We all know that getting wrinkles is part of growing older. As your skin ages, the tissues that keep it supple—collagen and elastin—weaken and cause the skin to lose some of its resilience. But did you know that the No. 1 cause of wrinkles is sun exposure?

There are two types of aging. One is inevitable, the other is partially preventable. To the inexperienced eye, these two types might look exactly the same, but they are somewhat different.

Intrinsic (coming from the inside) aging is a continual, natural process that begins in the deep layers of your skin as early as your 20s. The rate at which intrinsic aging advances is controlled primarily by genetic factors.

Intrinsic aging is affected by changes in your body chemistry over time, as well as changes in the skin's life cycle. As you age, your skin becomes thinner and loses fat, making it look less smooth. Also, gravity takes its toll, causing skin to sag. The effects of intrinsic aging alone are subtle—primarily looseness in the skin and fine wrinkling—and can most easily be seen in areas of skin not exposed to the sun.

Superimposed on these subtle changes are changes due to extrinsic (environmental) influences, such as pollution and exposure to harsh irritants or chemicals. The major extrinsic cause of skin aging is photodamage, damage caused by chronic exposure to ultraviolet radiation from the sun.

Photodamage is a cumulative process that takes place gradually over decades. It begins with your first exposure as a child to sunlight and accumulates throughout life with each additional exposure, whether prolonged or incidental. The majority of the skin changes we think of as aging are actually caused by exposure to the sun.

The visible signs of photodamage include erythema (inflammation), sunburn and tanning. These are acute reactions that occur within hours and days of exposure. Chronic photoaging occurs over many years of exposure to ultraviolet radiation and is characterized by skin roughness, mottled hyperpigmentation (i.e., brown spots), fine and coarse wrinkling, sallowness, looseness of the skin, freckles and telangiectasia (a visible network of enlarged and broken superficial blood vessels). The severity of these signs tends to increase with the cumulative amount of sun exposure.

Adopting a comprehensive program of skin care can reduce skin damage from the sun. The most important step is to minimize sun exposure by avoiding outdoor activities at midday and by wearing protective clothing, specifically hats and tightly knit, dry clothes. You should also apply sunscreen or moisturizer with a sun protection factor (SPF) rating of 15 or higher to all exposed skin (including the lips, ears and scalp).