What is SPF

What is SPF and why is it so important?

1st April 2024 | Author: Abigail Beall

From how it works to what the numbers mean, here’s everything you need to know about SPF and why it’s so important 


What does SPF stand for? SPF stands for Sun Protection Factor.

How does SPF work? Sun protection creams contains active ingredients that either absorb, or reflect UV rays to stop them getting into your skin and body.

What SPF should I use? This depends on your skin phototype but the higher the SPF the rating the better.

How long does SPF last? This also depends on your skin phototype and SPF rating but you should ideally reapply every two hours regardless.

It’s drummed into us – and rightly so – that using SPF to protect our skin from sun damage is crucial to avoid cancer, at one end and premature ageing at the other.

Even in the depths of winter, it’s important to make an SPF part of your daily routine.

However, there’s been a lot of confusing, and often wrong, information being shared online recently, as well as content that could be classed as fear-mongering.

So to cut through the noise and dispel some of the myths, we’ve put together this guide to explain what SPF is, how it works, and why it’s so important?

What is SPF? 

SPF stands for Sun Protection Factor and refers to how well a cream, oil or lotion will protect you from the sun. In particular, it’s a relative measure of how long a sunscreen will protect you from ultraviolet rays, specifically UVB.

Light from the sun includes a broad spectrum of wavelengths, from visible light, the light we see, to infrared, the light that warms the planet, and ultraviolet (UV). If you think about a rainbow being the spectrum of visible light, infrared has a longer wavelength than red, and ultraviolet has a shorter wavelength than violet (hence their names).

“SPF is the most important step in our skincare regimes, and the SPF number is a measure of how well a sunscreen will protect our skin from harmful UVB rays,” said Dr Charlene DeHaven, iS Clinical’s Clinical Director. Without it, expensive skincare products become redundant as SPF is a sole defender against premature ageing, redness, inflammation, dryness and most importantly, skin cancer.

How does SPF work?

UV light comes in two variations, UVA and UVB – well, technically there is a third type called UVC but its short wavelength can’t penetrate the atmosphere so doesn’t reach us on Earth’s surface.

UVB causes sunburn and increases the risk of skin cancers by penetrating the upper layers of the skin. UVA also causes skin cancer, but in the deeper layers of the skin. Up to 80 per cent of these UV rays penetrate through clouds, even in the winter, so wearing SPF is vital for any skin exposed to sunlight at any time of the year. Even if you’re only popping out for a few minutes.

Sun protection creams tend to have two parts to them, an active ingredient – something doing the protecting, and an emulsion – an oil or cream that makes it easy to apply. Active ingredients then fit into two categories, UV absorbers and UV reflectors.

“UV absorbers are chemicals that absorb UV radiation and convert it to a very low level of heat,” explained Terry Slevin from Cancer Council Western Australia (CCWA). “So low most don’t notice it, but a small proportion of people do report sunscreens make them feel uncomfortably warm.” UV absorbers usually contain carbon.

If you’re using a UV absorber, look for one labelled “broad spectrum”, which will protect from both UVA and UVB. UV reflectors are usually oxides, such as zinc oxide or titanium dioxide, which scatter UV radiation and prevent it from reaching the skin.

READ NEXT: The science of moisturiser

Different types of SPF

There are two main types of sunscreen formulations – chemical and mineral.

Chemical sunscreens

  • How they work: Chemical sunscreens absorb UV rays, convert them into heat, and release them from the skin.
  • Ingredients to look for: Oxybenzone, avobenzone, octisalate, octocrylene, homosalate, and octinoxate.

Mineral Sunscreens

  • How they work: Physical sunscreens sit on top of the skin and reflect the sun’s rays. They are often best for sensitive skin.
  • Ingredients to look for: Contain mineral ingredients like zinc oxide and/or titanium dioxide.

♥︎ What do SPF numbers mean?

what SPF should I use formulamamabella | mamabella

An SPF number i.e. SPF 15; SPF 30, SPF 50, gives an indication to how long the sun’s UV radiation would take to redden your skin, which is measured by how much UV light is allowed to pass through, versus how long it would take if you weren’t wearing any SPF cream.


The Skin Cancer Foundation recommends a minimum of two tablespoons of “water-resistant, broad-spectrum sunscreen with an SPF of 30 or higher” applied 30 minutes before going out.

It also needs to be reapplied every two hours – regardless of how easily or not you burn – and always immediately after swimming or sweating.

SPF 20 allows one-twentieth of UV to reach the skin, or 5 per cent, blocking the other 95 per cent. On average, cream with an SPF 30 rating lets in around 3% of UVB rays, while an SPF 50 cream allows 2%.

This may seem like an inconsequential number but it means that SPF 30 creams allow 50% more radiation to reach your skin.

Theoretically, wearing lotion or creams rated SPF 30 would protect you from skin damage for up to 30 times longer than without protection.

READ NEXT: Best moisturiser with SPF

As an example, if you burn within 10 minutes of being in the sun, SPF 30 would protect you for around five hours, but it may protect someone else, who is less genetically likely to burn as quickly, for longer.

It’s not a hard and fast rule, however.

The intensity and wavelength distribution of UVB rays varies throughout the day and depends on where you are in the world. This rating also doesn’t apply to UVA rays.

To rate UVA protection, SPF creams additionally feature a star rating. It should be noted, however, that this star rating refers to the ratio of UVA protection compared to that same cream’s UVB protection.

It’s confusing, but in summary, it means you should opt for creams with high SPF and a high star rating.

