‘Slip, slop, slap’ has long been the catchphrase of summers across our nation.
This is for good reason, as one Aussie dies of melanoma (skin cancer) every five hours, according to a report from the Australian Institute of Health and Welfare (AIHW). Meanwhile, two in three will be diagnosed with a type of skin cancer by the age of 70, one of the highest rates in the world, according to the Cancer Council.
Exposure to ultraviolet (UV) radiation in sunlight causes the majority of skin cancers in Australia. So, it’s not surprising sunscreen has become such a vital component of our sun protection routine.
But what do we really know about how sunscreen works, what’s in it, how to use it or whether it’s actually good for us?
We’ve done the hard yards to break down the science and debunk common myths of sunscreen, so you can better understand how it can help in the fight against burns and skin cancers.
How does sunscreen work?
Put simply, most sunscreens work by reflecting and filtering ultraviolet radiation (UV rays) emitted by the sun.
Physical or reflecting sunscreens, as the name suggests, create a physical barrier on the skin by using active mineral ingredients like zinc oxide and titanium dioxide to reflect UV rays instead of absorbing them, according to the Cancer Council. Most physical sunscreens are broad spectrum and look opaque when applied to the skin.
These types of sunscreens are typically used in natural or organic sun protection products and are best for sensitive skins. The main advantage is that it also protects from the sun right upon application.
On the other hand, chemical or absorbent sunscreens absorb sun rays to prevent UV radiation from penetrating skin cells. These types of sunscreens usually contain synthetic chemicals like oxybenzone and octinoxate.
These sunscreens are generally clear when applied to the skin, but are more likely to cause irritations or allergies and require about 20 minutes after application to take effect.
The Cancer Council of NSW also says sunscreens can often have up to six or more active ingredients.
The ABCs of UV radiation
Anytime you’re outside or in the sun, you’ll be exposed to two main harmful types of UV radiation: UVA and UVB.
UVA radiation penetrates deeply into your dermis (the thick layer of skin below your outer layer of skin called the epidermis) and plays a major part in tanning (it’s used in tanning beds), skin ageing, sun damage and wrinkling. This type of UV ray is present at any time during daylight hours and accounts for 95% of the UV radiation reaching the surface of the earth.
What’s more, UVA rays can penetrate clouds and glass windows, so you’re still exposed to skin damage even on overcast days or while travelling in the car.
UVB radiation has higher energy and is the principal cause of sunburn, skin reddening, skin cancers and damages mostly the epidermis. UVB also causes direct damage to DNA but rays vary according to location, time of day and season. Broad-spectrum sunscreens are designed to filter both UVA and UVB rays, according to the Skin Cancer Foundation. 
You might have also heard of UVC radiation, but this doesn’t reach the earth’s surface as the ozone layer mostly absorbs it.
Screening sunscreen: Is it actually good for us?
Most skin cancers are caused by sun exposure, and sunburn is behind 95% of melanomas, according to the Cancer Council. Troubling figures like this emphasise the importance of sun protection, particularly when the UV Index – the scale that highlights the danger of UV radiation and skin damage – reaches three and above.
Sunscreen is specifically designed to prevent the sun’s ultraviolet radiation from reaching the skin. Research published in the Australian and New Zealand Journal of Public Health in 2015 found that in 2010, more than 1,700 cases of melanoma and 14,000 cases of non-melanoma skin cancers were prevented by regular sunscreen use in the previous decade.
Despite this, there are questions circulating from some health advocates and consumers about the true safety of sunscreen.
In fact, only 55% of Aussies believe sunscreen is safe for everyday use, according to the Cancer Council’s 2017 National Sun Protection Survey. What’s more, 17% of respondents believed sunscreens contained ingredients that were harmful to their health if used frequently, while 20% said people who used sunscreen regularly were not getting enough Vitamin D from the sun.
Nanotechnology in sunscreens has been particularly controversial over the years. By using smaller nanoparticles (that are less than 100 nanometers thick) instead of bulkier particles of mineral UV filters, sunscreen can appear transparent on the skin, instead of leaving a white residue.
