A research team reports that unless sunscreen is reapplied often, the sunscreen itself can become harmful to the skin.
Sunscreen works by blocking ultraviolet light. When skin is exposed to the ultraviolet radiation in sunlight, it can generate harmful compounds called reactive oxygen species or ROS. ROS can react with cell walls, lipid membranes, mitochondria and DNA, leading to skin damage. One effect is an increase in the visible signs of age.
However, the molecules in sunscreen that block ultraviolet light can themselves penetrate into the skin, leaving the epidermis exposed to ultraviolet radiation.
In addition, some of these molecules, including octylmethoxycinnamate, benzophenone-3 and octocrylene, generate ROS molecules in skin themselves when exposed to ultraviolet radiation. This occurs when the molecules have both penetrated into the skin, and additional sunscreen has not been applied to block the ultraviolet light.
About 95 percent of the visible signs of aging are associated with the ROS molecules created by exposure to ultraviolet light.