For ample protection against non-melanoma and melanoma skin cancers, people over the age of six months should use sunscreen daily, year-round and in any weather. Infants should be kept out of the sun or protected with clothing and an umbrella or stroller hood.

Keep in mind that sunscreen needs to be applied on overcast days, since 70 to 80 percent of the sun's rays penetrate clouds and fog. In addition, according to the World Health Organization, ultraviolet radiation levels, or UVR, rise by 10 to 21 percent for every 1000 feet in elevation gain. Sun reflection from sand, water, snow or concrete also magnifies UVR effects by up to 80 percent.

Apply sunscreen one-half hour before going outside. Because it tends to be rubbed or washed off by sweat and water exposure, sunscreen should be reapplied at least every two hours, and immediately after swimming or heavy sweating. At least one ounce, or two tablespoons, is needed to cover the entire body surface.

Since facial skin is delicate and highly exposed, it’s important to apply sunscreen to the face. Many sunscreens are now incorporated in facial moisturizing creams. If the face is untouched and there is no sweating, it’s adequate to apply sunscreen just once at the beginning of the day. Remember that if you’re involved in outdoor activities, you must still reapply sunscreen frequently.

Most sunscreens provide protection against UVB rays. Make sure that your sunscreen has a sun protection factor, or SPF, of 30 or higher. Some sunscreens also provide protection against UVA rays, although the SPF rating usually refers only to UVB protection.

Since both UVA and UVB exposure can lead to skin cancer, broad-spectrum sunscreens that combine UVA and UVB protection offer the most safety. The shelf life for sunscreen is at least two years and many have an expiration date stamped on the container.

There is strong evidence that sunscreens protect against the development of the precancerous skin condition actinic keratosis, or AK. Studies in the United States and Australia show that regular use of sunscreen significantly decreases AK development. Since AK can be a precursor to squamous cell carcinoma, it’s presumed that sunscreen may help prevent that condition, as well.