SUMMER may do wonders for your spirits, but it's likely to be very hard on your skin. From assaults by searing sun, drying wind, excessive bathing and penetrating chlorine to irritations triggered by heat, sweat and common chemicals, your skin is under constant attack from typical summertime activities.

The skin is the body's largest organ, far more complex than surface appearances suggest. It serves many vital functions: It regulates body temperature, directs blood flow, shields internal organs from chemical and physical attack, holds in body fluids, keeps out invading microorganisms and manufactures vitamin D. And since it embodies the sensation of PERSONAL HEALTH touch and carries the identifying marks of individuals, the skin has also been said to serve as the principal organ of sexual attraction.

The skin is also the body's most vulnerable organ, unprotected from attack from without. Skin damage wrought by the elements can usually be prevented if you appreciate the risks and follow simple preventive measures.

In a nation where millions of sun worshippers pursue a glowing tan and millions of others do outdoor exercise, there's no use pretending that people will stay out of the sun for the sake of their skin. But if you're going to expose your skin to the elements, it needs protection. Here are some tips for preserving the integrity of your skin this summer.

Sun Injury. If properly cared for, your skin can add years to your life. No matter how youthful you may be in spirit or in fact, you're likely to look old if your skin is wrinkled, leathery or loose. The very method many people use to improve their appearance, suntanning, actually undermines it in the long run by speeding these effects of aging.

The effects of sun exposure are cumulative and some people suffer permanent damage after just a decade or two of sun abuse. The ultraviolet rays of the sun damage the elastic fibers beneath the surface of the skin, eventually causing a loose, wrinkled appearance. Dark patches, often called liver spots or age spots, also form as permanent blemishes.

In some people, repeated sun exposure causes cancer, lifethreatening melanomas as well as the usually curable superficial skin cancers.

Sunlight can also trigger photosensitivity reactions, itchy or burning rashes that result from ultraviolet rays interacting with a wide variety of chemicals found in drugs, soaps, perfumes and cosmetics. These include tetracycline and sulfa drugs, the thiazide diuretics, deodorant soaps and the oils of bergamot, citron, lavender, lime, sandalwood and cedar that are used in many perfumes and cosmetics.

In recent years, a revolution has taken place in the suntan lotion industry. Transparent chemicals have been discovered that can block out most of the burning rays of the sun and still permit penetration by the tanning rays. The burning rays are more damaging to the skin's infrastructure. With exposure to the burning rays partly blocked, tanning takes longer, but it can be accomplished without painful sunburn and with less injury to the structure of the skin.

Sun screens, as these agents are called, come in a range of strengths. Don't be fooled by the name or advertising claims. Though a lotion may be called a sun ''block'' (or ''bloc''), it may keep out only a small percentage of the damaging rays. Check instead for the Sun Protection Factor (S.P.F.) number. This will range from 2 for very minimal protection to 15 for the maximum blocking effect currently available in these products.

If you have fair skin and burn easily without ever tanning, use an agent with an S.P.F. of 10 to 15. Those who tan easily can usually get by with an S.P.F. of 4 to 6.

The best products contain PABA (para-aminobenzoic acid), esters of PABA or benzophenones. Other sun-screening agents include cinnamates, salicylates and anthranilates. PABA and its chemical relatives work by absorbing the burning ultraviolet rays.

The benzophenones and anthranilates absorb both the burning rays and the tanning rays; the latter are the major cause of photosensitivity reactions. PABA and PABA esters can sometimes cause photosensitivity reactions, so the first time you use a sun product, it's wise to try it on a small part of your body and protect the rest of your skin with clothing. Sun screens should be reapplied after swimming or heavy sweating.

Further protection can be obtained by restricting your sunbathing to the hours before 10 A.M. (11 daylight time) and after 2 P.M. (3 daylight time), when the atmosphere partly blocks the sun's more damaging rays. But you can get burned through loosely wooven clothing and wet T-shirts, under beach umbrellas or a cloudy sky (80 percent of the burning rays penetrate a thin cloud layer).

For areas under heavy sun attack (such as your lips, nose and tops of your ears), you may need an opaque sunscreen (a combination of zinc oxide, titanium dioxide or talc with an opaque substance like red veterinary petrolatum) for adequate protection during prolonged sun exposure.