You’ve probably already been told how important it is to use sunscreen every single day. But, as summer is in full swing, and vacations, pool days and beach trips lead to more time in the sun, your sunscreen knowledge is being put to the test. This time of year, there’s no shortage of sunscreen on the shelves. With so many options, what kind should you be buying? Is SPF 100 a waste of money? And do you really need to reapply it every two hours? Keep reading for the answers to these and other commonly asked sunscreen questions.

What does SPF stand for and what does it protect against?

SPF, or sun protection factor, measures protection against the sun’s UVB rays. It does not measure protection against UVA rays. While UVA rays don’t cause a sunburn, they do penetrate the skin more deeply than UVB rays and can lead to darkening, wrinkles and an increased risk of skin cancer. Sunscreens that are “broad-spectrum” offer both UVA and UVB protection.

Does a higher SPF mean more protection?

According to TIME magazine, SPF 15 filters about 93 percent of UVB rays, SPF 30 filters about 97 percent and SPF 50 filters about 98 percent. With only a 1 percent filtering improvement between SPF 30 and SPF 50, a higher SPF number does not mean it is significantly more effective. TIME says that dermatologists typically recommend SPF 30 because it is “readily available at many different price points.”

However, a recent article published by the Associated Press says Consumer Reports found many sunscreens did not meet their SPF claims and recommends using SPF 40 or 50 to ensure getting at least SPF 30.

Sunscreens labelled as higher than SPF 50 are not known to protect against more than 98 percent of UVB rays and should be avoided, as the higher label could lead you to believe you can stay out in the sun longer.

What ingredients should I look for?

An easy way to pick a sunscreen based on the safety of its ingredients is to check out the Environmental Working Group’s annual guide. It reviews products based on their labels and gives a toxicity rating. The organization gives their picks for the best and worst sunscreens.

How much should I be applying and how often?

An ounce (about 2 tablespoons) of SPF 30-50 should be applied on exposed skin at least every 2 hours--more if swimming or sweating a lot, and even if the label says “water-resistant.” Dr. Darrel Rigel of the American Academy of Dermatology’s sunscreen regulations task force says spray sunscreens can be tricky. Two coats should be applied and then rubbed in for even coverage.

Is sunscreen enough to prevent skin cancer?

Just using sunscreen is not enough to prevent skin cancer. Sunscreen should be paired with other skin protecting measures, such as avoiding direct sun between 10 a.m. and 2 p.m. and wearing protective clothing.