These days, many people incorporate sunblock into their everyday skin care routines (public awareness around skin cancer prevention has gotten pretty good, after all). But are we wearing enough of it? Or, alternatively, is it giving us all cancer, as some rumors would suggest?

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To set the record straight on the effectiveness of SPF, we turned to the experts and asked them to dispel some myths.

Here’s how sunscreen works.

Sunlight produces two types of that we have to protect against: UVA, which penetrates to the middle layer of the skin, and UVB, which reaches the outer layer of the skin.

There are two types of sunscreens. Chemical sunscreens include active ingredients such as oxybenzone, avobenzone, octisalate, octocrylene, homosalate, and octinoxate, and work by absorbing UV rays’ chance of penetrating the skin and changing UV energy into imperceptible heat, says , M.D., a dermatologist at Skin Associates of South Florida and the University of Miami.

Physical (or mineral) sunscreens, on the other hand, block UV rays from penetrating the skin. These include active ingredients such as zinc oxide or titanium dioxide.

What exactly do those SPF numbers mean, anyway?

"The sun protection factor [SPF] is a measure of the fraction of sunburn-producing UV rays that reach the skin," says , M.D., Ph.D., associate professor of dermatology at the University of Pittsburgh. "For example, SPF 15 means that one-fifteenth of the burning radiation will reach the skin."

A certain SPF number won’t work the same for everyone—it depends on you are to burn. If your skin begins to burn after 10 minutes in full sun without any protection, a sunscreen of SPF 15 would provide 15 times the protection of no sunscreen.

A pretty woman rubbing suntan lotion into her legs SPF only indicates how much protection you're getting from UVB rays, since they're what cause you to burn. The only way to know a sunscreen also protects against UVA rays is if the label says "broad spectrum," which means it protects against both.

How much sunscreen is enough sunscreen?

Studies show most people don’t apply enough sunscreen to achieve the full SPF of the product they’re using. Neale R, Williams G, Green A. Archives of dermatology, 2002, Nov.;138(10):0003-987X. You need at least a shot glass full of sunscreen for the whole body, Ferris says, following a recommendation by the . Sunscreen should be applied liberally, all over exposed skin.

For the face, you need about a nickel-size amount. While some brands recommend a pea-size amount, that likely isn’t enough, Ferris says. "A pea-size amount would likely not cover the face, particularly because you need to also cover the neck and ears," she says. "Don't forget the ears, feet, hairline, and hard-to-reach areas on the back and along bathing suit straps."

If you don’t wear enough sunscreen, you’re likely to burn more quickly and get more sun damage.

"SPF 70, for example, at half the recommended amount, will still give you SPF 35, but SPF 15 at half the recommended amount will only be SPF 7.5, which won't give adequate protection," Ferris says.

You need at least SPF 30 for daily use.

"And SPF 50 if you are going to be outdoors for a prolonged period of time or at the pool or beach," Ferris says.

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But when it comes to increasing SPF numbers, there's a point of diminishing returns. SPF 15 filters out of all incoming UVB rays, SPF 30 keeps out 97 percent, and SPF 50 keeps out 98 percent. But in order for these percentages to remain accurate, you need to reapply.

One application is not going to last all day.

That’s because sunscreen (like makeup) rubs off with water, sweat, and touch. Dermatologists recommend reapplying at least every two hours or after swimming or sweating. This step is important. "Not only will it wash off, it actually breaks down in sunlight and becomes ineffective," Ferris says.

But what about those "toxic" ingredients in sunscreens?

"There is an undercurrent of concern about sunscreens that doesn't yet have adequate basis," Dosal says. But the truth is there are few, if any, real risks to using sunscreen; in fact, studies show both chemical and physical sunscreens are . So let’s tackle the sunscreen-related concerns we’ve seen pop up on health blogs recently.

Sunscreen Myth No. 1: Sunscreen usage leads to vitamin D deficiency.

