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Sunscreen myths: The real science behind sun protection

From when exactly to use sunscreen to whether beach umbrellas really work,que que deu na mega-sena de hoje this is what scientific research tells us about staying safe in the Sun.

During the hot, sticky days of summer, sunscreen is an important accessory for most of us when going outside. But there are also a surprising number of myths about it. These are often perpetuated by wellness influencers offering their own sunscreen "alternatives", but some are shared by more authoritative sources, while others seem to have just crept into common acceptance.

So what does the skin protection factor (SPF) really mean in terms of protection? Does ultraviolet A (UVA) alone cause ageing? And do sunscreens really require 20 minutes after applying before they "activate"? Here's what the research and leading experts say we really need to know.

The higher the SPF rating, the more protection provided. But the number indicates a different value than many people think. "SPF does not tell you how long you can be in the Sun without getting sunburn," the US Food and Drug Administration (FDA) notes. "SPF does not mean that if you normally burn in one hour and you apply an SPF eight sunscreen, it will take eight hours for you to burn."

Instead, it's a ratio: the amount of ultraviolet (UV) radiation that would cause the skin to burn with sunscreen, divided by the amount of UV radiation that would cause the skin to burn without sunscreen. This generally works out as the percentage of sunburn-causing radiation that's still transmitted: a sunscreen with a rating of SPF 25, for example, allows in one-twenty-fifth of sunburning rays, or 4%, while SPF 50 allows in one-fiftieth, or 2%. In other words, a SPF 25 blocks 96% of UV radiation and a SPF 50 blocks 98%.

It's important to note that this is calculated by laboratory tests, where sunscreen is applied at an amount much higher (2mg/cm2 of skin) than most people apply, so most of us are getting more UV exposure than these numbers indicate.

Either way, sunscreen needs to be reapplied regularly throughout the day (at least every two hours, as well as after swimming or sweating) for it to work. For the best protection, look for both a high SPF rating as well a sunscreen that offers "broad spectrum" protection. In the UK, the UVA star rating, which indicates a sunscreen's effectiveness against long-wave UVA rays, where five is the highest level of protection.

The spectrum of UV light can be broken down into smaller bands of different wavelengths. UVA has a longer wavelength that can penetrate the skin more deeply, while UVB waves are slightly shorter and only penetrate the outer layers of the skin. However, this doesn't mean that UVB doesn't cause deeper damage.

It was long thought that UVB was the main cause of skin cancer, but in recent decades research has indicated that damage caused by UVA may also lie behind the development of skin cancers. This has led to sunscreen formulations that block both types of UV radiation.

When either form of UV hits the skin, the skin's cells respond by releasing mediators like cytokines – which cause inflammation deeper down, contributing to long-term damage, ageing and cancer risk, says Antony Young, professor emeritus of experimental photobiology at King's College London and a lifelong researcher of sunscreen efficacy.

Both UVB and UVA also contribute to ageing and to tanning, says Mary Sommerlad, a consultant dermatologist based in London and a spokesperson for the British Skin Foundation, a UK health charity. In fact, she says, if she has a patient with a condition like melasma, where there are darker patches of skin, she tells them they need to protect themselves from the broad UV spectrum – UVA included – because UVA will "drive the darkening of the skin", she says.

Many of us skip the hat and sunscreen when we see cloud cover. But that may be a bad idea.

This is because, while clouds do scatter radiation from the Sun, the impact on surface-level UV radiation levels can vary hugely. Very thick clouds can attenuate around 99% of the UV radiation that reaches the Earth's surface, but thinner cloud cover or scattered clouds might have no effect at all. Some types of cloud formations can mean that even more UV radiation reaches the ground in some spots than on a clear day.

When it comes to understanding Sun protection, what matters most is the UV index – a measure of the level of UV radiation that public health organisations recommend people use, much like the temperature, to help plan their days (and clothing).

When the UV index is three or greater, public health organisations suggest wearing a hat, protective clothing and sunscreen – and seeking out shade. But what kind of shade is important, experts warn.

Take the ubiquitous beach umbrella. Because it's likely a couple of meters above your head, "your skin is exposed to virtually the whole of the open sky around you", says Brian Diffey, emeritus professor of photobiology in dermatological sciences at the University of Newcastle in the UK and inventor of sunscreen's UVA star rating. "You're getting a sun protection factor equivalent of about five." One trial, for example, found that while 25% of people using sunscreen got sunburn, 78% of people using just a beach umbrella did.

Other types of shade that have been found to offer little protection include the roof of a veranda (if it still gets afternoon sun) or a grandstand. Leafy trees may offer more protection – a dense-canopy oak, for example, may offer the equivalent of up to SPF 20. Still, many types of trees may only provide a SPF of about five or lower.

Generally speaking, the more sky is covered by an overhang, or the denser the canopy and the more trees clustered together, the greater the protection.

