Sun protection includes seeking shade when outdoors, avoiding mid-day sun, using sun protective clothing, wearing a wide brimmed hat, wearing sun glasses and application of sunscreen.
What is the difference between a physical and a chemical sunscreen? UVA/UVB protection? Broad spectrum sunscreen?
Sunscreens (or UV filters) are classified into organic UV filters (or chemical sunscreens) and inorganic UV filters (or physical sunscreens). Chemical sunscreens primarily function by absorption, while physical sunscreens reflect and scatter UV radiation.
Chemical sunscreens absorb UV radiation photons, resulting in temporary excitation to a higher energy state, from which the energy may be diffused via photochemical reactions, with subsequent release of heat. In the United States, commonly used organic UVB filters are Octinoxate, Salicylates, Octocrylene and Ensulizole. Common organic UVA filters include Benzophenones, Avobenzone and Ecamsule. As not all organic filters are photostable, they are combined for adequate sun protection. New broad spectrum organic UVA and UVB filters exist but are not yet available in the United States, including Silatriazole (Mexoryl XL), which is the first broad spectrum photostable organic UV filter. Among the sunscreens, Benzophenones are the most common cause of contact photoallergy.
Physical sunscreens work via reflection and scattering. By dispersing UV radiation energy in various directions, inorganic UV filters reduce the photon energy encountered. The most commonly used physical sunscreens contain titanium dioxide and zinc oxide. The physical sunscreens protect in the ranges of visible light, UVA and UVB. Although they provide broad spectrum coverage, overall, inorganic UV filters are less efficient at protecting against UV radiation compared to the newer organic UV filters.
What ingredients should I stay away from if I have sensitive skin?
If you have sensitive skin, it is best to use physical sunscreens which do not usually cause allergic contact and photoallergic dermatitis. Contact allergies to sunscreens may not always be caused by UV filters directly but may be related to the inactive ingredients including preservatives and masking fragrances.
How do we measure sunscreen efficacy?
UVB protection is measured by Sun Protection Factor (or SPF), which is the ratio of time to generate a sunburn with or without the use of the sunscreen being tested. UVA protection measurements are more challenging as sunburns from UVA alone at energy levels in natural light are rare. Other experimental methods which utilize artificial higher intensity UVA, or other outcome measures besides sunburns, do not measure UVA protection in a natural environment.
How do we properly apply sunscreen?
The efficiency of a sunscreen depends on proper application. Sunscreens with an SPF of 30 or higher are typically recommended for adequate UVB protection. As a guideline, one ounce (equivalent to contents of a shot glass) of sunscreen provides adequate full body coverage. As most individuals under-apply sunscreen, a sunscreen with an SPF of 30 or higher is recommended. (Studies about the application patterns of sunscreens demonstrate that a sunscreen of SPF 50 provides an SPF of 12.5 under real consumer conditions.) Frequent reapplication of sunscreen maintains its efficacy. In general, sunscreen should be applied 20 minutes prior to initial sun exposure, reapplied initially 20 minutes later, and then reapplied every 2-3 hours when outdoors.
Can sunscreen use prevent skin cancer?
In a large, 4.5-year, randomized controlled Australian study, the daily use of SPF 17 sunscreen reduces the development of actinic keratoses (precancerous lesions) and squamous cell carcinomas. Recent clinical studies suggest that the combination of a film forming medical device containing photolyase with UV filters with very high SPF (>100) can prevent the recurrence and development of new actinic keratoses. Photolyase contains DNA repair enzymes with the potential to reverse photo-induced DNA damage.
Use of antioxidants in protection from visible light and infrared radiation
In addition to UV radiation, visible light has been found to cause skin discoloration, redness and produce oxygen radicals which may cause DNA damage. Recent research suggests that infrared radiation likely contributes to photoaging and skin cancer development.
Topical anti-oxidants, combined with the use of UV filters, are recommended to neutralize oxygen radicals and prevent skin cancer and photoaging. The most commonly used antioxidant is topical vitamin C.