With summer on the horizon, we know it’s time to slather on the sunscreen and reapply, but how much do we really know about this magical lotion that’s said to protect against sunburn and prevent skin cancer? For all the mysteries that still remain about the sun and how it affects us, we can rest assured knowing that we have these 10 facts about sunscreen covered:

1. There is No Evidence that the Chemicals in Sunscreen are Not Safe: Despite ongoing controversy over the safety of the chemicals in sunscreens, most scientists and doctors agree that there is no evidence that sunscreen ingredients are harmful to humans. The argument that sunscreen ingredients are carcinogenic, block vitamin D or alter the body’s hormone system will require further research and experimental studies before any conclusions are drawn.
2. The Higher the SPF Number, the Smaller the Difference: It’s a common misconception that a higher SPF number means you are doubly or triply protected. A higher number does indicate more protection, but it doesn’t give you two to three times as much protection as one with an SPF 15. Sunscreens with SPF 15 filter out roughly 93 percent of UVB rays and SPF 30 sunscreens filter about 97 percent. The protection slightly increases as the SPF number gets higher, but only by one percent (98) for SPF 50 and two percent (99) for SPF 100 sunscreens. The fact is no one sunscreen will protect you completely.
3. SPF Numbers Only Refer to UVB Ray Protection: The sun protection factor (SPF) number on sunscreens only measures protection against UVB rays, the skin-burning rays. There is no current FDA-approved rating system for measuring protection from UVA rays, which cause aging of the skin. To ensure coverage against UVB and UVA rays, you should use a "broad-spectrum" sunscreen, or one that contains avobenzone (Parsol 1789), ecamsule, zinc oxide or titanium dioxide.
4. There’s a Difference Between Waterproof and Water-Resistant Sunscreen: Sunscreens that are labeled "waterproof" or "water-resistant" are slightly different in their chemical makeup and water tolerance. Water-resistant sunscreens can maintain their SPF level after 40 minutes of water exposure, and waterproof sunscreens can maintain their SPF level after 80 minutes of water exposure, according to the FDA. If you’re plan on being in the water or participating in outdoor activities, you should choose a water-resistant or waterproof sunscreen for optimal protection.
5. Sunscreen Does Not Cause Vitamin D Deficiency: For years, people have blamed sunscreen and dermatologists’ pleas to stay out of the sun as the leading cause for vitamin D deficiency in Americans. However, we now know that there is little to no evidence that shows sunscreens cause vitamin D deficiency, and people can get the recommended amount of vitamin D from other sources than just the sun, such as taking dietary supplements and eating foods like salmon, milk and eggs.
6. Sunscreen is Not Fail-Safe: We now know that sunscreen is not fail-safe because it is not a guaranteed protection against all of the sun’s harmful rays. There is also no supporting evidence that sunscreens protect you from developing malignant melanoma, the deadliest kind of skin cancer. Sunscreen alone will not fully protect you from the sun or from developing skin cancer. In addition to wearing sunscreen, you should also seek shade, wear protective clothing, avoid peak hours of sun exposure and monitor the UV index.
7. One Ounce of Sunscreen is Needed to Cover Your Body: We now know that one ounce, equivalent to a full shot glass, is the recommended amount of sunscreen needed to cover your exposed skin. You should apply sunscreen liberally and reapply every two hours, especially after perspiring, swimming or towel-drying. Sunscreen is something you definitely don’t have to go easy on. Don’t forget to protect the often-missed parts of the body, like the lips, ears, hands, feet, neck and scalp.
8. Everyone Should Use Sunscreen, Regardless of Skin Color: People of all races and ethnicities are at risk for developing skin cancer, and should wear sunscreen to protect themselves from UV radiation. We know that people with fair skin and a large number of freckles and moles have a greater chance of burning and developing skin cancer, but people with darker skin can also burn and develop skin cancer as well. Dark-skinned individuals are also more likely to be diagnosed with skin cancer in the later stages when it is more dangerous and could be fatal.
9. Sunscreen is Needed in All Types of Weather: Whether it’s sunny, cloudy or snowing outside, you still need sunscreen to stay protected all year long. Ultraviolet rays can do a significant amount of damage even when the sun is not at its hottest. According to the Skin Cancer Foundation, people experience some of the severest sunburns because they did not protect themselves on cloudy days, where up to 40 percent of the sun’s radiation can get through. So, whether you’re hitting the slopes or riding the waves, you need to wear sunscreen in every season and every type of weather.
10. Sunscreens Have a Maximum Shelf Life of Three Years: According to the FDA, all sunscreens have to be stable at their original strength for at least three years, unless otherwise indicated by an expiration date. Sunscreens lose their effectiveness after three years, especially when the bottle is exposed to direct sunlight, extreme changes in temperature or left open. Sunscreen is said to have a shelf life of three years. However, if you’re using the appropriate amount of sunscreen daily, you shouldn’t have bottles of sunscreen sitting around for more than one year!