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How to wear sunscreen the right way: Your guide to SPF
Summers come and go, yet some people still may not know how to wear sunscreen correctly.
This could be concerning since these ultraviolet ray-shielding topicals can help prevent skin cancer, which is the "most commonly diagnosed cancer" in the country, according to the U.S. Department of Health and Human Services.
The health agency reports that approximately five million Americans get treated for skin cancer each year.
HOW TO KNOW IF YOUR SUNSCREEN IS EXPIRED
If you need a refresher on how you’re supposed to wear sunscreen, here’s a quick guide on what SPF is and how it works, the differences between sunscreen types and which sunscreens are better for your face and body.
What is SPF?
Sun Protection Factor – commonly known as SPF – is the measure of how much UV radiation is needed to produce sunburn on protected skin compared to unprotected skin, according to the Food and Drug Administration (FDA).
With sunscreens, the SPF rating scale indicates the level of sunburn protection a person can receive – if applied correctly.
"As the SPF value increases, sunburn protection increases," the FDA says. "[But,] SPF is not directly related to time of solar exposure but to amount of solar exposure."
The amount of UV radiation that comes down from the sun varies throughout the day, so an hour of sun exposure at 9 a.m. could be equivalent to 15 minutes of sun exposure at 1 p.m, the FDA states on its webpage about SPF.
How is this possible? The sun’s rays are at their strongest between 10 a.m. and 4 p.m., according to the United States Environmental Protection Agency.
WHAT SPF SHOULD YOUR SUNSCREEN HAVE? FIND OUT HOW IT MAY HELP PREMATURE AGING AND SKIN CANCER
This means people who go outside during the 18 other hours of the day are exposed to a lower amount of UV radiation, but health experts universally agree that sunscreen should be worn for added protection.
How do I find the right amount of SPF?
Per the FDA’s sunscreen testing, the agency recommends 2 milligrams of sunscreen for every square centimeter of exposed skin.
The FDA also recommends applying sunscreen 15 minutes before going out and reapplying the product at least every two hours. Water-resistant sunscreens should be used if a person decides to go swimming or does an activity that’ll cause profuse sweating. Reapplication should be done on dry skin at least every two hours.
While the FDA is clear about how much sunscreen should be used for maximum protection, few people apparently wear their sunscreen at the recommended amount – if at all.
"Generally, there has to be a balance because what happens is that we do know that the average public, including ourselves, when we put on the sunscreen, we don't put on that two-milligram-per-centimeter-square, which is the one that is used for testing purposes mandated by the FDA," said Dr. Henry Lim of Henry Ford Health – a not-for-profit health care organization in Detroit.
"So automatically, the end-use SPF is going to be lower than the labeled SPF because of the inadequate application as compared to the amount that is used for testing," Lim continued.
注释：
topical: adj
表示"【医】局部的"，means "pertaining to the surface of a body part"，如：The use of topical anestlatics should be closely monitored.局部麻醉药的应用必须慎重。
premature: adj
表示". 提前的；过早的；"，means "too soon or too hasty"，如：This disease produces premature aging of the brain. 这种疾病使大脑过早衰老。
profuse: adj
表示"丰富的；慷慨的"，means "produced or growing in extreme abundance"，如：He made profuse apologies. 他再三道歉。
mandate： v; n
表示"授权; 要求 "，means "make mandatory"，如：Britain was mandated to govern the former colony of German East Africa.英国授权代管德国在东非的前殖民地。You should follow the doctor's mandate. 你该听从医师的要求。
not-for-profit: adj; n
表示"非利益化的; 不是为了利润的"，means "an organization chartered for other than profit-making activities"，如：We undertake the social study not for profit but for the welfare of the employees. 我们从事这项社会调查并非出于盈利,而是为了员工们的福利。
end-use: n; adj
表示" 最终用途", 如：Multi-market end uses from comfortable flannels and fleeces to luxury blends. 多样的终端市场用途,从舒适的法兰绒和瑶粒绒到豪华的混纺织物。
中文简要说明：
为了避免出门在外被艳阳晒伤老化，增加罹患皮肤癌风险，不少人都会擦上防晒乳来保护皮肤，不过如何正确使用却是众说纷纭。美国食品药物管理局（FDA）和皮肤科医生提供防晒乳的用法、防晒系数的计算方式，让你在这个夏天能够尽情享受户外玩乐时光。
根据FDA的定义，防晒系数（SPF）指的是涂抹防晒用品后，可以推迟皮肤被紫外线B光（UVB）晒伤的能力。举例来说，原本10分钟会被晒伤的皮肤，擦了SPF15的防晒用品后，可以延长到150分钟才会被晒伤；若擦的是SPF30的产品，则要到300分钟后才可能被晒伤。
乍看之下，防晒产品的系数愈高，的确能带来更好的保护力，不过除了时间长度之外，「紫外线含量」也是决定能否有效防晒的重要因素，在早上9点晒一小时太阳，可能相当于下午1点钟晒15分钟的照射量。既然紫外线如此危险，应该如何正确使用防晒产品呢？FDA建议，每1平方公分皮肤应涂抹2毫克防晒乳，而为了方便大众记忆，皮肤科医生也提供了「9分法则」：
◆ 在脸上、头部和颈部涂抹1茶匙。
◆ 上臂和手臂加起来1茶匙，左、右各1。
◆ 身体前面、后面各1茶匙。
◆ 左、右下肢各2茶匙。
美国底特律非营利保健组织Henry Ford Health的林医生（Henry Lim）说，按照上述原则，全身涂抹防晒产品的用量大约是31公克，相当于装满一口酒杯，「大多数人不会用到这么多…通常只有抹建议用量的一半而已。」
除了足量涂抹，FDA也建议民众出门前15分钟就该擦防晒，且至少每2小时就要补擦一次。若进行游泳或其他大量出汗的活动，应该使用防水型防晒用品，并且每2小时在干燥的皮肤上重新涂抹，才能确实抵抗紫外线的伤害。
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