According to the American Cancer Society, more than 2 million people are diagnosed with skin cancer in the United States each year. There are more than 2,000 over-the-counter sunscreen formulas on the market today. How can you tell which sunscreens are the safest, most effective, and represent the best value for your money? In most cases, the answer comes down to the difference between the two types of filtering ingredients. Chemical or Physical?The UV radiation in sunlight consists of UV-A, UV-B, and UV-C rays. UV-A and UV-B are both responsible for photoaging, skin cancer, sunburn, tanning, and wrinkling. UV-C is not a factor in skin health, as it is absorbed by the Earth's atmosphere and does not reach us in significant amounts. Broad-spectrum sunscreen protects against both UV-A and UV-B. This protection can work in one of two ways: chemical or physical. Chemical UV FiltersWork by absorbing UV radiation; Require application 30 minutes before sun exposure; Provide partial protection from UV spectrum; May irritate the skin and eyes; Not regulated for safety by the US Food and Drug Administration (FDA)--some may even be carcinogenic; Not photostable (exposure to sunlight degrades effectiveness); Avobenzone is the most commonly used chemical filter ingredient. Physical UV FiltersWork by reflecting UV radiation; Start protecting immediately upon use; Provide full broad-spectrum protection; Non-irritating to skin and eyes; Safe, as particles do not penetrate the skin; Highly photostable (exposure to sunlight does not change effectiveness). Zinc oxide and titanium dioxide are the most commonly used physical filter ingredients. Clothing and shade structures also count as physical filters. How Stable Is It?One of the most important factors in the effectiveness of a sunscreen formula is also one of the least known to the general public. Photostability is an ingredient's ability to remain effective after exposure to sunlight. Many people are aware that this is an issue for numerous skin care ingredients, but may be surprised to learn that some active ingredients in sunscreen--a product whose sole purpose involves being exposed to sunlight--are not photostable. In addition, the FDA's new rules do not require sunscreen ingredients to be tested for photostability. Yet, many consumers expect that their sunscreen will protect them for longer than one hour. Physical filters such as zinc oxide and titanium dioxide are photostable. Studies have shown that these ingredients suffer no degradation after more than two hours of sun exposure. However, the chemical filter avobenzone is not at all photostable, and degrades almost completely in less than one hour. Even worse, avobenzone also degrades on contact with other UV filters such as zinc oxide or titanium dioxide, and with metal ions such as iron oxide, which is commonly found in makeup. This goes a long way toward explaining why many consumers experience sunburn even after applying sunscreen as directed. Health ConcernsEffectiveness is not the only thing to consider in any product being applied to the face or body. Significant health concerns have also been raised about many sunscreen ingredients. Here are some issues to consider. Avobenzone has been found to generate free radicals beyond acceptable safety levels after sitting on the skin for just one hour, and children and pregnant women have been advised not to use products containing it. Octocrylene, which is known to act as an endocrine disrupter, is used in many sunscreens as a stabilizer. It can also cause skin irritation. According to the Archives of Dermatology, "Octocrylene appears to be a strong allergen leading to contact dermatitis in children and mostly photoallergic contact dermatitis in adults." Chemical UV filters can also have harmful effects on the environment. Octocrylene does not seem to be effectively contained in wastewater treatment plants, and studies in Switzerland have indicated that it accumulates in fish. Oxybenzone, a chemical UV-B filter often used in combination with avobenzone, has been found to negatively impact reef ecosystems and biodiversity. Physical UV filters, in contrast, have an excellent safety profile. The FDA has long considered zinc oxide to be a safe ingredient for both external use and as a food additive, even in infant formula. Considering all these factors, physical UV blockers represent the best choice overall. The main challenge in getting consumers to use sunscreens based on physical filters is purely cosmetic: zinc oxide and titanium dioxide tend to feel thick and greasy, and are visible on the skin, leaving a white residue. However, new advances mean there are now an increasing number of sunscreens that use these ingredients in formulations that allow for clear application. When evaluating a sunscreen, the most important considerations should be safety and effectiveness. Carefully examine the ingredients and make use of all available information to make the best choices for yourself and your family. Jason Barbaria is director of marketing at Dermagenics, a skin care line that includes sunscreen, cleansers, and moisturizers.