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How Does Sunscreen Work?

Ian Fortey by Ian Fortey Updated on September 19, 2022. In Beach

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Cancer researchers, doctors, the media and the government have all been telling us for decades to use sunscreen. By age 70, one in five Americans will have developed some kind of skin cancer. Your chances double if you’ve had just five sunburns at any time in your entire life. That’s because the effects of the sun are cumulative, you can never get over sun exposure and start fresh. Two people die of skin cancer every single hour in America. BUT! If you detect skin cancer early enough the survival rate is nearly 100%. The key to it all is responsible use of sunscreen. So how does sunscreen help? Let’s take a look.

Chemical Sunscreen vs Mineral Sunscreen

Before I get into the science of exactly what the sun does to your skin and how sunscreen prevents it, it’s good to cover what exactly sunscreen is. There are two kinds of sunscreen available on the market today, mineral and chemical. Chemical sunscreen is hands down the most popular because of a handful of reasons.

  • Chemical sunscreens are the most readily available in stores
  • Chemical sunscreen has been around longer on the mass market
  • Chemical sunscreen is cheaper
  • Chemical sunscreen is easier to use

Those are a lot of great reasons to stick with chemical sunscreen. It’s the kind most of us grew up with, things like old school Banana Boat or Coppertone.

Chemical sunscreen works by using chemicals to protect you from the sun’s harmful rays. They absorb UV radiation and UV light. There are a large number of active ingredients that can do this but these are some of the most common chemical sunscreen ingredients:

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

When you put on a chemical sunscreen, the UV rays from the sun still penetrate your skin. The radiation hits these chemicals and undergoes a chemical reaction. It causes the radiation to change state and become heat. That heat then dissipates and your skin radiates it away.

Chemical sunscreens can be very effective at preventing sun damage and this is the way they accomplish it. The wide variety of chemical sunscreens make them really appealing to most people. They can come in all kinds of scents, with moisturizers and antioxidants for sensitive skin, and that makes them a great choice for many people.

How Does Zinc Oxide Work in Sunscreen?

Zinc oxide is an ingredient in the other kind of sunscreen, or more accurately sunblock. Zinc oxide and titanium dioxide are the main minerals used in mineral sunscreens. These are also call physical sunscreens. Unlike chemicals, which absorb into your skin, mineral sunscreens form a physical barrier on your skin. This can be as a cream or a powder sunscreen. The sun’s rays hit the mineral particles and they reflect some of UV rays, so they never reach your skin at all. But contrary to popular belief, zinc oxide also absorbs the UV rays and then converts it to heat in a process called band-gap absorption. But because it remains on top of your skin, the sun can’t penetrate. It’s kind of like putting a shield on top of your skin that can’t be penetrated by the sun’s rays.

Mineral sunscreens are less popular right now than chemical sunscreens and that’s for a couple of reasons.

  • Mineral sunscreens are sometimes more expensive
  • There are usually fewer mineral sunscreen options at stores
  • Mineral sunscreens are thicker and harder to apply
  • Mineral sunscreens often leave a white cast or film on your skin

You can see how mineral sunscreen would be less popular than chemical sunscreens as a result of these potential drawbacks.

Which Sunscreen is Better?

Mineral sunscreen is actually the better option. If used properly, mineral sunscreen is safer for the environment and it offers better protection. A properly applied layer of mineral sunscreen can prevent nearly all of the sun’s rays from even touching you. But, as I often say, any sunscreen is better than none. And many chemical sunscreens are still very good quality and prevent a lot of sun exposure. So it’s not that chemical sunscreens are bad, it’s just that, based on a lot of factors, mineral sunscreens are a little better. Also worth remembering is that tanning oil is designed not to prevent burns but to help you tan. It can increase the effects of the sun.

What Does Reef Safe Mean?

Reef safe sunscreens are ones made without the majority of those chemicals I listed above. Although those chemicals have not been shown to be dangerous to humans, there is evidence they are bad for the environment. Fish and other sea life have begun to absorb them.

