Q. Can I use the sunscreen I bought last summer, or do I need to purchase a new bottle each year? Does it lose strength?

A. Unless indicated by an expiration date, the FDA requires that all sunscreens be stable and at their original strength for at least three years.

While you can use the sunscreen that you bought last summer, keep in mind that if you are using the appropriate amount, a bottle of sunscreen should not last you very long.

Approximately one ounce of sunscreen, enough to fill a shot glass, is considered the amount needed to cover the exposed areas of the body properly.

Q. What is the difference between UVA and UVB light wavelengths and will a sunscreen protect me from both?

A. Sunlight consists of two types of harmful rays - UVA rays and UVB rays.

The UVB rays are the sun's burning rays (which are blocked by window glass) and are the primary cause of sunburn and skin cancer.

UVA rays (which pass through window glass) penetrate deeper into the dermis, or base layer of the skin. They also contribute to sunburns and skin cancer.

Both UVA and UVB rays can cause suppression of the immune system which helps to protect you against the development and spread of skin cancer.

Since PABA and PABA esters only protect against UVB light, check for a broad-spectrum sunscreen that also screens UVA rays.

Ingredients like benzophenones, oxybenzone, sulisobenzone, titanium dioxide, zinc oxide, and avobenzone (Parsol 1789), extend the coverage beyond the UVB range and into the UVA range, helping to make sunscreens broad-spectrum.

Q. What is an SPF?

A. SPF stands for Sun Protection Factor.

Sunscreens are rated or classified by the strength of their SPF. The SPF numbers on the packaging can range from as low as 2 to greater than 50. These numbers refer to the product's ability to screen or block out the sun's burning rays.

The sunscreen SPF rating is calculated by comparing the amount of time needed to produce a sunburn on sunscreen protected skin to the amount of time needed to cause a sunburn on unprotected skin.

For example, if a sunscreen is rated SPF 2 and a fair-skinned person who would normally turn red after ten minutes of exposure in the sun uses it, it would take twenty minutes of exposure for the skin to turn red.

A sunscreen with an SPF of 15 would allow that person to multiply that initial burning time by 15, which means it would take 15 times longer to burn, or 150 minutes.

Dermatologists strongly recommend using a broad-spectrum sunscreen with an SPF of 15 or greater year-round for all skin types.

Q. Does SPF 30 have twice as much sun protection as SPF 15?

A. SPF protection does not actually increase proportionately with a designated SPF number.

In higher SPFs, such as an SPF of 30, 97 percent of sunburning rays are absorbed, while an SPF of 15 indicates 93 percent absorption and an SPF of 2 equals 50 percent absorption.

Research Note

Recent research suggests that high SPF sunscreens are an appropriate choice for very sun sensitive individuals (skin types I and II). One study determined that skin protected by an SPF 15 sunscreen and then exposed to 15 times the minimum dose of sunlight normally required to cause redness produced 2.5 times the number of sunburn cells seen in SPF 30 protected skin with the same dose of sunlight.

These results suggest that prevention of redness does not necessarily mean prevention of all sun-induced damage. More research is currently underway on the protective effects of sunscreens on different skin types.

Q. Does the SPF tell how well a sunscreen protects against UVA or UVB rays?

A. The SPF number on sunscreens only reflects the product's screening ability for UVB rays.

At present, there is no FDA-approved rating system that identifies UVA protection. Scientists are working to create a standardized testing system to measure UVA protection.