SPF stands for sun protection factor and is the unit of measure used in sunscreen to indicate the relative amount of time you can endure exposure to the sun’s rays before they begin to cause damage. Let’s take a look at what that means in practical terms and how SPF is even determined.

How Sun Protection Factor Works

One of the hardest things to understand about SPF is that it is a relative thing. You will have less time in the sun on a sunny July day in the early afternoon than you will on a sunny January morning just after sunrise. There’s a tendency to want to equate it with something concrete like time, but it can’t work that way. You can’t get an SPF 100 sunscreen and say “this will let me spend 100 minutes in the sun” because that’s not exactly how it works.

How SPF Protects From UVA Rays and UVB Rays

When people say sunscreen protects from the sun, what they specifically mean are UVA and UVB rays. The ultraviolet light produced by the sun is a form of radiation, just like x-rays and gamma rays. The difference between UVA and UVB is the wavelength of the light. UVA rays have a longer wavelength and penetrate deeper into the exposed skin. UVB rays have a shorter wavelength and therefore affect skin closer to the surface. Both wavelengths are shorter than visible light, which is why they fall on the ultraviolet spectrum and we can’t see it with our eyes.

SPF refers to the amount of UV radiation that will burn you when you have sunscreen on compared to when you have none on. So an SPF of 30 lets you handle 30 times the amount of sun exposure as you could with no sunscreen. High SPF sunscreens offer more protection from harmful rays and can prevent more skin damage. But you need to remember that this is not a duration of time and that’s where the relative part comes in. You can’t necessarily spend 30 times longer in the sun, you can just absorb 30 times the radiation.

The amount of UV rays you’re exposed to can change based on things like cloud cover, time of year, and time of day. The sun you experience at 8 in the morning in January is very different from the sun you’re exposed to at one in the afternoon in July. So, like we said, while you could handle hours of sun exposure on that January morning, you would probably burn in 15 minutes in July.

This is very relevant when you factor in something like tanning beds. People still believe tanning beds are safer than sun tanning when the exact opposite is true. You can get up to 5 times as much UV exposure from a tanning bed than you would get naturally. That means if you put on an SPF 30 sunscreen and went in a tanning bed, it’d be equivalent to something like an SPF 6 and you could suffer damage very quickly.

SPF 30 will prevent 97% of UV rays from hitting you. SPF 50 will prevent 98% of them. But SPF 15 only stops 93%. That still sounds good, right? Well, consider this. All of those SPF numbers were created for us in lab tests. To determine how protected a person is, the tests involve using a 2mg per square cm thick layer of sunscreen on the skin, which is what is the “recommended” usage. However, in practical tests it’s been shown most people never use anywhere near that much sunscreen. Most don’t even know that’s how thick it’s supposed to be. And, in fact, most of us are getting about 40% of the coverage we should be getting from sunscreen.

Based on those numbers, using an SPF 15 sunscreen means you’re basically using an SPF 6 and that is barely worth putting on at all. Better than nothing? Sure. But still barely anything. And that’s why we say SPF 30 is the bare minimum you should be using. If you’re going to spend the time and money on sunscreen, get something stronger. It makes more sense economically, and it will do the job of protecting you from harmful UV rays that much better.

How Does UVA Relate to Skin Cancer?

SPF only tells you about UVB protection. When you read the SPF rating on sunscreen it only lets you know how well it protects against UVB rays and, in fact, it may not offer UVA protection at all. This is thanks to the fact that, for many years, people thought UVA rays were less dangerous. UVA was long associated with premature skin aging while UVB was thought to be the one that causes burns and therefore cancer.

Later research showed both kinds of the sun’s uv radiation can cause cancer. However, the sunscreen industry was established when UVB was the big enemy. That means not all the ingredients in sunscreen will protect against UVA and some brands may offer no protection at all.