UV radiation also can be reflected off surfaces like glass, sand, concrete and water – so it's important to watch out for sneaky reflected UV, too.

In general, even for fair-skinned people who are sensitive to the Sun, the risk of UV damage when the UV index is two or below is limited and extra protective measures aren't needed, according to the World Health Organization.

That means that, in some parts of the world, most people won't need to use sunscreen all year. In the UK, for example, the UV index is less than three from about mid-October to mid-March, says Diffey. For people who spend most of the day indoors, an entire winter's day exposes them to the UV equivalent of less than one minute of what they'd get sunbathing at the height of summer. "If you're in the office and it's gloomy and it's raining and it's December – it's nonsense," Diffey says about using sunscreen in the winter. "You're doing something which is of no benefit to you."

Of course, context matters: studies have found that in some cases, snow cover can increase sunburn-causing UV by more than 60%. So, if it's the winter but you're out, say, skiing all day, it's important to protect exposed skin from the Sun.

There's another caveat, too, points out Sommerlad: the UV index can change quite quickly. "If you're out and the cloud suddenly lifts and you're not protected, you can burn," she says. In other words, you're likely less safe relying on the UV index to forego additional protection on, say, a summer day than during a time period when the UV index is consistently low – such in the middle of winter.

While mineral-based sunscreens do have a white cast, because they reflect a portion of the visible light spectrum, that's not indicative of their effectiveness in protecting the skin from UVA or UVB – the only UVR spectrums at the Earth's surface, and those that cause skin damage and cancer. And because mineral sunscreens work primarily by absorbing UV radiation – reflecting or scattering only 5% of UV rays – there's no reason they'd have to appear "reflective" in order to work. (Read more about the differences between mineral and chemical sunscreens.)

While it's true that our bodies synthesise the hormone called vitamin D3 through exposure to UVB from sunlight, that doesn't mean using sunscreen is likely to make much of a difference. Few of us coat our entire bodies in sufficient quantities of sunscreen, regularly enough, to get even the full level of protection indicated on the bottle by the SPF rating – meaning most of us are getting more UV exposure than we think.

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For people with lighter skin, when lying in the Sun on a clear, summer day it often takes only a few minutes of exposure (depending on factors like UV index and quantity of skin exposed) for vitamin D3 synthesis to not only kick in, but to reach its maximal level. However, in other contexts, such as walking around an urban environment, fully clothed, it might be closer to an hour.

Regardless, after that level is reached, it's not a case of "more exposure, more vitamin D". Instead, DNA damage increases – but vitamin D3 stores remain the same. This is probably why past research has found that sunscreen users are no more likely to be vitamin D-deficient than non-sunscreen users – although, Diffey notes, much of this research has taken place in sunny countries with relatively low SPFs.

Most sunscreen labels say sunscreen should be applied 15-20 minutes before exposure. But that doesn't mean sunscreen is ineffective during the wait. In fact, studies have shown that sunscreens can be effective immediately, something many manufacturers also say. Instead, 15-20 minutes gives the formulation enough time to dry into a consistent, protective "film" that's harder to smear, or to sweat or wash off, notes Michelle Wong, chemist and author of the book The Science of Beauty, who frequently tackles sunscreen myths online.

Fifteen minutes may actually be a conservative estimate for how much time that takes, with one small study finding that the sunscreen tested created this film after just eight minutes. Still, following the manufacturer's instructions is always better.

The melanin in darker-toned skin provides some natural protection from UV, which is why melanoma incidence tends to be much less in black populations than fair-skinned ones. However, people with darker skin tones can still get both sunburns and skin cancer, and Sun exposure can cause them to develop hyperpigmentation disorders like melasma – which is part of why dermatologists like Sommerlad recommend that people with all skin tones use sun protection.

Rates of melanoma are on the rise among light-skinned people in countries like the US, UK and Australia, but that doesn't mean that sunscreen is the cause. For one, cancers of all kinds are more likely among older populations and populations in these countries are ageing. We also know that the more frequently someone sunburns or uses tanning beds, the greater their risk of skin cancer – and both sunbathing and tanning have been particularly popular over the last few decades.

One study of skin cancers in Australia found that around two-thirds of melanomas and "essentially all" basal and squamous cell carcinomas result specifically from UV exposure, while a study on all patients diagnosed with melanoma in Sweden between 1960 and 2004 found that melanomas were most likely to be on the torso and limbs for people under 70 – indicating that UV exposure was a main cause. "If you're health conscious and want to protect your longevity and your health, then protecting yourself from sunburns is one of the best things you can do," says Sommerlad.

But there is one roundabout way in which sunscreen might have led to an increase in skin cancer, experts note: it can make us feel invincible.

"When people put on a sunscreen, they think they've got this invisible barrier against the Sun on themselves, and they tend to change their behaviour and spend more time in the Sun," says Diffey. "But I don't believe that sunscreens per se are carcinogenic."

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