Have you ever been in the store wondering which sunscreen to buy? You wondered what the devil does SPF mean? You’re not alone. This guide to sunscreen and SPF will teach you all about sun safety and make you better prepared.

Dermatologists recommend daily use of a broad-spectrum sunscreen with an SPF of at least 15 to protect against damaging ultraviolet (UV) rays that cause sunburn, increase your risk of skin cancer and cataracts, and suppress your immune system. Some people say that an SPF of 15 allows you to stay in the sun 15 times longer than normal without getting burned, or that an SPF of 30 is twice as effective as an SPF of 60. Both explanations are wrong, so what does SPF really mean?

Types

The sun produces three kinds of UV rays: A, B and C. The earth’s atmosphere blocks out all the C-type radiation. The atmosphere does not stop type-A rays, which penetrate deep into the skin and cause damage. The atmosphere can block some of the burning type-B rays, but not all.

How They Work

Sunscreens form a barrier between the sun’s energy and your skin. Some of the ingredients reflect rays, and others absorb them, cutting back the amount of dangerous energy that reaches your skin. It takes time for this barrier to sink in—so you should apply sunscreen about 20-30 minutes before going outside.

What is SPF?

SPF is an indication of effectiveness in blocking burning UVB rays. It stands for Sun Protection Factor. It’s a relative scale: SPF 30 blocks more UVB rays than SPF 15, but not twice as many. The SPF number does not measure UVA-blocking effectiveness at all. To make sure you are protecting yourself from both types of UV rays, be sure you buy a product labeled “Broad Spectrum.”

When to Wear Sunscreen

The American Academy of Dermatology recommends using sunscreen every day on exposed skin. If you find some types of sunscreens irritating, keep trying different ones until you find one that works.

How much to use

The American Academy of Dermatology estimates that most people only use about 25-50% of the recommended amount of sunscreen. An “average-sized” person should use one ounce of sunscreen to cover their entire body. If you buy an eight-ounce tube of sunscreen, you should use one-eighth of the container for each application.

Cover all exposed skin with sunscreen. Have someone help you with the difficult spots— example: back and between the shoulder blades.

Reapplying

According to the Centers for Disease Control and Prevention, you should reapply sunscreen at least every two hours you stay in the sun, and more often if you are sweating or swimming. If you are using the recommended amounts and reapplying as often as you’re supposed to, one bottle should not last long.

Expiration

The chemical components of sunscreen can break down over time and lose effectiveness. Some sunscreens have printed expiration dates. If you buy one without a date, you can write the date of purchase on the bottle: Sunscreens are supposed to last for three years. If the product has obviously changed color, smell or consistency, it’s time to buy new.

No sunscreen can filter out 100% of harmful rays, so look for the shade in the spring and summer, especially during the peak UV hours of 10-4pm. Cover your skin: wear broad-brimmed hats, sunglasses, and lightweight clothing like linen that shades the skin without trapping heat.

Still have questions? Post it in the comments below!

For more information:
The Environmental Protection Agency presents a very helpful guide to sun protection, available online

The CDC has a helpful guide to selecting, applying and using sunscreen

Image by Joe Shlabotnik via Flickr.