The skin, being the body’s first environmental defense, is also exposed to other sources of free radical damage.
Still, the vast majority of each cell’s damage comes from its own internal metabolic creation of energy.
Other sources of free radical damage to the skin include solar damage, ozone, pollutants, applied substances (for example, some sunscreens) and other toxins.
Also, smoking is critical in damaging cells and tissues.
Every puff of cigarette smoke contains enough free radicals for a "free radical hit" to every cell of the body.