I’ve seen so many different takes on sunscreen that I had to do something about it. Here is my best effort to summarize the current research.

What does sunscreen do?

Without getting into technical details, sunscreen reflects or scatters UV radiation (mainly from the sun), offering protection against sunburn (Norval & Wulf 2009).

Consequently, sunscreen also reduces “the severity of solar elastosis, the development of solar keratoses, new naevi, SCCs and cutaneous photodamage, and the reactivation of herpes labialis,” all of which are pretty nasty things caused by overexposure to the sun.

With that out of the way, let's move on to a bigger question.

Is sunscreen really effective against skin cancer?

There are different types of skin cancer. In this section, we'll focus on malignant melanoma (“the skin cancer with increased risk of death” Lindqvist & Olsson 2016). The current consensus is that overexposure and episodic sunburns are the leading causes of melanoma. Sunscreen seems to be effective in preventing melanoma in white children (Gallagher et al., 2000). However, as the author puts it, melanoma is rare and that makes it hard to study.

So, sometimes, we see strange findings.

There's one paper that everyone keeps talking about. Between 1988 and 1990, Westerdahl et al. compared 400 melanoma patients to 640 healthy “controls.” The researchers asked them about their sunscreen use and personal factors. They found that not only was sunscreen ineffective against melanoma but there was also a positive link between them! Could sunscreen be harmful and cause skin cancer?

Well, not really.

Westerdahl and his colleagues provide several hypotheses for how to explain these surprising results:

They tried to control for sun exposure (people who use more sunscreen are more in the sun generally and thus more at risk), but they admit their method was not perfect. More on this later.

Sunscreen usually protects only one kind of UV radiation, and other types might be dangerous. More on this later.

Maybe sunscreen contains carcinogenic agents? More on this later.

"Sunscreen may increase melanoma risk by interfering with vitamin D synthesis." More on this later.

First off, it sounds logical that people who use more sunscreen are more in the sun. You don't apply sunscreen before locking yourself in your basement to play D&D. So, people who use more sunscreen are also more at risk of overexposure.

In their literature review from 2009, Norval & Wulf found that sunscreen users generally expose themselves more frequently and for more extended periods to sunlight than non-sunscreen users. Once they've applied sunscreen, most users believe they're protected and forget about it (Lindqvist & Olsson 2016). Finally, Gallagher writes:

Sunscreens are most commonly used by those with a strong propensity to burn in the sun because of fair skin. These same people are also at elevated risk of melanoma because of that same sun sensitivity, and techniques to measure sun sensitivity in population-based studies are unfortunately relatively crude. Thus it is highly likely that there is some degree of uncontrolled confounding from this factor in virtually all retrospective studies, and this may well be the reason why a number of the investigations show a direct rather than an inverse relation between use and melanoma risk.

Lack of control for sun exposure already seems to be a strong hypothesis for explaining the results, but there might be more!

There are two types of UV radiation that harm your skin: UV-A and UV-B. Most sunscreens protect against UV-B (the SPF index indicates how much you'll be protected against UV-B), but not so much against UV-A.

The Skin Cancer Foundation says, “[UV-A] exposure causes genetic damage to cells on the innermost part of your top layer of skin, where most skin cancers occur.” That means most sunscreens are in themselves only half effective in reducing skin cancer. It also concurs with the previous results. If you use sunscreen, you're more often in the sun. If you're more often in the sun, you're more exposed to UVA. Even if you efficiently block all UV-B, the other rays are still out to get you. Always try to get a full-coverage sunscreen!

Westerdahl et al. don’t say what type of sunscreen they used, but given how old this study is, I’m pretty sure their stuff wasn’t as good as it is today.

How dangerous is skin cancer?

We've seen that the sun can cause skin cancer and that sunscreen is probably effective in preventing skin cancer. But I've (too) often read that skin cancer is not that prevalent or not that dangerous. Thus, we shouldn't wear sunscreen, as the benefits of vitamin D outweigh the risks of skin cancer. We'll investigate the relationship between sunscreen and vitamin D in the next chapter, but let's focus first on the claims about skin cancer.

