Exogenous factors in skin aging.

The dramatic alteration in the appearance of the skin with aging is related to both intrinsic (genetic) and exogenous factors. While intrinsic aging is an insidious degenerative process predictable in outcome, the superposition of environmental factors is neither universal nor inevitable. There are distinct morphologic and histological features differentiating intrinsic and extrinsic aging of the skin. The most well appreciated environmental factors affecting skin aging are sun exposure and smoking. Recent advances in molecular biology have increased our understanding of the mechanisms by which exogenous factors contribute to the cutaneous aging. The skin is equipped with numerous inherent mechanisms that protect and defend against accelerating aging. But the efficacy of these mechanisms decreases significantly over a lifetime. In this review, we summarize the features of extrinsic aging and biochemical steps involved in this process.