FURTHER READING: What SPF should I use? Everything you need to know about choosing an SPF for your skin

Why SPF is important: Cancer

Skin cancerNHS

Exposure to UV light is the biggest risk factor for the three most common types of skin cancer, with or without sunburn. These skin cancers are called basal cell carcinoma, squamous cell carcinoma and malignant melanoma. Malignant melanoma, often referred to as simply melanoma, is the deadliest form of skin cancer.

Basal cell carcinoma (BCC)

Basal cell carcinoma (BCC) is the most common type of skin cancer. It grows slowly and is easily cured if caught early enough. However, it is still a serious condition and if untreated, can become life-threatening. BCC occurs when basal cells in the outer layer of the skin are damaged and start to grow out of control. They can look like red patches, shiny bumps, scars or open sores.

Squamous cell carcinoma

Squamous cell carcinomas are the second most common form of skin cancer and are caused by the abnormal growth of squamous cells, another type of cell in the skin’s top layer. SCCs look like red patches, sometimes itchy sores, rough patches, warts or growths. These can appear all over the body, and again, if allowed to grow they can become dangerous.

Malignant melanoma

Malignant melanoma is the most dangerous type of skin cancer and it begins in melanocytes, the cells that produce melanin. This type of cancer can spread much more quickly than BCC and SCC, to other organs, making it the most life-threatening of the three.

Melanoma is usually spotted by something new, changing or unusual on your body. For example, an existing mole that has started to change shape or a new one growing rapidly. The signs of melanoma can appear anywhere on your skin but usually somewhere that has had a lot of exposure to the sun. The Skin Cancer Foundation has a comprehensive list of early warning signs for melanoma.

The signs of all skin cancer types often look different from one person to another, so make sure to see a doctor if you are concerned about any changes to your skin’s appearance.

Why SPF is important: Ageing

We can’t see UV, but it can be incredibly damaging to our skin. This is because exposing skin to UV light causes the production of melanin, the pigment that makes skin darker and gives us a tan, while also promoting the creation of molecules called cyclobutane pyrimidine dimers (CPDs).

These CPDs bond with the molecules linking skin’s DNA together, particularly in the skin’s upper layer.

These bonds cause the skin to become thicker, and the connective tissues to break, which makes the skin less elastic and loose. This is why sun exposure causes wrinkles and saggy skin. It can also cause dark patches, know as age spots or liver spots.

In extreme cases, like sunburn, skin becomes inflamed a few hours after exposure to too much UV light. How susceptible someone is to sunburn is genetic. Melanin is produced to defend skin against the sun, but some people produce more than others – which is why some people tan quicker. People with lighter skin are more prone to sunburn. However, a tan and sunburn are both signs of UV damage to your skin. It can also lead to age spots, also known as dark spots or liver spots. Read more in our how to get rid of age spots guide.

DNA damage can also cause cells to grow abnormally, causing cancer.

READ NEXT: Best anti-ageing creams

♥︎ UV rays and eye care

How to protect your eyes from UV damageGetty Images/iStockphoto

The damage that UV rays can have on our eyes, as well as our skin, is often overlooked. Yet it’s equally damaging. It has been linked with a number of eye, and sight-related conditions ranging in severity, including:

Macular degeneration

Macular degeneration is a condition where the retina becomes damaged. This can lead to gradual sight loss. It’s most associated with older people but UV light has been shown to accelerate the process and the condition.


Cataracts are “clouds” that appear on the eye lens. These clouds can obscure your vision and impact your ability to focus on objects and they require surgery to remove. Studies have shown that exposure to UVB rays increases the risk of cataracts forming.

Pterygium or Surfer’s Eye

Pterygium is a benign growth that forms on the conjunctiva of your eye as a result of UV light. It’s nicknamed Surfer’s Eye because people who surf and are out in the sun for long periods of time are particularly at risk of developing pterygium due to exposure to both direct and reflected UV rays.


Typically caused by UVB light reflecting off snow, photokeratitis is caused by high short-term exposure to the rays. Essentially, the eye becomes sunburnt from this light, causing blurry vision or temporary sight loss.

The best way to protect your eyes from UV damage is to wear sunglasses that provide at least 99% UV protection and which comply with European safety regulations. According to Feel Good Contacts, “sunglasses with darker lenses offer the most protection, while wrap-around frames offer full coverage of your eyes from angles around the lenses.”

For extra protection, you can buy polarised sunglasses which protect against glare and halos from reflected rays. This can be extra useful when driving, or when near water.


Below are common questions asked about SPF.

What does SPF mean?

SPF means Sun Protection Factor and refers to the level of protection that a product provides against the UV rays and damage from the sun.

What SPF should I use?

The best SPF for you is one you’re most likely to use and which fits into your skincare routine and lifestyle. However, when looking for an SPF you should opt for one that is:

  • Broad-spectrum
  • With an SPF rating to suit your skin’s phototype
  • At a price you can afford.

You can read more about what SPF is best for you in our What SPF should I use? guide.

What SPF is sunblock?

Sunblocks typically contain active mineral ingredients, such as titanium dioxide or zinc oxide, that block UV rays. Sunblock sunscreens create a barrier on top of the skin to reflect the sunlight and its harmful rays.

Products with SPF 30 and higher are generally considered sunblock because they block around 97% of UVB rays.

As a guide:

  • SPF 20 allows one-twentieth of UV to reach the skin, or 5%, blocking the other 95%
  • SPF 30 lets in around 3% of UVB rays, blocking 97% of rays.
  • SPF 50 allows 2% to reach the skin, blocking 98% of rays.

Higher SPF levels offer increasing protection, but no sunscreen can block 100% of UVB rays and it’s important to apply a generous amount of sunscreen every two hours, or more frequently if you’re swimming or sweating, to keep protection at its highest.

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