  • Coral that becomes oversaturated with the chemicals in sunscreen will bleach and die off.
  • Green algae will absorb these chemicals and it has been shown to interfere with photosynthesis and growth. The algae is a food source for many forms of aquatic life, and it also is a very important part of the producing and balancing levels of oxygen.
  • Chemicals in sunscreens have been shown to have an effect on the fertility rates of some fish species. Even more unusual, some have caused female fish to start developing male characteristics.
  • Creatures like sea urchins and mussels have also been affected by the chemicals. In specific they affect fertility and have led to birth defects in both species.
  • Dolphins have also shown that they are absorbing these chemicals and transferring them over to their young.

Reef safe sunscreens don’t use these sunscreens. Most often they are mineral sunscreens and use non-nano zinc oxide and non-nano titanium dioxide which is much safer for the environment. Some beaches, like those in Hawaii, will not let you in the water unless you have reef safe sunscreen.

What Does SPF Mean?

SPF on a sunscreen stands for Sun Protection Factor. It means that you are able to spend more time in the sun than if you had no protection on your skin at all. The number of SPF indicates how many more times you can safely stay in the sun. Therefore, SPF 15 would mean you can stay 15 times longer than if you had no sunscreen on. SPF 50 means 50 times longer.

Another way to look at it is how much UV radiation gets to your skin. An SPF 30 sunscreen will only allow 3% of UV rays to pass through. That means you are protected from 97% of them, which is pretty good. For SPF 50, just 2% get through and you are protected from 98% of UV rays.

To many people the difference between SPF 30 and SPF 50 seems very small, but it’s a matter of perspective. Think of it this way – SPF 30 lets in 50% more UV rays than SPF 50. A higher number is always better.

What Are UV Rays?

The damaging part of the light from our sun comes in the form of ultraviolet radiation, a kind of electromagnetic radiation, what we call UV rays. These are beyond the visual spectrum of light so we can’t actually see UV rays, but they are definitely there.

UV rays are just one part of the electromagnetic spectrum. Light that we can see is one of the regions, but so is infrared light, radio waves, microwaves, x-rays and finally gamma rays. Gamma radiation is incredibly dangerous and comes from exposure to nuclear materials. X-rays are also dangerous but we usually have controlled exposure to those.

The radiation comes at the earth in different wavelengths and those wavelengths determine what kind of UV rays we’re dealing with. There are actually three kinds of UV rays – UVA, UVB and UVC. UVC rays are filtered out by our atmosphere so we don’t have to worry about those.

What is the Difference Between UVA Rays and UVB Rays?

The two kinds of radiation that can potentially cause skin problems are UVA rays and UVB rays. UVA rays are usually associated with skin aging. They have a longer wavelength and penetrate more deeply into our skin. They also carry some risk of cancer. UVB rays are the ones we are most likely to have trouble with in a noticeable way because UVB is what causes sunburns. The shorter wavelength doesn’t penetrate as deeply but it does cause more immediate damage.

A good sunscreen needs to protect you from both UVA and UVB rays. How do you know if it does? Look for the term “broad spectrum protection” on the label. That means it protects against both kinds of radiation and it’s very important.

If a sunscreen claims to be SPF 50 but it’s not broad spectrum, it will only protect against UVB rays. The UVA rays will still penetrate your skin. So you won’t get a sunburn and it will appear as though the sunscreen has worked well, but you are still at risk from premature aging and also some kinds of skin cancer.

What Happens To Your Skin When Exposed to Sun?

The darker your skin tone the less likely you are to notice the effects of the sun. This is because the pigment in skin, called melanin, is actually a natural sunscreen. You tan in the sun because your body is making more melanin to combat the sun. That is literally what a tan is. It’s not the sun cooking you like the oven cooking a turkey. It’s your body producing the melanin pigment so that it can absorb the UV radiation and turn it into heat, the same way chemical sunscreens do. But it can only do so much. At some point you can overwhelm your body’s ability to react to the sun. That’s when a sun tan becomes a sunburn.

How Does the Sun Cause Skin Cancer?

When your body can no longer combat the exposure to UV radiation, it becomes overwhelmed. The radiation begins to damage the cells and the actual DNA of the calls can mutate as a result. At this point your immune system will try to repair the damage. Blood rushes to the site of the growing sunburn which causes that swelling and inflammation you get.

