As the last weeks of summer melt into autumn, you might be tempted to leave sunscreen out of your morning skincare routine.
But as temperatures start to drop in southern Australia, UV radiation stays high, so it's a good idea to continue slathering on sunscreen daily.
For decades now, we've known sunscreen dramatically lowers a person's risk of developing skin cancer, with some of the first evidence coming from (aptly) the Sunshine State — Queensland.
Specifically, we have residents of the southeast Queensland town of Nambour to thank.
In the 1990s, more than 1,600 Nambourians participated in a randomised controlled trial. Some slathered on sunscreen every day for 4.5 years while others only used it when they wanted to.
Years later, the daily sunscreen users had substantially lower rates of melanoma and a common form of skin cancer called squamous cell carcinoma than the discretionary group.
In the years since, other studies have also supported sunscreen's cancer-preventing abilities.
So exactly how can energy from the Sun cause lethal changes to our skin cells, and how does something as seemingly innocuous as a lotion help shield us from it?
The ABCs of UV and skin cancer
When it comes to skin cancer, ultraviolet or UV radiation is the main culprit. This light is invisible to our eyes, but the damage it inflicts is very real.
UV is split into three subtypes: UVA, UVB and UVC.
We don't have to worry about UVC. It's absorbed by Earth's atmosphere, and doesn't reach us on the ground.
But we do have to worry about UVB. It's responsible for sunburn and most sun-related cancers.
Imagine a UVB photon, spat out by the Sun, hurtling towards Earth. You step outside with bare skin in that photon's path.
It will pass through your skin's outermost layer of dead cells and into the epidermis layer below, where the cells are alive.
And if that UVB photon collides with a cell's genetic material — its DNA — it can transform a healthy cell into a cancerous one.
Normally, a double strand of DNA in a cell is structured like a twisted ladder. But if it's hit with UVB, the energy imparted by the photon breaks a couple of the ladder's rungs.
The broken rungs don't reattach properly, creating kinks in your cell's DNA ladder called "photolesions".
The good news is our cells can deftly find and fix this kind of DNA damage, QIMR Berghofer Medical Research Institute medical epidemiologist David Whiteman says.
"A cell will, 99.999 per cent of the time, repair that damage within 24 hours.
"The problem is that when you have billions or trillions of UVB hits, 99.999 per cent is just not good enough, and you're still left with excessive DNA damage."
Then next time you go out in the Sun, those already-damaged cells sustain even more DNA damage, and photolesions accumulate.
"So by the time you get to 40 or 50 years old, you might have a whole bunch of cells in your skin that have damage in key genes," Professor Whiteman said.
"And if those key genes regulate growth, it can lead to uncontrolled growth and cancer."
Another type of UV radiation, called UVA, can also nudge healthy cells into cancer territory.
UVA is less energetic than UVB, so it can't directly cause kinks in your DNA ladders. But it penetrates deeper into your skin, and can corrupt cells in more roundabout ways.
One way paradoxically involves melanin, the pigment that darkens skin. UVA radiation triggers a chemical reaction that produces DNA-damaging molecules.
While this is a far slower process than UVB's direct DNA hits, UVA exposure can be dangerous.
This is perhaps best shown by multiple studies showing tanning beds — which use UVA but not UVB — increase a person's risk of developing skin cancers, including melanoma.
Over our lifetime, cumulative DNA damage from UVB and UVA means your likelihood of developing skin cancer "is a numbers game", Professor Whiteman says.
"The longer you live in a high-UV environment like Australia, if you have a particularly susceptible skin type and you don't protect yourself, eventually you'll damage enough cells to lead to skin cancer."
What about premature ageing?
Getting a few wrinkles is a normal part of getting older. But most hallmarks of ageing skin are not caused by the inexorable march of time. They're brought about by the Sun.
One study estimated that in Caucasian people, UV radiation is behind 80 per cent of visible signs of ageing such as leathery skin, pigmentation and sagginess.
The worst offender when it comes to premature ageing is UVA.
It encourages our body to make molecules that chop up collagen, a protein in our skin that gives it strength.
We also start to produce an abnormal, less-stretchy form of elastin, another skin protein.
Blood vessels cop UV damage too. Capillaries can become twisted and elongated, Professor Whiteman says.
"The whole landscape of the skin's environment becomes less elastic and more fibrous and thick."
So like the elastic in your favourite pair of tracksuit pants, your skin starts to lose its youthful suppleness, and can't snap tautly back into place.
"And that's where people end up with crow's feet, redness, leatheriness, and so on."
