Whether you're a skincare devotee, or an absolute newbie, the most reliable and loyal friend in your skincare arsenal is SPF.

But what is SPF? While we know it's important, most of us don't actually give much thought to how sunscreen actually works.

So we decided to take a deep dive into the complex world of sunscreen science to unveil everything you need to know about sunscreen, from the secrets behind the mysterious "+", to just how different SPF50 and SPF30 are, and finally, which sunscreen is best for you.

So, what is SPF?

First, let's demystify the acronym. You'd be forgiven for thinking the "S" in SPF stands for sunscreen, but not quite. SPF refers to Sun Protection Factor.

This is why "SPF" is always followed by a series of numbers. These numbers, from 15-50 are the SPF rating, that is, the level of sun protection you're receiving while wearing sunscreen.

The higher the number that follows SPF, the better level of protection you're receiving from the sun's ultraviolet rays.

What is SPF protecting us from?

Sunscreen is packed with ingredients designed to protect us from the sun's UV radiation. There are 2 types of UV rays we should be concerned about when talking about sun protection — these are UVA and UVB rays.

UVA rays are typically associated with skin ageing. Present rain, hail or shine, they have longer wavelengths than UVB rays, meaning they can reach you on cloudy days and through windows.

This is why it's important to finish your skincare routine with a broad spectrum high SPF sunscreen — that means applying SPF after your moisturiser — even if you're planning on spending your whole day indoors or the weather outside looks bleak.

On the other hand, UVB rays are associated with sunburn and skin cancer [1]. So how do you protect exposed skin from both UVA and UVB rays? Broad-spectrum sunscreen provides broad-spectrum protection, shielding skin from both forms of ultraviolet radiation.

How does SPF work?

There are 2 kinds of sunscreen, chemical and physical, and they protect the skin in slightly different ways.

Chemical filters are called "UV absorbers", while physical filters are called "UV reflectors." UV absorbers are used in chemical sunscreens. They absorb UV rays converting them to a very low level of heat.

UV reflectors are found in physical sunscreens. They are made up of compounds like zinc oxide and titanium dioxide, and they deflect and scatter UV rays, think of them like tiny mirrors [2].

While there are 2 categories of filters, there are many different filters within these categories.

There are currently 33 active ingredients approved for use by the Australian Therapeutic Goods Administration (TGA) the body that regulates all sunscreens in Australia, and most sunscreen products will use a mix of filters, as each one works slightly differently. By using a combination, sunscreens can offer more protection [3].

What does SPF30 versus 50 mean?

The SPF rating on your sunscreen indicates the amount of radiation your skin absorbs from the sun's rays while wearing sunscreen.

The numbers refer to the amount of time it would take prolonged sun exposure to cause sun damage if the product were applied correctly. So, theoretically, wearing an SPF30 sunscreen should mean you take you would take 30 times to burn sitting in the sun than if you were wearing no sunscreen (this is slightly misleading, though, more on that later).

So, how big is the difference between SPF30 and SPF50? Things get a little complicated here, so bear with us.

An SPF30 rating indicates a sunscreen has been shown to filter roughly 96.7% of the UVB rays and 3.3% of UV rays. An SPF50 rating means the sunscreen provides an estimated 98% filter for UVB radiation [4].

You may think there's not much difference between a 30 and a 50 but as noted by the Skin Cancer Foundation, this very small percentage actually means an SPF30 is allowing 50% more UV radiation onto your skin [5].

In summary? Although any sunscreen is better than no sunscreen at all, it is always better to reach for an SPF50+ broad spectrum and water-resistant sunscreen if you can.

What does the plus mean in SPF50+?

Noticed the mysterious 'plus' sign after the number on your sunscreen and wondered what it's all about? The plus sign on Australian SPFs indicates "more than" [6].

The Australian Therapeutic Goods Administration restricts advertising of sun protection factors over 50. This is why even though you may see higher SPFs, even up to 100 on shelves in Europe and parts of Asia, you won't see them in Australia.

However, in Australia, you will see some SPF 50 sunscreens with a '+' at the end. This means the company has tested the sunscreen and registered at least a 60.

Some SPF companies do this to allow for the fact that slight variations in batches mean sunscreens may have slightly variable SPF ratings. When you purchase a sunscreen like Software's Daily Facial Sunscreen SPF50+, you can consider the '+' a tick of assurance that you are getting at least SPF 50.

What to be aware of when wearing sunscreen

While sunscreen is the most important product in your skincare routine and should be used daily, you shouldn't let the fact you're wearing sunscreen give you a false sense of security.

Studies have shown that people tend to spend longer periods of time in the sun, with otherwise unprotected skin, when they are wearing sunscreen — and there have been correlations drawn between this behaviour and higher rates of skin cancer [7].

No sunscreen provides 100% protection from skin damage, and they are not fail-safe. As flagged above, sunscreen is tested under "ideal" conditions. This means an exact amount of sunscreen is applied completely evenly, it is allowed time to settle, and it is not rubbed, exposed to sweat or water, or your foundation and concealer.

In reality, sunscreen is usually applied imperfectly, meaning it's stated SPF protection is usually somewhat below that on the tube. Even if it is applied perfectly, it still allows some rays in and needs to be reapplied liberally throughout the day.

Your best insurance against sun damage is sun avoidance. That means wearing protective clothing during the day and making sure you seek shade when the UV is highest, between 10am and 4pm.

It's also essential to ensure you are applying sunscreen correctly. Sunscreen should be applied 20 minutes before you go into the sun as this allows its protective barrier to settle.

Your SPF should be applied liberally to ensure maximum protection. How liberally? The Cancer Council recommends 5mls — around 1 teaspoon — for each part of the body (arms, legs, front, back and face) [8].

Whether using a water-resistant spf or not, you should apply sunscreen every 2 hours and reapply your sunscreen after swimming or sweating. And, don't forget that like any other product, your sunscreen can expire and become less effective, so be sure to check this before using.

What to look for when choosing a sunscreen

So, shopping for sunscreen? It's important to find a sunscreen that's perfect for your skin type and lifestyle, as it will be the most consistent feature in your skin routine (it's the one skincare product you're putting on your face every 2 hours).

While high SPF sunscreens are ideal, those with oily or sensitive skin sometimes opt for SPF30 sunscreens or physical sunscreens. If you have very dry skin, you may want something more hydrating.

Fortunately, plenty of lightweight, non-greasy and hydrating formulas on the market tick all of these boxes. One such is Software's Daily Sun Defence SPF50+, which has been formulated to provide lightweight protection with maximum UVB and UVA protection.

Non-comedogenic and fragrance-free, this sunscreen works for all skin types, and importantly, it's Australian-made, which means it has been tested and formulated to perform under the harsh Australian sun.

So, there you have the science behind our best-loved skincare sidekick. Now, whether you're hitting the beach, running errands or out to brunch you have the information you need to ensure you're well protected. Stay sun-smart, skincare lovers!

Photo credit: Getty Images