These concerns appear to be unfounded. A 2017 study showed short-term sunscreen usage doesn’t affect circulating vitamin D levels and therefore does not increase the risk for osteoporosis. Libon F, Courtois J, Le Goff C. Archives of osteoporosis, 2017, Jul.;12(1):1862-3514.

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"You don't put yourself at risk of vitamin D deficiency if you wear sunscreen," Ferris says. "In one large study, the amount of vitamin D in the bloodstream was no different in people who reported high versus low rates of sunscreen use. Also, vitamin D supplements are cheap and safe."

Sunscreen Myth No. 2: Active ingredients in chemical sunscreens are carcinogenic.

Dosal says this a common misinterpretation of an ingredient found in many sunscreens: retinyl palmitate. "At lower doses, RP in mice prevented cancer, which is why we use an oral form of RP to prevent cancer in high risk patients," she says. "But in very high doses—way higher than anything we see in real life—the mice had an increased risk of tumors. Dose is the issue, so we can't make blanket statements about certain ingredients."

A lead researcher in that study actually there is no evidence supporting the idea that sunscreen causes cancer.

"A glass of red wine a day may have cardioprotective effects, while eight a day is going to damage your liver," Dosal says.

Another study (with humans, not rats) indicates that RP is photoprotective. Serri R, Iorizzo M. Clinics in dermatology, 2009, May.;26(6):0738-081X. "Again, we see a case where the animal studies raise the concern, but the human studies are being ignored," Dosal says. "I think it’s sexier to be alarmist. As far as carcinogens, the only thing people need to be worried about is UV radiation, which is a known carcinogen."

Sunscreen Myth No. 3: Oxybenzone causes hormone disruption.

"Concerns over oxybenzone stem from where rats were fed the sunscreen ingredient and subsequently had some endocrine dysfunction, but the with real-life application showed no hormone disruption," Dosal says. "Yet you consistently find on the internet that your chemical sunscreen will cause hormone dysregulation, which simply hasn't been shown. We definitely need more studies to prove that [normal, topical use of oxybenzone is harmless], but to date, the data is very good."

Sunscreen Maybe-Not-a-Myth No. 4: Sunscreen is damaging the world’s coral reefs.

The verdict seems to still be out on this one. In a , researchers found that oxybenzone is killing the coral and that the highest concentrations of oxybenzone were found in reefs most popular with tourists.

"Oxybenzone is damaging our coral reefs, so that may be a good reason to avoid it," Dosal says. But the study was conclusive, and we know other factors are of greater danger to the reef. However, if you’re concerned, just opt for any of the numerous derm-approved sunscreens that don’t contain the ingredient. these physical sunscreens: , , , , and .

The bottom line: Chemical and mineral sunscreens are both safe.

Both chemical and mineral sunscreens are effective in blocking UV rays, but Ferris recommends a sunblock that’s broad spectrum and uses both chemical and physical actives. "Combination products are usually best, as you want to cover the full spectrum of UVA and UVB and also have a product that is stable enough to give protection over several hours," she says.

The biggest problem might be choosing the right sunscreen that doesn’t leave a white cast or a greasy residue.

The two types are . For a (usually) odorless, colorless, thin sunscreen, go the chemical route, but note that these can cause irritation or stinging and take about 20 minutes to start working. On the other hand, physical sunscreens tend to leave a white cast (which isn’t pretty if your skin tone isn’t very light), but you don’t have to wait at all between applying and safely going outdoors.

"Mineral sunscreens tend to be a little thicker, and need more rubbing in to avoid the whitish discoloration, while chemical sunscreens are often more cosmetically elegant," Dosal says. "A test showed the mineral sunscreens fared a little worse when it came to being waterproof and providing protection after being in the water."

As long as you’re applying enough, research indicates you don’t have much to be scared of when it comes to sunscreen safety. Unlike many other cosmetics, rest assured "sunscreens do undergo evaluation by the FDA for safety," Ferris says.

Julissa Treviño is a writer and journalist currently based in Mexico. Her work has appeared in Columbia Journalism Review, The Dallas Morning News, Racked, Man Repeller, Teen Vogue, and CityLab. Follow her .

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