How Does Sunscreen Work, and Should You Use It?

Mother applying sun protection sunscreen on her baby girl's face.One of my missions is to get more people enjoying the great outdoors without fearing the sun. We should respect the sun, sure. Sunlight is a powerful agent that, like so many enjoyable and beneficial things in life, can become harmful in excessive doses. But the sun is also a great life force, powering every living thing on the surface of this planet.

As you may know, I’m not a huge fan of sunscreen. I just don’t think it’s all that necessary. If you’ve had enough sun for one day, and you’re worried about burning up, using physical barriers like shirts, hats, umbrellas, and shade trees to impede the sunlight is my preferred course of action.

Our ancestors used various methods to protect themselves from blistering sun rays, but modern sunscreens were only invented in the last century. Since then people have become obsessed, with the encouragement of doctors, to slather their skin with powerful chemicals every two hours to avoid even a whiff of color, even as deadly skin cancer rates have risen since the mid-twentieth century.

So no, I’m not on the side of Big Sunscreen. I certainly avoid the chemical compounds that most commercial sunscreens contain. These chemicals act as carcinogens, at least in animal models, and harm the oceans’ ecosystems.

Still, in the event that the only thing standing between you and a second-degree sunburn is the application of some sunscreen, that’s an obvious choice. You should always opt for safer sunscreen ingredients, though. That’s what we’re talking about today.

How Does Sunscreen Work?

Mineral vs. chemical sunscreen

The sunscreens you’ll see on your supermarket or pharmacy shelves work in one of two ways.

Chemical sunscreens contain chemicals that are absorbed into the skin. When UVA and UVB rays hit the skin, they react with these chemicals and dissipate as heat. Common chemical sunscreen active ingredients are oxybenzone, avobenzone, octocrylene, octisalate, and homosalate.

Mineral sunscreens sit on top of the skin and provide a barrier using zinc oxide or titanium dioxide to block UVA and UVB rays from penetrating the skin. That’s why these are also called physical sunscreens.

Pros and cons of chemical sunscreens (mostly cons)

The only good thing I have to say about chemical sunscreens is ease of use. They go on smoothly and are usually clear on the skin. That’s not enough to weigh all the cons in my book.

Con: endocrine disrupting UV filters

Most of your typical commercial sunscreens use chemical UV filters like benzophenone and oxybenzone that in addition to blocking UV possess a hidden feature: endocrine (hormone) disruption.

Certain forms of benzophenone, for example, inhibit the action of thyroid peroxidase, an enzyme necessary for the production of thyroid hormone. Another study showed that applying sunscreen containing benzophenone-2 for five days lowered T4 and T3 thyroid hormones in rats.
Other researchers applied a UV filter called octyl-methoxycinnamate to rat skin and found that amounts typically present in sunscreen were enough to disrupt hormonal function and exert other, non-endocrine health effects.

That might not be a problem if UV filters in sunscreen weren’t designed to be absorbed into the skin, and therefore the body, but they are. The only way the chemical sunscreens work is if they are absorbed into the skin—and into systemic circulation. Two studies in 2019 and 2020 demonstrated that common chemical sunscreen ingredients are readily absorbed into the body. More worryingly, even after a single application, these chemicals were present at levels higher than the FDA’s limit for requiring additional safety testing.

Con: imbalanced UV protection

Another downside of chemical sunscreens is that they’re selective screeners. They tend to block UVB while allowing UVA passage.

UVB rays penetrate the epidermis, the upper layers of our skin, and trigger vitamin D production. UVA rays, on the other hand, penetrate more deeply into the basal section of the dermis, which is where most skin cancer develops. Excessive UVA exposure is also associated with wrinkling, immune suppression, oxidative stress, and related aging. Research shows that concurrent exposure to UVB actually serves to counteract skin damage and inflammation from UVA. We need both together. Blocking one while exposing our skin to the other is a recipe for danger.

Con: environmental concerns

There’s clear evidence that chemical sunscreen ingredients are damaging to coral reefs and sea life. This is such a concern that Hawaii, the U.S. Virgin Islands, Aruba, and other countries have banned chemical sunscreens in their waters. Only mineral sunscreens are allowed. Other regions are sure to follow suit, so if you have a seaside vacation planned, look for non-chemical sunscreens with the Protect Land + Sea Certification seal.

Pros and cons of mineral sunscreens (mostly pros)

Physical sunscreens contain either zinc oxide or titanium dioxide. These are the only sunscreen active ingredients recognized as safe by the FDA. The active ingredients in chemical sunscreens are permitted for use by the FDA, but as of 2019, the agency admits that there isn’t enough evidence to give them the GRASE (generally regarded as safe and effective) stamp of approval. Yikes.

