Summarize the following:
“SPF” refers to a sunscreen’s “sun protective factor,” or how effectively it blocks UVB rays. The SPF number reflects the amount of time it takes to sunburn wearing the sunscreen vs. not wearing the sunscreen.  For example, an SPF of 30 means that you can spend 30 times as long in the sun before burning compared to not wearing any sunscreen at all. So, if you would usually begin to burn after 5 minutes in the sun, an SPF of 30 would theoretically allow you to spend time outside for 150 minutes (30 x 5) before you burn. However, your unique skin, your activities, and the sun’s intensity all cause variation in how effective sunscreen is, so you may need to use more than other people. The SPF number can be tricky, because its protection doesn’t increase proportionally. Thus, SPF 60 is not twice as good as SPF 30. SPF 15 blocks about 94% of UVB rays, SPF 30 blocks about 97%, and SPF 45 blocks about 98%. No sunscreen blocks 100% of UVB rays.  The American Academy of Dermatology recommends an SPF of 30 or higher. The difference between extremely high SPFs is often negligible and not worth the extra money. SPF refers only to the ability to block UVB rays, which cause sunburn. However, the sun also emits UVA rays. UVA rays cause skin damage, such as signs of aging, wrinkles and dark or light spots. Both increase your risk of skin cancer. A broad-spectrum sunscreen provides protection from both UVA and UVB rays.  Some sunscreens may not say “broad-spectrum” on the packaging. However, they should always state whether they protect against UVB and UVA rays. Most broad-spectrum sunscreens contain “inorganic” components such as titanium dioxide or zinc oxide, as well as “organic” sunscreen components such as avobenzone, Cinoxate, oxybenzone, or octyl methoxycinnamate. Because your body expels water through sweat, you should look for a water-resistant sunscreen. This is especially important if you are going to be very active, such as running or hiking, or if you’ll be in the water.  No sunscreen is “waterproof” or “sweat proof.” In the US, sunscreens cannot market themselves as “waterproof.”  Even with water-resistant sunscreen, reapply every 40-80 minutes or as directed on the label. Some people prefer spray sunscreens, while others prefer thick creams or gels. Whatever you decide, make sure you apply a thick, even coating. The application is as important as the SPF and other factors: if you don’t apply it properly, the sunscreen won’t do its job.  Sprays may be best for hairy areas, while creams are usually best for dry skin. Alcohol or gel sunscreens are good for oily skin.  You can also purchase wax sunscreen sticks, which are good for applying near the eyes. This is often a good choice for children, as it avoids getting the sunscreen in the eyes. They also have the benefit of not spilling (such as in a purse) and can be applied without getting lotion on your hands. Water resistant "sports type" sunscreens are often sticky, so they are not good choices for applying under makeup.  For acne-prone individual, take care in selecting your sunscreen. Look for those that are designed specifically for your face and will not clog pores. These often have higher SPF (15 or higher), and are less likely to clog pores or increase acne breakouts.  Many acne-prone individuals find that zinc oxide-based sunscreens tend to work best. Look for "non-comedogenic", "will not clog pores", "for sensitive skin", or "for acne-prone skin" on the labels. If you see any allergic reaction or skin problem, buy a different sunscreen. Repeat the process until you find the right sunscreen, or talk to your doctor about recommended brands if you have sensitive skin or allergies. Itching, redness, burning, or blisters are all signs of an allergic reaction. Titanium dioxide and zinc oxide are less likely to cause allergic skin reactions.

Summary:
Look at the SPF number. Choose a “broad-spectrum” sunscreen. Look for a water-resistant sunscreen. Decide what you like. Go home and try a small portion around your wrist.