DR Murad says...

''For decades skin researchers have made a distinction between intrinsic and extrinsic aging. Skin, like every other organ in your body, begins to age when you are about twenty as a result of internal cellular changes, many of which are genetically preprogrammed. But skin is different from other organs in that it is also exposed to external injury, mainly environmental damage by sunlight and pollution. Environmental factors also include those things we do to ourselves, such as eating poorly, drinking too much alcohol, smoking,and taking certain medications. These environmental factors cause biological changes that accelerate the natural intrinsic aging process.
It's been estimated that 80 to 90 percent of the visible signs of aging are caused by environmental injury, and that the remaining 20 percent are due to internal aging factors that we once thought to be byound our influence.
I do think it's safe to assume that much of the dammage we see as aging is the result of injury from the outside-that is, overexposure to the sun and polluants in the atmosphere.
I don't believe in a simple division into inside and outside factors. Rather I believe these destructive forces are mutually dependent. Everything in the external environment has some impact on how the body ages internally. And the body's diminished ability to cope with injury affects its vulnerability to external injury.''