Nanoparticles have been used in sunscreens since the 1990s and concerns have been raised about whether zinc oxide nanoparticles can be absorbed into the epidermis and cause DNA damage.
However, a recent study by the University of South Australia and the University of Queensland found nanoparticles in sunscreens could not penetrate the skin.
Investigators studied five volunteers aged 20-30 years who applied zinc oxide nanoparticles every hour for six hours over five consecutive days and found nanoparticles remained within the superficial layers of the skin and did not cause cellular damage.
A 2014 laboratory study conducted by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and other research institutions also concluded that if nanoparticles in sunscreen could enter the skin, they would be broken down by the immune system before reaching the bloodstream.
A literature review by the Therapeutic Goods Administration (TGA) also found no evidence that nanoparticles could reach viable skin cells.
In Australia, the TGA regulates the manufacture and composition of sunscreens to help ensure only approved ingredients and chemicals that have been assessed for quality and safety are used in each product.
How does the SPF rating system work?
The SPF labelled on sunscreen stands for Sun Protection Factor and is simply a measure of how long you can stay in the sun with sunscreen applied before you show redness or burn.
Usually tested in a laboratory (on humans), the SPF is calculated by how much time it takes for intense ultraviolet (UV) radiation to burn skin that has been liberally ‘slopped’ with sunscreen, compared to unprotected skin.
So, if it takes 10 minutes for unprotected skin to show redness, then skin with SPF30 sunscreen correctly applied should take 30 times longer or 300 minutes (5 hours) to burn.
The plus sign on sunscreens like SPF15+ or SPF30+ indicates the SPF provides greater protection from the sun than the number displayed.
A common misconception is that SPF50+ offers vastly better protection than SPF30 but not quite, as SPF50+ filters out 98% of UVB radiation, only slightly more than SPF30 at 96.7%
In Australia, sunscreens must have an SPF of at least four and the highest rating is 50+.
How to use sunscreen properly
The Cancer Council of Australia recommends using an SPF30 sunscreen or higher with broad spectrum, that is water resistant and TGA approved.
Sunscreen should be applied generously on clean dry skin, 20 minutes prior to sun exposure to give it time to create the intended protective barrier.
According to the Australian Radiation Protection and Nuclear Safety Agency, the average adult needs 35ml of sunscreen for a full body application, equivalent to more than half a teaspoon to each arm and the face, and just over one teaspoon for each leg, the front of the body and back.
Sunscreen should be re-applied every two hours or any time after swimming, sweating or towel drying.
Check the expiry date on your sunscreen and always use other forms of sun protection measures including sun protective hats, clothing and sunglasses.
Using sunscreen on babies younger than six months old is not recommended due to skin sensitivity and a higher risk of a skin reaction, while the Cancer Council also recommends keeping babies under 12 months away from direct sunlight with protective clothing, hats and shade.
The Cancer Council recommends getting a skin check every three to six months. Generally, you can visit a bulk-billing GP for a routine check and Medicare rebates are available if you require a specialist for a biopsy.
 Australian Institute of Health and Welfare- Cancer data in Australia (2018).
 Cancer Council- Skin cancer (2018).
 Cancer Council- How does sunscreen work? (2018).
 Skin Cancer Foundation- UVA and UVB (2017).
 Skin Cancer Foundation- UVA and UVB (2017).
 Cancer Council- Sunscreen FAQs (2018).
 Cancer Council-Almost half of Australians confused about sunscreen (2017).
 Cancer Council WA- Cancer myth: Nanoparticles in Sunscreen (2018).
 Therapeutic Goods Administration- Sunscreens: information for consumers (2017).
 University of South Australia- Keep slapping on that sunscreen and ignore toxic claims (2018).
 Therapeutic Goods Administration- Literature review on the safety of titanium dioxide and zinc oxide nanoparticles in sunscreens (2017).
 Therapeutic Goods Administration- Sunscreens (2018).
 Cancer Council- Sunscreen FAQs (2018).
 Australian Radiation Protection and Nuclear Safety Agency- Sun protection using sunscreens (2018).