1. Antiaging Tips & Secrets Slideshow Pictures
2. Take the Sun Safety Quiz!
3. Take the Skin Quiz

Medical Author:

Melissa Conrad Stöppler, MD

Melissa Conrad Stöppler, MD

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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In this Article

• What is sun protection?
• How is sunburn best prevented?
• What is sunscreen?
• What is meant by SPF?
• Are all sunscreens equally effective against UV radiation?
• How do sunscreens work, and which sunscreen ingredients protect against both types of UV radiation?
• How should skin sunscreens be applied?
• Do water or perspiration wash off sunscreen?
• Can sunscreens cause a skin reaction?
• Should everyone use sunscreen protection?
• Can the labels on sunscreen products be trusted in the U.S.?
• Do all tanning products contain sunscreens?
• What kind of sunglasses should be worn?
• Is sunscreen protection necessary in the winter?
• Are a good sunscreen and sunglasses enough?
• Sun Safety FAQs
• Find a local Dermatologist in your town
• Sun Protection and Sunscreens Index

SPF, an abbreviation for sun-protection factor, is a number such as 15, 30, or 50 that indicates the degree of sunburn protection provided by sunscreens. SPF is related to the total amount of sun exposure rather than simply the length of sun exposure. It is a common mistake to assume that the duration of effectiveness of a sunscreen can be calculated simply by multiplying the SPF by the length of time it takes for him or her to suffer a burn without sunscreen, because the amount of sun exposure a person receives is dependent upon more than just the length of time spent in the sun. The amount of sun exposure depends upon a number of factors including the length of exposure, time of day, geographic location, and weather conditions.

A common mistake is applying too little sunscreen, which can drastically reduce the effective SPF of the product. About 1 ounce (5-6 teaspoons) of sunscreen is recommended to cover the entire body. Further, sunscreen must be reapplied every two hours when staying outdoors for a prolonged period of time. Sunscreen should also be applied at least 30 minutes before going outdoors.

People with sensitive skin who burn quickly and must spend a lot of time outdoors should always apply a broad-spectrum sunscreen with an SPF of 30 or more. The U.S. FDA is also considering a ruling that would limit the maximum SPF value on sunscreen labels to "50 +" because many scientists believe that there is insufficient evidence to show that products with SPF values higher than 50 provide greater protection for users than products with SPF values of 50.

A major limitation of the SPF value is that these numbers are determined from a test that measures protection against sunburn caused by ultraviolet B (UVB) radiation. Therefore, SPF values only indicate a sunscreen's UVB protection and do not provide any information on the product's effectiveness in blocking the ultraviolet A (UVA) rays that contribute to the development of skin cancers. New rulings issued in 2011 (see below) will require more accurate labeling of sunscreen products.