It's true that melanoma, the most dangerous skin cancer, is also the least frequent one. Gallagher gives us these base rates:

Melanoma has a fatality rate of 15% (Gallagher 2005). It's not as high as other cancers, but it's still something. It kills around 10,000 people each year in the U.S. (Jacobsen 2019) and 2,000 in the U.K. (Norval & Wulf 2009). If you compare these figures to other health risks, skin cancer is not the scariest thing. But death is not the only bad thing. The other types of skin cancer are treatable, but the surgeries are often disfiguring — which is a pretty bad thing. Furthermore, they are costly. It is estimated that skin cancer treatment imposes a burden of $800 million per year in the U.S.A. (Norval & Wulf 2009).

Does sunscreen block vitamin D?

And now, on to the fun part!

The biggest claim about sunscreen is usually that it blocks vitamin D production in the body, and that vitamin D deficiency is more dangerous than skin cancer. Let's unwrap this.

First, vitamin D is a hormone manufactured by the skin with the help of sunlight. It is a crucial factor in overall health. Almost every disease or disorder you can think of has been linked with low vitamin D levels (cancer, diabetes, high blood pressure, depression…) (Jacobsen 2019).

Many studies have shown that exposure to sunlight has protective effects against all types of cancer and cardiovascular diseases (Lindqvist et al. 2016van der Rhee et al. 2013Tuohimaa et al. 2007…)

Furthermore, it appears that UV-B provides for more than 90% of vitamin D production in the body (Norval & Wulf 2009). So, if sunscreen is effective in blocking UV-B, wearing sunscreen will prevent vitamin D production, right? Right…?

In 1995, Marks undertook a randomized, double-blind controlled trial (the strictest type of study) with 113 subjects, half of whom applied SPF 17 sunscreen every day. The other half used placebo cream. Everyone could wear whatever they wanted and avoid the sun around midday. In the end, both groups had similar sun exposure (measured by sun badges) and… similar increases in vitamin D blood concentration (measured by blood samples taken at the beginning and end of the study)!

The conclusion was that sunscreen usage could not be interpreted as a risk factor for vitamin D deficiency. Even if it should be in theory or a strictly controlled environment, that's not the case in real life. People don't apply sunscreen uniformly everywhere on their bodies, and not at the perfect interval. That leaves more than enough time and space for UV rays to trigger vitamin D production.

Does sunscreen contain harmful substances?

This is… ambiguous. First, I'm not qualified enough to read biology papers about hormones and chemical reactions in the body. Then, most studies on the topic conclude, “more research is needed.” Anyway, here is what I could find.

One of the most common components of sunscreen is oxybenzone. In a 2008 study, Calafat, Antonia et al. found that more than 95% of the people who applied sunscreen had oxybenzone in their urine. That means it goes through the skin and is absorbed into the bloodstream. Bloodstream absorption is not necessarily unsafe, but that calls for caution.

So far, oxybenzone has only been linked with some rare cases of photoallergy. In 2020, Suh et al. reviewed 29 papers about oxybenzone and octinoxate (another sunscreen component). The abstract reads:

Studies show that elevated systemic level of BP-3 (oxybenzone) has no adverse effect on male and female fertility, female reproductive hormone level, adiposity, fetal growth, child's neurodevelopment, and sexual maturation. However, the association of BP-3 level on thyroid hormone, testosterone level, kidney function, and pubertal timing has been reported and prompts further investigations to validate a true association. The systemic absorption of OMC has no reported effect on thyroid and reproductive hormone levels. In conclusion, current evidence is not sufficient to support the causal relationship between the elevated systemic level of BP-3 or OMC and adverse health outcomes.

Following this, the Food and Drug Administration said they would reevaluate the potential adverse’ potential adverse health impacts. Watch this space, I guess?

Anyway, unless a randomized-controlled trial shows strong links between oxybenzone and hormone disruption, I don't think we should be too worried.

Is sunscreen harmful to the environment?

