We had a great response to our first look at common questions about sunscreen. A few of you asked some great questions that I will expand upon in this piece. The newest numbers estimate approximately 3.6 million skin cancers to be diagnosed in the US alone last year.  Not to mention that wrinkles and brown “lentigines” are directly related to the sun as well. The first thing to remember will not surprise you…nobody likes to wear sunscreen (even dermatologists). But the truth remains that the number of people affected by skin cancer is growing every year, and melanoma is occurring in younger individuals.

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

 

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.  

 

What is the difference between UVA and UVB (ultraviolet) light wavelengths and will a sunscreen protect me from both?

 

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, avobenzone (Parsol 1789) and ecamsule (Mexoryl SX), extend the coverage beyond the UVB range and into the UVA range, helping to make sunscreens broad-spectrum.

 

What is an SPF?

 

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 deflect 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 30 or greater year-round for all skin types.

 

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

 

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 deflected, while an SPF of 15 indicates 93 percent deflection and an SPF of 2 equals 50 percent deflection.   

Research Note

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. Journal of the American Academy of Dermatology, 1990, 22(3): 449-452; Kaidbey, K.H.

— Dr. Ross Campbell at Campbell Dermatology at Cowles Clinic specializes in skin cancer treatment and cosmetic treatments to help you look your very best.