Mineral sunscreens do protect against UVA as well as UVB. Also, chemical sunscreens that contain things like avobenzone, oxybenzone, and dioxybenzone offer good protection. In fact, zinc oxide and avobenzone are considered the best protections against UVA while there are many chemicals that protect against UVB. There are potential downsides to the UVA blocking chemicals though and more research is being done to see if they are harmful to the environment as well as to humans. That said, there are reef safe sunscreens that are potentially more safe for use and for the environment. These are often mineral sunscreens.

A sunscreen must say it’s broad spectrum if you want it to protect against UVA and UVB. Mineral sunscreens do this already, as we said, but you need to look for that on chemical sunscreens as well. The SPF on a chemical sunscreen label only refers to UVB if it doesn’t say it’s broad spectrum.

Are There Different SPF Rules?

Different countries regulate SPF differently on labels. As I mentioned above, SPF is determined based on using a 2mg per square cm layer of sunscreen and lab tests have shown you may only be getting 40% protection. In Australia, however, for a sunscreen to call itself SPF 50 it needs to actually be better than SPF 50.

You may see sunscreens that say SPF 50+. In Australia, it needs to actually be about SPF 60 or higher to get the SPF 50+ rating. In other words, SPF 50 is what it has to be at the very least.

American sunscreen currently does follow these regulations but you may still see labels that read SPF 60+. This means that the formulation is at least SPF 60 but there’s not a definite upper range being established. I’d recommend you treat anything labeled as SPF 60+ as just being SPF 60.

How Do They Determine SPF?

A scientific formula established in lab testing is used to determine the SPF of any given sunscreen. We’ve explained how sunscreen works in this article but there’s still some science left to discuss regarding SPF. We know that SPF refers to how much longer you can stay in the sun with the sunscreen on compared to being unprotected. In order to get a specific number for a specific formula, it needs to be tested practically.

The SPF number of sunscreen is determined by a mathematical formula. A patch of untreated skin is exposed to UV rays. The number of seconds it takes to turn red as a result is then recorded. Next, the number of seconds it takes a patch of skin treated with the sunscreen to turn red is recorded.

Let’s say it takes you 20 seconds to start to turn red on your untreated skin. With sunscreen on it takes 10 minutes, or 600 seconds. To get the SPF you would use the formula 600/20. In this case the SPF would be 30 because your skin takes 30 times longer to suffer effects of the sun with it than without.

How Does SPF Relate to Skin Tones

The relative power of sunscreen is especially relevant to people with certain skin tones. Not only is SPF relative to the power of the sun during your exposure, it’s also relative to you personally. Consider two people having a day at the beach in mid-July. One has very dark skin and the other is extremely pale. If they both put on the same sunscreen, who is going to burn first? It’s absolutely going to be the person with the pale skin.

SPF improves your skin’s ability to withstand the sun so you need to be aware of what that mans personally. People with darker naturally have more melanin in their skin. Melanin is your body’s natural uv fighting pigment. It can absorb UV rays and protect skin just like the compounds in sunscreen do. The more of it you have the more naturally protected you are. This is why people with darker skin tones are less prone to sun burns, because their body is naturally able to defend against sun more easily.

An extremely pale person with blonde or red hair and blue eyes has very little melanin in their body. When they are exposed to sun they may not even tan and instead will burn right away. A tan happens when melanin forms in your skin, that’s where the pigment of a tan comes from. But if you don’t make much or any, you skin the tan and just burn.

So an SPF 30 sunscreen will seem to better protect someone with darker skin because their darker skin already offered more protection. This is important to remember if you are comparing how long your sunscreen’s SPF works for you compared to someone else with a different skin tone. It will seem to fail faster for those with lighter skin tones because they have no natural protection from their own melanin.

The Bottom Line

SPF refers to sun protection factor and explains how much longer you can stay in the sun with sunscreen on compared to if you have no sunscreen on. You need to take into account the intensity of the sun and your own skin’s sensitivity to the sun to determine exactly how much time SPF will give you. Broad spectrum sunscreen is necessary to offer protection from both UVA and UVB rays as SPF only refers to how well a sunscreen protects against UVB rays. Remember, all exposure to sun is damaging and the effects are cumulative so you should always be wearing sunscreen when you go out. As always, stay safe and have fun.