Everyone has that friend who applies the highest SPF they can find anytime they step outside. While you can’t fault them for practicing sun safety, it’s normal to wonder if wearing SPF 100 is really necessary — especially since it feels like every summer there’s conflicting advice on what level of SPF is sufficient enough. But a study done in 2018 and published in the Journal of the American Academy of Dermatology found that "SPF 100+ sunscreen was significantly more effective in protecting against sunburn than SPF 50+ sunscreen in actual use conditions."Even with this knowledge, it’s still a heated topic among dermatologists. One of the most important things to know before buying sunscreen is how the number rating system works. The reason? If you have a clear understanding of how sunscreen protects your skin, then you’ll have more success in keeping sun damage at bay. Below, Women's Health rounded up your most burning questions to help you understand proper sunscreen practices and whether or not you should have SPF 100 in your beach bag:The SPF number indicates how long you can stay in the sun without getting burned.SPF, short for sun protection factor, is a term that hit the skincare scene in the early 1960s as a way to measure a sunscreen’s effects against UVB light. “For example, if you typically burn after 15 minutes of sun exposure, applying an SPF of 50 means that it will take 50 times longer for you to burn in the sun," explains dermatologist Dr. Orit Markowitz, the Director of Pigmented Lesions and Skin Cancer at Mount Sinai. To break it down even further, SPF 15 is said to block 93% of UVB rays whereas SPF 100 blocks around 99%. While that 6% difference doesn’t seem too drastic, it's the difference between the amount of rays that are getting through over time that can add up. For example, SPF 15 still lets 7% of UVB rays through while SPF 100 only lets 1% through, meaning your skin is being exposed to seven times the amount of UVB rays if you use a lesser SPF number. “And if you extend that over a lifetime, that's an enormous amount of damage,” says dermatologist Dr. Steven Q. Wang, chair of The Skin Cancer Foundation’s Photobiology Committee and director of dermatologic surgery and dermatology at Memorial Sloan Kettering Cancer.It is not true that a higher SPF number is better.While a higher SPF does block more rays, it doesn’t necessarily mean it’s a better option. “Choosing between applying a chemical vs. a physical sunscreen, the amount of water resistance and the thickness of the sunscreen itself are much more indicative of the type and length of protection you can expect from a sunscreen product,” says Dr. Markowitz.Another issue people gain when choosing a high SPF is a false sense of security. “When people use SPF 100 they think they're invincible — that they can just stay in the sun for much, much longer time. I’ve also found that people don't put on enough sunscreen, so the actual SPF is really half the real one,” explains Dr. Wong. “You don't want to give people the false sense of security and direct them to risk seeking behavior, so this is one bad aspect of using really high SPF product.” No matter the SPF number, aim to reapply every two hours – and do multiple layers. While it depends on environmental factors like how hot and humid it is, along with other variables like sweating or water exposure, aim to reapply SPF every two hours (or any time after getting wet). It’s also crucial that you’re applying enough sunscreen. “Some practical advice is that when you put sunscreen on it should be like painting a house, so you should do two coats of paint,” says Dr. Wong. “Put the first layer of sunscreen on to cover your entire body and then repeat. Not only will it ensure you applied enough, it'll also ensure you catch any spots you may have missed the first time." Bottom line: A higher SPF number lasts longer.Even though you don’t have to have a triple digit SPF number, at the end of the day, it’s better to lather up with a formula that has higher protection since it’ll provide longer lasting results.