Oxybenzone is also a common culprit for coral bleaching, and some countries have started banning sunscreen that contains the chemical. However, the evidence is not as strong as previously believed. Because I'm lazy, I will quote the Wikipedia paragraph on the topic here:

Media reports link oxybenzone in sunscreens to coral bleaching although some environmental experts dispute the claim. A small number of studies have been released which linked coral damage to oxybenzone exposure. A 2015 study published in the Archives of Environmental Contamination and Toxicology led to ban of oxybenxzone containing sunscreen in Palau. However, the purported link between oxybenzone and coral decline is widely discussed within the environmental community since most studies on the subject have been conducted in a lab environment. A 2019 study of UV filters in oceans found far lower concentrations of oxybenzone than previously reported, and lower than known thresholds for environmental toxicity.

Bottom line: How much sunscreen should you use?

All in all, should you wear sunscreen? If so, how much should you use?

Looking at the current evidence, it doesn't seem like sunscreen is harmful in itself. It usually does its job quite well and protects you from bad consequences. Applying sunscreen is beneficial because you will be less hesitant to go to the sun.

However, there's definitely a trade-off somewhere. A vitamin D deficiency still seems to be overall more dangerous than skin cancer.

I want to make the following recommendation (remember, I'm not a skin doctor, nor a doctor, I went to business school, so don't trust me): if you're going for a “lunch on a terrace in Stockholm,” don't wear sunscreen. Get this glow. However, if you're going to the beach in Sidney in December, please wear sunscreen, and try to get shade whenever possible. The most important thing is to avoid overexposure and sunburns. Apply common sense. Adjust this advice based on your skin type and your sensitivity.

Side note: vitamin D supplementation should not be used to replace sun exposure. A huge study (Manson et al. 2019) with 25,871 participants showed that vitamin D supplements had no cancer or cardiovascular disease incidence. There's clearly something in vitamin D produced organically that we haven't figured out how to synthesize.

I probably have forgotten or overlooked some studies and arguments. Please let me know if this is the case, and I'll do my best to update this article based on new evidence.

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Side note: vitamin D supplementation should not be used to replace sun exposure.

Strongly agree, but I don't think it is because vitamin D supplements aren't "real" vitamin D. It is very likely that sun exposure triggers other important processes.

Like nitric oxide production, which lowers blood pressure: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5593895/

Also endorphins (opiate type things). 

This is why a friend described taking heroin as like being "bathed in golden sunlight". 

I very appreciate the article, it was very good start for me, but someway I actually wanted to know more... :) Here's a short summary of what I found:

  • Whenever you need protection (sunscreen, sunglasses) depend on so-called UV Index. UV Index varies during the hours of day (it's much higher during midday) and time of year. You can find actual value at local meteorological office. If the index is greater than 2 during the time of day/year you are outside, you need protection. Except you are near water or snow, which may increase intensity greatly.
  • E.g. in my country (Central Europe) you are safe from around begin of October to the end of February. In other countries, you may need protection around the year.
  • You may need more protection depending on your skin type (search for Fitzpatrick test) - lighter skin needs more protection
  • Now if you need protection, it's a BIG difference, whenever you have sunblock or not. E.g. one hour with SPF30 equals about 2 minute without protection (against UV-B rays... which primarily cause sunburn or tan)).
  • Its more complicated with UVA rays, which primarily cause skin aging (but no or just a little tan). Both UVA and UVB cause skin aging, but UVA much more than UVB. UVA is more difficult to filter/block.
  • You need to make sure, the sunblock you are using is marked "Broad Spectrum SPF [value]" (USA) or has UVA seal in circle (Europe). If the seal is present, then the sunblock has protection against UVA in proportion to declared SPF against UVB. There are various other systems measuring UVA/UVB protection. The actual "proportion" UVA/UVB is not really much clear to me...
  • E.g. if the "UVA in circle" (Europe labeling) is present - then you should have at least 1/3 of declared UVB SPF also against UVA. Very roughly, SPF30 then lets through 1/30 UVB and 1/10 UVA rays.
  • There is not much real difference between SPF30 and SPF50 against UVB (should be a difference of ~1.3% of rays which get through), but it could be more against UVA, as only a proportion of UVA is blocked.
  • Further: SPF factor value is only valid if you apply enough of the lotion... most people use too little. Also, most people don't use sunblock on all uncovered skin.
  • If found many references, that you need to reapply the sunblock each 2 hours OR after swimming, sweating ... E.g. if you use day cream in the morning which has SPF30 ... I have no idea (didn't succeed to find any reference) how big your protection is, if you go outside from office during noon, when you need most of the protection... It seems it is much lower than original SPF30 or even maybe zero (?).
  • Regarding vitamin D... according to Skin Cancer Foundation and American Cancer Society... The danger of not using sunblock is clearly higher than not getting vitamin D from sun exposure (as you may safely get vitamin D from different sources than sun).
  • Cancer: lifetime risk of getting melanoma is about 2.5% (1 in 40) for white people.... which doesn't look alarming, but your individual risk could be higher (e.g. if you have lighter skin type or did already a lot of sun "bathing" previously).
  • Most important info/quite a surprise/against "common wisdom": there is nothing like a healthy or safe way to tan. Every time you tan, you damage your skin. As this damage builds, you speed up the aging of your skin and increase your risk for all types of skin cancer. ....except using a self tanning lotion could be a safe way to add color to your skin.
  • Doesn't matter whenever you used sunblock or not: if your skin changes color after being exposed to sun - it's evidence of genetic damage. Even if no sunburn happened. Even if there is no color change, damage still could happen as UVA cause only little skin coloring effect.