How sunscreen works
It's sunscreen's job to stop harmful UV radiation from getting to and tinkering with our skin cells.
Sunscreens with a higher SPF, or sun protection factor, let less UV through.
Go to the chemist or supermarket, and you'll find two main types of sunscreen on the shelves, University of Queensland dermatology researcher Yousuf Mohammed says.
Chemical sunscreens, also called organic sunscreens, need to be rubbed into and soaked up by your top layer of skin before you head outside.
They contain compounds with names like ethylhexyl triazone and butyl methoxydibenzoylmethane that absorb UV radiation and convert it to heat.
Then there are mineral or physical sunscreens. As their name suggests, they create a physical barrier between Sun and skin, Dr Mohammed says.
"They contain zinc oxide or titanium dioxide particulate matter that sits on the skin's surface, where they work … by blocking, scattering and reflecting UV radiation to prevent skin damage."
Those zinc and titanium nanoparticles are the reason mineral sunscreen can be quite thick — otherwise, they would sink to the bottom of the tube.
Sunscreen formulations for children can cost more than the regular old stuff, but they do tend to have less irritating ingredients, and are usually (but not always) mineral ones, as kids have more sensitive skin than adults.
And while we should protect our skin from UV throughout our life, there's plenty of evidence that childhood is a particularly critical time to be sun-smart, Professor Whiteman says.
"Epidemiological studies, over many years, show people who lived in a high-UV environment like Australia for their first 10 or 15 years suffer DNA damage to their skin.
"That sets them up with a higher lifetime risk of melanoma."
Is sunscreen safe?
Evidence to date shows sunscreen is safe when applied properly, and its potential benefits considerably outweigh any potential risk.
Mineral sunscreens might be best if you have sensitive skin, Dr Mohammed says, as chemical sunscreens tend to cause more reactions.
When some active ingredients in chemical sunscreens do their job of soaking up UVA and UVB, they start to break down.
"These degraded products can no longer protect the skin against UV and, if they penetrate the skin, can cause cell damage," Dr Mohammed says.
And while a 2019 study in the US found some ingredients in chemical sunscreens can make their way into the bloodstream, it's not known if they pose any health risks. The researchers wrote their "results do not indicate that individuals should refrain from the use of sunscreen".
Loading...Mineral sunscreens have had a bad rap in the past too, with studies suggesting that zinc and titanium nanoparticles might get through the skin into the body too.
But they were conducted in cells grown in a lab, not in humans.
Australia's Therapeutic Goods Administration (TGA) in 2017 updated its review of the evidence surrounding the safety of mineral sunscreens.
The TGA found zinc oxide and titanium dioxide nanoparticles in sunscreen "do not penetrate or minimally penetrate the [outer layer of dead skin cells] ... and underlying layers of skin. This suggests that systemic absorption, hence toxicity, is highly unlikely.
"In conclusion, on current evidence, neither [titanium dioxide nor zinc oxide nanoparticles] are likely to cause harm when used as ingredients in sunscreens and when sunscreens are used as directed."
More recently, a 2018 study in people, led by Dr Mohammed, found zinc oxide nanoparticles "accumulated on the skin surface and within the skin furrows but did not enter or cause cellular toxicity in the viable epidermis".
Overall, according to the TGA, sunscreens are not likely to cause harm, but the risk of skin cancer from UV radiation is real.
What people do wrong
Right. We know how sunscreen works, and why it's important. So slap a bit on and you're OK to head out into the sunshine, right?
Well …
An SPF 50+ sunscreen won't be SPF 50+ if it's not used correctly.
Common mistakes people make include:
- putting it on too late (sunscreens should be applied 15 minutes before going out)
- not putting enough on (you should use a pea-sized blob of sunscreen for an area of skin the size of your palm)
- forgetting to reapply, especially after swimming, sweating or towelling off.
An overcast day isn't automatically safe from damaging radiation either. When the sun's behind a cloud, the ultraviolet radiation index can easily be high enough to warrant sunscreen.
You should also check the use-by date on the bottle or tube, Dr Mohammed says.
If it's expired, chuck it out. UV absorbing compounds in sunscreens can go off with time.
They can also degrade with heat. That tube of sunscreen that lives in your glovebox, and has withstood many a sweltering day? There's a good chance it no longer provides the protection stated on the label.
Another classic mistake Dr Mohammed says people make with sunscreen is to rely on it alone.
No sunscreen will fully block all UV radiation, so it should be used alongside the rest of the "slip, slop, slap, seek and slide" slogan.