Decades ago, studies started bubbling up, reporting that not only was the use of sunscreen not found to be protective against skin cancer, but it was associated with significantly elevated risk.

But subsequent studies offered the obvious explanation of this paradox.

People who use sunscreen are more likely to be sunbathers.

That's like finding those who use motorcycle helmets are more likely to die on the highway.

It's not that sunscreen and helmets aren't protective, it's just that they're associated with engagement in risky activities.

But you don't know until you put it to the test.

As I noted in my last video, randomized controlled trials have shown that regular use of sunscreen can arrest visible signs of skin aging, including biopsy-proven reductions in UV-related skin damage.

But are their interventional trials proving sunscreen can prevent cancer?

Yes. Organ transplant recipients are highly susceptible to skin cancers because they have to be given immune suppressants to prevent organ rejection.

A group of 120 organ recipients were equally informed about sun protection measures, but half were given free, broad-spectrum SPF 50 plus sunscreen for daily application to their head, neck, forearms, and hands.

After two years, there were 9 new basal carcinomas in the control group versus only 2 in the sunscreen group.

That sounds good, but it may have just been a fluke.

In contrast, there was a highly significant difference in the proportion of patients who got new invasive squamous cell carcinoma, 8 new cases in the control group versus 0 in the sunscreen group.

But wait until you hear about what happened to their actinic keratoses, the precancerous growths that can turn into skin cancer.

They all started out covered in them, a total of 191 detected in each group.

In the subsequent 24 months, the control group developed 82 new ones.

How many did the sunscreen group develop? Negative 102.

More than 100 precancerous growths in the sunscreen group spontaneously regressed and vanished completely, compared to 82 new growths developing in the control group without a single one disappearing.

Their bodies, even in their immunocompromised state, could heal itself once it just stopped being bombarded with so many cancer-causing rays.

Other randomized control trials have shown similar findings, but generalizability is limited.

The studies all used high-risk populations, for example, those living in subtropical climates or with a personal history of precancerous lesions.

So cancer prevention efficacy in the general population would be expected to be less.

On the other hand, the studies could also have underestimated the impact since they were relatively short-term, not exceeding four years, and ethically had to allow the control group participants to slather on their own sunscreen, which would dilute the difference between groups.

For maximum effectiveness, sunscreen needs to be applied properly, which apparently rarely happens in the real world.

In a study of nearly 5,000 skiers and snowboarders, almost no one, only about 4%, were fully compliant with sunscreen recommendations.

First, there's the amount.

The FDA standard is 2 mg per square centimeter.

What does that mean?

Use the teaspoon rule.

One teaspoon for your face, head, and neck, one for the front of your torso, another for your back, one teaspoon for each arm, and two teaspoons for each leg.

That's a total of 9 teaspoons.

That's about the total volume equivalent of a golf ball or a shot glass to help you visualize it.

Unfortunately, the average sunscreen user may only apply a quarter of the recommended amount.

This is why high SPF sunscreens are often recommended, like 50 plus SPF.

Although the FDA recommends a minimum SPF of only 15 to prevent skin cancer, under normal consumer conditions, even an SPF of 50 may effectively give you only an SPF of 12.5 because most people don't put on enough.

Randomized, double-blind, head-to-head, split-face experiments, where you're randomized to apply one sunscreen to one side of your face and another sunscreen to the other side, show that in real-life use SPF 100 plus sunscreen works significantly better than SPF 50 plus sunscreen.

More than 50% of the participants were sunburned more on their SPF 50 plus side, compared to only about 5% on their SPF 100 plus side.

Now, a potential downside of higher SPF sunscreens is that they could provide a false sense of security.

Those randomized to an SPF 30 sunscreen ended up spending more cumulative time in the sun than those receiving an SPF 10 sunscreen, as much as five times longer in some cases.

What else?

Well, people tend to remember to use sunscreen on a sunny day at the beach, but sun protection is needed even on cloudy days, since the UV rays are not dampened as much as visible light.

There's even a phenomenon known as cloud enhancement, where overcast skies can sometimes result in even more UV reaching the Earth's surface compared to clear skies.

Sunscreen labels often suggest waiting at least 15 minutes after application before going outdoors, but when put to the test, sunscreen was found to start working immediately, with a full effect apparent by minute 10.

However, if water resistance is required, it may be prudent to wait the full 15 to 30 minutes after application before taking a dip.

The terms waterproof or sweatproof appear to be meaningless marketing, as no difference in retention was noted between quote-unquote waterproof products and those that were merely labeled water-resistant.

Both were better than non-water-resistant products, though, which lost nearly all their protective effects within 20 minutes of water immersion.

The suggestion that sunscreen should be reapplied every two to three hours only applies under conditions in which it is rinsed off by water or sweat or rubbed off by friction from clothing or sand.

Even after allowing sunscreen to dry first for 20 minutes, between 15 and 60% of its protective effect can be lost after contact with sand.

Otherwise, if the recommended amount is applied and the sunscreen layer is not disturbed,

SPF can be maintained for as long as 8 hours.