There are many types of sunscreen on the market with many different symbols.

Many American adults may be struggling to understand sunscreen labels, according to a small 2015 survey published in the journal JAMA Dermatology. Researchers surveyed 114 participants and found that only 43% understood the definition of “SPF,” while only 22.8% of participants correctly identified the term that indicates how well a sunscreen protects against sunburn.

To help you avoid similar confusion, here’s a comprehensive guide to understanding sunscreen labels.

What is a water-resistant sunscreen?

According to the U.S. Food and Drug Administration (FDA), no sunscreen is completely “waterproof.” Under current FDA regulations, sunscreens that may have been labeled water-resistant or sweat-resistant are now labeled as “water-resistant” for 40 or 80 minutes, meaning they remain effective for up to 40 or 80 minutes of use.

Dr. Adam Friedman, director of dermatological research at Montefiore Medical Center in New York, says water-resistant sunscreens are products that only protect the skin from the sun for a limited period of time when a person is exposed to water or sweat.

“Choose a sunscreen that is water-resistant for 80 minutes and reapply after getting out of the pool, ocean, or even after a workout,” Friedman says.

What is a broad-spectrum sunscreen?

Sunburn is damage to the outer layer of the skin caused by ultraviolet rays: ultraviolet A (UVA) and ultraviolet B (UVB). These are types of light that fall between visible light and X-rays on the electromagnetic spectrum, and both types of radiation can be harmful.

UVB rays can cause redness and sunburn, while UVA rays can penetrate deeper into the skin and accelerate skin aging. Both types of skin damage increase the risk of skin cancer. Dr Paul Banwell, a cosmetic surgeon and founder of the Skin Cancer and Oncology Unit (MASCU) in East Grinstead, UK, says the label “broad spectrum” indicates that the sunscreen protects against both UVA and UVB rays.

What is SPF?

Apply sunscreen to baby

According to the FDA, the sun protection factor (SPF) indicates how much sun protection a product provides. Note that this measure is only for UVB rays, and there is no specific rating for UVA rays. (Again, a sunscreen’s “broad spectrum” label is what confirms that it protects against both UVA and UVB rays.)

The SPF value indicates the level of UVB exposure needed to cause a sunburn when using a sunscreen compared to the level of UVB exposure that would cause a sunburn without any skin protection.

“So if you apply a sunscreen with an SPF of 50, you can expect your skin to burn 50 times longer than if you were wearing nothing. An SPF of 15 protects against 93% of UVB rays, while an SPF of 50 protects against 98% of UVB rays,” Banwell said in an email. “Higher SPF sunscreens will block slightly more UVB rays than lower SPF sunscreens, but no sunscreen will provide 100% protection,” he says. “It’s also important to note that a higher SPF doesn’t mean the product can be reapplied less often,” he adds.

Under current FDA regulations, products with SPF 2 to SPF 14 or that aren’t broad spectrum must carry a warning label stating that they have been shown to help prevent sunburn but do not protect against skin cancer or premature aging.

To get the best possible protection, “generously apply” about a shot glass’s worth of broad-spectrum sunscreen to your entire body, covering all exposed skin; Friedman recommends that sunscreen have an SPF of at least 30 to 50. Do this about 15 to 20 minutes before sun exposure and every two hours thereafter, he says.

What is the difference between chemical sunscreen and mineral sunscreen?

The American Academy of Pediatrics says that for babies under 6 months old, sunscreen should be applied to small areas of the body, such as the face, if protective clothing is not available. For children 6 months and older, sunscreen can be applied to all exposed skin on the body.

There are two main types of sunscreens on the market: chemical sunscreens and mineral sunscreens. Chemical sunscreens, also known as organic or synthetic sunscreens, work by absorbing UV rays and converting them into heat that can be released from the skin. Physical sunscreens, also known as inorganic or mineral sunscreens, work by clinging to the skin to form a shield. Banwell says physical sunscreens effectively scatter and reflect harmful UV rays away from the skin.