Some of the sources I used - https://www.who.int/news-room/questions-and-answers/item/radiation-the-ultraviolet-(uv)-index - https://www.sunsmart.com.au/uv-radiation/what-is-uv - https://www.skincancer.org/skin-cancer-prevention/sun-protection/vitamin-d/ - https://www.cancer.org/latest-news/are-you-getting-enough-vitamin-d.html - https://www.aad.org/media/stats-sunscreen

A huge study (Manson et al. 2019) with 25,871 participants showed that vitamin D supplements had no cancer or cardiovascular disease incidence.

Might be misreading but should this be “had no reduction on incidence”?

A better time until sunburn calculator: https://www.lesswrong.com/posts/vX2GP2fiFnkrWfNpw/a-better-time-until-sunburn-calculator (with some related discussion)

Limitations of the study of sunscreen which make it inconclusive -

1. They only measured blood levels during summer. They would have declined in winter, and were not actually that terrific in either group even at the end of summer, though not at the levels of frank deficiency. Differences would have widened over winter and with ongoing use/non-use of sunscreen over time, as vitamin D is stored in body fat.

2. The study was small and short term and thus major effects could show up as N.S. E.g the 50% greater increase in the placebo group of over 70s was not found "statistically significant". The confidence intervals were very wide so the result should be seen as inconclusive and tending towards sunscreen reducing D levels rather than definitively showing no major effect over time.

My advice is to get regular enough sun exposure that you're not at risk for sunburn.

I only use sunscreen about once a year for unusual exposures.

One downside is that when I stopped using sunscreen, I got more tick bites. I don't think I got more ticks on me. Most likely what changed is they became faster at finding a place to bite.

My advice is to get regular enough sun exposure that you're not at risk for sunburn.

Are you sure this is the correct thing to do, though?

I believed this myself for a long time and this seems to be the common wisdom:
Get a natural tan -> you will get fewer sunburns -> therefore you are less at risk of cancer
So what I thought was that it is better to have tanned skin than pale skin (if your skin is naturally pale) and I should purposefully tan my skin to 'strengthen' it.

However, recently I have read things that seems to suggest that this is actually not true. Unfortunately I haven't found great sources for this, but also nothing to the contrary. If you look at point number 3 here, it suggests what I mean.

If I understand that correctly, the argument is as follows:
- Every time your skin changes color you are already doing damage to your skin
- Yes, your skin gets more resistant to sunburns, however in order to regularly keep your skin tanned, the aggregated damage to your skin still puts you at higher risk of skin cancer and other problems

So the course of action would be
- Get enough sun for Vitamin D production (but this seems to be possible without tanning)
- Don't let your skin get to the point of getting tanned by using sunscreen
- Of course, don't get sunburnt

Most skin cancer information sites seem to support this interpretation (example), however I feel like a lot of people still seem to think tanned skin is healthy, at least I used to.