Sunscreens are an essential tool for preventing skin aging and skin cancers. Sun damaged skin shows wrinkling, is less elastic and sags as well as appears sallow and leathery. In addition, broken blood vessels and even blackheads may form.
Ultraviolet (UV) rays are non-visible rays transmitted from the sun. Those UV rays that reach the earth’s surface are divided into UV-A and UV-B, depending on their wavelength size. UVA and UVB radiation irreversibly damages our skin and an effective sunscreen should protect against both types of radiation.
A list of several active ingredients is found on sunscreens because the combination is necessary to protect against this broad spectrum of radiation. Typically, there will be either salicylates or cinnamates, which are the UVB protectors. Avobenzone has been added to many sunscreens, thus extending coverage to include the UVA range. Titanium dioxide or zinc oxide protect against both UVA and UVB radiation.
Sun Protection Factor (SPF) is a measurement of the sunscreen’s ability to protect against UVB rays. An SPF of 15, when properly applied, will extend the time one can stay in the sun without burning by a factor of 15. If you are fair skinned and otherwise burn after 10 minutes of sun, you are protected for 10 minutes x 15=150 minutes. An SPF 15 shields about 90% of the UVB rays from our skin. An SPF 30 protects us against approximately 95% of those UVB rays. Higher SPF’s do not deliver substantially more protection.
The exact SPF of a sunscreen is determined by dividing an individual’s lower back into small test squares. Some squares are exposed to varying amounts of UVB light without sunscreen. The next day, sunscreen is applied to other squares and allowed to dry. The SPF is calculated by determining the amount of light needed to cause the same amount of skin redness as the day before, when no sunscreen was applied.
Under these test conditions, the sunscreen is applied thickly and evenly. There is no wind, humidity, sweating or excessive heat. With actual use, the SPF is likely not as effective as the number indicated on the bottle and we should not rely on the sunscreen to protect us from the effects of staying out all day in the sun without a shirt or a hat.
Water resistance is determined by applying a thick layer of sunscreen, allowing it to dry for 20 minutes, then applying a second layer with another 20 minutes allowed for drying. The test individuals then swim for 20 minutes, dry their skin and rest for 20 minutes, then swim for another 20 minutes. If the SPF is the same before swimming as after, the sunscreen is considered water resistant. Note that there were two applications plus adequate drying time in these tests. That is the recommended manner that sunscreen should be applied prior to swimming. Sunscreens can and do eventually get washed off in the water and should be reapplied regularly throughout the day as well.
Finally, sunscreens should be discarded after their expiration date, since their stability and activity cannot be guaranteed.
Dr. Lisa Benest offers extensive procedures in non-invasive cosmetic skin care, using the latest and most up-to-date lasers and medical equipment. Visit us for a free consultation with our skin care specialist!