When used correctly, sunscreen can help protect against sunburn and damage to skin from UV radiation exposure. When choosing sunscreen, look for a broad-spectrum, water-resistant, sunscreen of at least SPF30.
But what do we mean by 'broad-spectrum' and 'SPF'? To help you make sense of these and other terms we've put together a list of frequently asked questions about sunscreen.
On this page, you will find information on:
- How does sunscreen work?
- What is a broad-spectrum sunscreen?
- What is SPF?
- How much protection does sunscreen give?
- How long can I stay in sun with sunscreen on?
- What is water-resistant sunscreen?
- Does sunscreen really prevent skin cancer?
- When and how do I put sunscreen on?
- What is the Ultraviolet Index (UVI)
- I put on sunscreen and I still got burnt. Why?
- How do I choose a sunscreen?
- How much sunscreen should I apply?
- Why do I need to apply sunscreen 20 minutes before going outdoors?
- Is sunscreen safe to use?
- Does sunscreen expire?
How does sunscreen work?
There are two main parts to all sunscreens. The active ingredient and the emulsion.
The active ingredient does the sun protection work. These come in two categories: UV absorbers and UV reflectors.
- are chemicals that absorb UV radiation and convert it to a very low level of heat. So low most don’t notice it, but a small proportion of people do report sunscreens make them feel uncomfortably warm.
- some absorb the UVB part of the spectrum, which is known to cause sunburn and contribute to skin cancer risk. Others absorb the UVA part of the spectrum. Recent research suggests the longer UVA wavelengths not only penetrate to deeper layers of the skin but contribute to skin cancer through compromising immune response to DNA damage.
- For that reason, sunscreen labelled “broad spectrum”, which contains both UVA and UVB absorbers, is recommended as it offers the best protection.
- are mostly made up of oxides, like zinc oxide and titanium dioxide, that absorb and scatter UV radiation.
There is normally more than one and often up to six or more active ingredients in most sunscreens.
The emulsion – the lotion, milk, cream, oil, foam or gel – is what carries the active ingredient. It is usually made up of some combination of oil and water, plus other goodies. These are important as they preserve the product so it lasts on the shelf or in your cupboard. They also help with water resistance, influence how the sunscreen feels and smells, and how well it binds to the skin.
What is a broad-spectrum sunscreen?
Broad-spectrum sunscreen gives extra protection because it filters out both UVA and UVB rays.
- UV radiation consists of UVA, UVB and UVC radiation. UVA penetrates deep into the skin, affecting the cells that lie deep under the skin’s surface. UVA causes ageing of the skin and long-term damage.
- UVB radiation penetrates the skin’s top layer, causing sunburn, and long-term damage. Both UVA and UVB contribute to the development of skin cancer.
- UVC radiation is absorbed in the upper atmosphere and does not reach the Earth.
What is SPF?
Sunscreen provides a screen, not a block. Think of a fly-screen door: air gets through but flies don’t. In the same way, the sun lotion or potion of your choice allows some small amount of UV radiation onto your skin.
SPF stands for sun protection factor. It’s the measure of how much UV gets through the screen. The higher the number, the less UV passes through.
An SPF of 30 allows one-thirtieth or 3.3% of UV to reach your skin. This means it filters 96.7% of UV. With an SPF of 50, 98% is filtered and one-fiftieth or 2% gets through.
So while the difference between SPF 30 and SPF 50 sounds like a lot – it is a pretty modest (1.3%) – difference in protection.
Put another way, if your unprotected skin would take ten minutes to show signs of burning, then properly applying SPF 30 sunscreen would slow the rate of burning to the point where it would take 30 times longer or 300 minutes in total. SPF 15 would take 150 minutes, while SPF 50, 500 minutes.
But this is perfect world stuff. If you extend your stay in the sun for 500 minutes (over eight hours!) only relying on sunscreen, you will very likely still burn!
SPF gives a general guide to sun protection but does not determine how long it will take for a person to be sunburnt. The amount of time it takes to be sunburnt depends on the level of UV radiation, and varies according to the time of day, the time of year, the weather, and the person’s skin colour.
How much protection does sunscreen give?
The protection a sunscreen offers is affected by its SPF rating, whether it is broad-spectrum, how evenly and how thickly you apply it, and how long you spend in the sun. The longer the time spent in the sun, the more UV radiation accumulates and the greater the potential for burning.
Even if you are not very active, sunscreen tends to rub off gradually and therefore needs to be reapplied regularly. This applies particularly to children because they tend to be active.
Remember though, you should not rely on sunscreen as your only form of sun protection. Over-exposure to UV radiation is the main cause of skin cancer, including melanoma. In addition to using sunscreen, seek shade and wear SunSmart clothing (including a wide-brimmed hat and close-fitting sunglasses). Read more about sun safety.
How long can I stay in the sun with sunscreen on?
It’s wise to stay in the sun no longer than is necessary to do your planned activity. Staying out longer just because you have the sunscreen “suit of armour” (which it is not) is a bad idea.
Even following all the best advice, the normal daily activity – wiping water from your eyes, scratching an itch, cuddling the kids, brushing against a tree or your best buddy – will remove sunscreen and diminish its performance. And remember it is screening, not blocking the sun.
And will you still get a tan if you put on sunscreen properly? Well, no. If sunscreen is properly applied to do its job of reducing UV radiation exposure, it prevents the biological process of tanning.