Mineral sunscreens are also broad spectrum: they block both UVA and UVB. Because they don’t dissipate UV rays as heat, they are better for people with skin conditions that can be exacerbated by heat. They’re generally less irritating for those with sensitive skin, too.

The biggest con to physical sunscreens is that they can be unsightly because they don’t absorb into your skin, often leaving a ghostly white cast that people don’t like. On the plus side, if you can see the sunscreen on your skin, you know it’s still working, unlike chemical sunscreens that may rub or wash off without you noticing. Manufacturers have also started to develop better formulations, including clear and tinted versions that are better suited for darker skin tones.

Physical sunscreen in nanoparticle form does rub in, but there’s conflicting evidence about the degree to which it’s absorbed and whether it matters from a health perspective. I wouldn’t want to inhale them in any case.

Zinc oxide and titanium dioxide nanoparticles are also not considered reef safe, so read labels carefully if you’re headed to the beach. Choose mineral sunscreens with regular, non-nano, active ingredients.

Sunscreen ingredients to avoid

Look at both the active and inactive ingredients lists on the back of the bottle or tube. If you see any of these, just say no.

  • Oxybenzone
  • Avobenzone
  • Octocrylene
  • Benzophenone
  • Homosalate
  • Octisalate
  • Octinoxate
  • Cinoxate
  • Dioxybenzone
  • Ensulizole
  • Meradimate
  • Padimate O
  • Sulisobenzone

PABA (aminobenzoic acid) and trolamine salicylate have been banned by the FDA, but you might see them in sunscreens if you’re traveling internationally. Strictly avoid these.

There are two other ingredient categories to avoid:


Parabens are ubiquitous preservatives used in cosmetic and skincare products, including sunscreens. They show up in our urine because humans can readily absorb parabens from topical application. Although the health effects haven’t been explicitly proven, human studies suggest a link between urinary paraben levels and certain health conditions, such as sensitivities to airborne and food allergies, elevated stress hormones in pregnant mothers and their newborn children (who, by the way, are showing up with parabens in their first urine!), and DNA damage to sperm.


Vitamin A in the diet is protective against sun damage, so manufacturers figured they’d start putting it in topical sunscreens. Except a 2012 study in hairless mice found that applying retinyl palmitate to bare skin and exposing it to UV increased tumor incidence and skin damage. Now, humans aren’t hairless mice, and the results from the 2012 paper may not apply to us. But even if retinyl palmitate isn’t carcinogenic, it’s useless. Avoid sunscreens containing retinyl palmitate, retinol, or vitamin A just to be safe.

Sunscreen Best Practices

Here’s what I recommend when it comes to enjoying the benefits of the sun while also protecting yourself against the harms of overexposure.

1. Use sunscreen alternatives first.

Given the option, I’ll always go for hats, clothing, and shade first when I feel myself baking. Lightweight wool garments are surprisingly suited to warm and cold environments alike and provide good sun protection.

2. Opt for zinc oxide or titanium dioxide sunscreens.

SPF 30 is probably as high as you need. There’s no harm in going up to SPF 50, but there’s also no benefit to going higher.

3. Apply wisely.

Don’t forget the backs of your hands, tops of your ears, and your part or areas of thinning hair if you’re not wearing a hat. Mineral sunscreens generally have good staying power, but reapply as needed.

4. Toss expired sunscreen.

The active ingredients can break down, and there’s no point in dousing yourself with stuff that isn’t even effective.

5. Don’t neglect the inside-out factors.

Sun protection isn’t just a surface-level issue. It’s always good to be proactive and to take steps to make your body more resilient against any potential damage. Consuming a Primal diet rich in antioxidants bolsters your skin’s ability to fight free radical formation.

I hear all the time from folks who go Primal and find themselves less prone to sunburns than they were before. I can’t say for sure what’s going on here, but my hunch is that it has something to do with the link between chronic inflammation and skin damage. A healthy Primal lifestyle probably factors in here, too. Our ability to repair UV-derived damage depends on a well-functioning circadian rhythm. Sleeping well and maintaining a good eating schedule both entrain your circadian rhythm appropriately.

Whatever it is, I’ve seen it often enough to believe that there’s something to this phenomenon. I spend tons of time in the sun and know for a fact that I am less likely to burn now than I was when I was younger. I’m still smart about my sun exposure. No hubris here. I know that in a battle of me versus one of the most powerful forces in the universe, I’m no match when it comes down to it.

But I also don’t fear the sun like it seems so many medical agencies and doctors want me to. I want all the vitamin D I can get. I rely on time outside on my bike or paddle board, hiking with Carrie, or just reading poolside to balance the unavoidable stresses of modern life. The sun recharges me, and I have no intention of avoiding it.

What about you? What steps do you take to make sure your fun in the sun doesn’t leave you burned?


About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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