Your body produces various proteins and enzymes to try to repair the damage. Some of this work will be successful. However, the UV radiation can alter the DNA in some cells in a way that actually allows the cell to grow and repair itself after a fashion. This prevents your body from healing the damaged cell. The damaged cell will continue to grow and this is what cancer is. It can grow and spread and become very dangerous if it is allowed to go for too long.

Every time you are exposed to the sun, the damage to your DNA will increase. It’s a cumulative thing, which is why after many years of exposure you can develop cancer. The DNA in the cell can become so badly damaged that it mutates at a certain point and cancerous skin cells are produced.

The Problems with Sunscreen

If sunscreen is so good at preventing skin cancer, why are there so many cases? There are three problems that you should be aware of.

The first big issue with sunscreen and the most problematic is that a lot of people don’t put sunscreen on at all. Many people will only use sunscreen if they are planning on hitting the beach for the day. But you are exposed to the sun literally every time you go out. You should really be using it all the time. I’ll keep saying it but any sunscreen is better than no sunscreen. So even if you don’t have time or the energy to use a strong mineral sunscreen, put something on.

The second problem with sunscreen is that some of it is really not very good. According to the Environmental Working Group, or EWG, as many as 75% of sunscreens either offer poor protection or have dangerous ingredients. This would include things like SPF 15 and lower sunscreen. Anything that low is just not good enough and therefore is essentially a waste of time and money. Always stick with SPF 30 or higher. Whenever possible, use a mineral sunscreen with zinc oxide or titanium dioxide as these are the ones that rank highest for their ability to prevent skin cancer based on EWG testing.

Finally, another big problem is that people do not use sunscreen properly. How many times have you just squirted some in your hand and rubbed it on? Most of us don’t look to see how much we actually need and that is a problem.

When researchers come up with SPF values, these are done in lab conditions and they follow very rigorous testing rules. To determine how well protected you are from the sun, they use a 2mg per cm squared thick coating of sunscreen. But research has shown that most people don’t use enough sunscreen and, in fact, are only getting about 40% effectiveness out of it. So your SPF 30 is now functioning at an SPF 20. And SPF 15 would be a terrible SPF 6.

If you read online reviews for many mineral sunscreens you’ll see a lot of people complaining that they burned right away. Two things have to be happening here. One, the sunscreen is not exactly what it say it is and it is using inferior ingredients. Or two, the person did not use it properly.

I mentioned earlier that a lot of people don’t like mineral sunscreen because it’s thick and you can see it on your skin. That’s a good way to tell if it’s working. If you can’t see it, it probably isn’t working. Unfortunately, most people don’t want to see it because it does look pretty noticeable. But it is a physical sunscreen and if it is not covering your skin, it can’t work.

There are some mineral sunscreens that have tints or promise “invisible” zinc and they are often better than regular old zinc oxide. But remember, if you rub in the sunscreen too much, you’re not rubbing it in, you’re rubbing it off. Chemical sunscreens can absorb, but mineral can’t. And if you rub it in so much it rubs off, then it won’t protect you at all.

It’s also very important to remember to reapply sunscreen exactly as the directions instruct you to. Many sunscreen brands will recommend you reapply after 80 minutes. You’ll likely also need to do it if you’ve been sweating a lot, or if you went swimming. If you wait 90 minutes or 100 minutes, that’s 10 or 20 minutes of sun exposure. UV radiation can become dangerous in as little as 5 minutes. So if you miss by even 20 minutes, you could already be developing a sunburn.

The Bottom Line

Remember, any sunscreen is better than no sunscreen. Whether you’re using chemicals to absorb uv rays or a mineral to block it, you need to protect your skin. A good rule of thumb is to ensure you have polarized sunglasses to protect your eyes as well. Wear protective clothing and a good sun hat.

It’s still possible to enjoy the sun even when you are staying sun safe.

About Ian

My grandfather first took me fishing when I was too young to actually hold up a rod on my own. As an avid camper, hiker, and nature enthusiast I'm always looking for a new adventure.

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