What is water-resistant sunscreen?
Water-resistant sunscreen is one that maintains its SPF in water (such as swimming or sweating), for a certain period of time. For example, a water resistance claim of two hours means the sunscreen should retain its full SPF protection for two hours in the water. However, it is wise to reapply sunscreen after any water sports, sweating or towel drying.
Does sunscreen really prevent skin cancer?
Yes. Sunscreen helps protect against UV damage (sunburn and tanning), as well as helping prevent skin cancer. Use sunscreen on uncovered skin.
There are three main types of skin cancer: melanoma (the potentially most dangerous form); squamous cell carcinoma (SCC), which often grows slowly, but if not treated can spread; and basal cell carcinoma (BCC), which is usually curable. Solar keratoses are rough, scaly spots that develop on skin that has had a lot of sun exposure. People with solar keratoses have an increased risk of skin cancer.
When used correctly, sunscreen can protect against sunburn and DNA damage to skin from UV radiation exposure. Sunburn, especially in childhood, is a risk factor for melanoma. Preventing sunburn may help reduce melanoma risk and skin damage.
There is evidence that regular sunscreen use may protect against squamous cell carcinoma (SCC). The information is less clear about the extent to which sunscreen prevents melanoma and basal cell carcinoma (BCC). Regular sunscreen users have shown a reduction in development of solar keratoses. Naevi (a type of mole), which frequently develop during childhood, increase the risk of melanoma. It has been shown that regular use of sunscreen in childhood can reduce the development of naevi.
When and how do I put sunscreen on?
At a microscopic level, the skin is a series of peaks and troughs. Layering on sunscreen around 20 minutes before going into the sun allows the product to flow into the troughs and bind properly to the skin.
Many sunscreens recommend reapplying every two hours. But another way to look at it is like painting a wall of your house. The first coat gets a reasonable coverage, but a reapplication 20-30 minutes after being in the sun – after the first coat has “dried” – gets you much more reliable coverage. And this will cover the bits you may have missed, or covered too thinly, on first pass.
Also, use it generously. Most people use too little (between a quarter and three-quarters) of the amount of sunscreen necessary to achieve the sun protection claimed on the label. A teaspoon per limb is a good rule of thumb. Add another teaspoon for your face, front and back. This comes to seven teaspoons (35ml) in all if you are at the beach in board shorts or a bikini.
Layer it on and spread it around. Reapply every two hours or more often if you are active (sweating, towelling off, skin making physical contact with anything that might rub it off), even if the bottle claims four-hour water resistance. And a good idea is to check if the lotion hasn’t passed its use-by date.
Use other things to protect your skin too. Hats, shade, clothing and even staying indoors at the highest UV periods. The closer to solar noon, usually between midday and 12.30 pm, the higher the UV.
What is the Ultraviolet Index (UVI)
The Ultraviolet Index (UVI) is an international, scientific measure of the level of ultraviolet radiation in the environment. The higher the number, the greater the risk of skin damage.
I put on sunscreen and I still got burnt. Why?
Make sure you are using enough sunscreen. The average-sized adult needs about 7 teaspoons of sunscreen for one full-body application. Put sunscreen on 20 minutes before you go outside and reapply it every 2 hours, as well as after swimming or sweating.
How do I choose a sunscreen?
As general guidance, the Cancer Society of NZ recommends the use of broad-spectrum, SPF30+ sunscreen. Broad-spectrum sunscreen reduces the intensity of both UVA and UVB rays. If you have fair skin that burns easily you should choose an SPF 50+ broad-spectrum sunscreen. Read more about how to choose sunscreen.
How much sunscreen should I apply?
Use a generous amount of sunscreen – most adults will need at least 7 teaspoons (35 millilitres) for a full-body application. Apply half a teaspoon to each arm, the face, neck and ears, and just over one teaspoon to each leg, the front of the body and your back. Read more about how much sunscreen to apply.
Why do I need to apply sunscreen 20 minutes before going outdoors?
Applying your sunscreen at least 20 minutes before you go out gives it time to sink into the skin’s surface. Sunscreen application is a bit like painting – your first coat is going to look a bit patchy, so it’s worth applying a second coat for better coverage.
Is sunscreen safe to use?
To date, there is no scientific evidence showing long-term side effects following regular use of sunscreen.
Short-term side effects may include reactions, such as skin irritation, stinging or a rash. If these side effects occur, try another brand and look for products that are fragrance-free, or labelled as suitable for sensitive skin. Products containing titanium dioxide and zinc oxide may be the most suitable.
Some sunscreens can have a detrimental effect on car and other pre-painted roof and cladding products, so try to avoid contact between the hands and these surfaces.
Does sunscreen expire?
Yes. Check the expiry date and storage conditions on the label. Most sunscreens last about 2 to 3 years. They should be stored below 30ºC. If left in excessive heat (e.g. in the glove box of a hot car or in the sun on the beach), over time, the product may not be effective.
- Literature review on the safety of titanium dioxide and zinc oxide nanoparticles in sunscreens. The Australian Therapeutic Goods Administration. Version 1.0, August 2013
- Dermal absorption of nanomaterials. Danish Environmental Protection Agency, 2013
Terry Slevin, Adjunct Professor, School of Psychology, Curtin University; Education and Research Director, Cancer Council WA; Chair, Occupational and Environmental Cancer Committee, Cancer Council Australia