it not only penetrates to skin’s lower layers but the damage “bounces back” like light reflecting from a mirror, giving every layer of your skin a double-whammy of damage. UVA radiation prompts tanning, yes, but the tan you see is direct evidence of your skin responding, as best it can, to the onslaught of damage it just endured.
To be truly effective and beneficial for skin, sunscreens must protect skin from both the sun’s UVA and UVB radiation. In the United States, there are four ingredients approved by the FDA that protect across the full UVA range: titanium dioxide, zinc oxide, avobenzone (also called Parsol 1789 and butyl methoxydibenzoylmethane).