What does the SPF rating really mean?

Summer’s almost here, so once again it’s time to reach for the sunscreen. For most people, the default is to grab the bottle with the highest SPF. Bigger is better, right? But what do those SPF numbers actually mean?

SPF stands for Sun Protection Factor, and the number beside it indicates how well the sunscreen protects skin against sunburn. It is not an indicator of how long you can stay out in the sun, rather, it indicates how much longer it takes untanned skin to start to redden with sunscreen applied compared to how long it takes to start reddening without it.

To work out the SPF of a sunscreen, laboratory tests are carried out on an untanned patch of skin (such as the buttocks) of human volunteers. Sunscreen is applied liberally to the skin, which is then exposed to simulated sunlight via UV lamps. Measurements are taken of how long it takes the skin to get a minimal burn when covered with sunscreen, and how long it takes to get the same minimal redness without it.

To get the SPF number, a simple formula is used. The number of seconds it takes a patch of skin to slightly redden when covered in sunscreen is divided by the number of seconds it takes to slightly redden when there is no sunscreen applied. Say it took 300 seconds for skin to burn with sunscreen, and 10 seconds to burn without it. 300 is divided by 10, which is 30. The SPF is 30.

Under current Australian regulations, sunscreens must have an SPF significantly higher than 50 in order to be rated at 50+ (the plus means the rating is ‘at least’ the value given). A rating of 51 won’t cut the mustard; the sunscreen needs to have an SPF of 60 or more to be compliant.

While the above laboratory test is pretty good at determining a sunscreen’s SPF, it only gives an indication of how well the sunscreen protects skin against UVB radiation, not the deeper-penetrating UVA. If you want protection against both, you’ll need to check that your sunscreen is ‘broad spectrum’. Sunscreens that have this on their label have undergone additional laboratory testing. The usual method is one known as persistent pigment darkening (PPD), but there isn’t a universally recognised way of measuring UVA protection.

There are a couple of other things that are good to know when it comes to a sunscreen’s SPF. The first one is that some UV radiation can always get through to the skin, no matter how high the SPF. No sunscreen can ever block sunburn and associated skin damage completely; sunscreen is only one part of effective sun protection, and not a front-line defence.

The second is that SPF is determined by tests in which a liberal amount of sunscreen is applied. That means that, to actually achieve the protection indicated on the label, you need to apply sunscreen thickly enough—and many people don’t. You also need to reapply the sunscreen every two hours, as it can come off through sweat or by being rubbed off by clothing.

Finally, be sure to check the ‘use by’ or ‘expiry’ date of any sunscreen before you use it. If it’s out of date, some of the active ingredients may have broken down and the sunscreen won’t work as well. And don't forget to shake it before applying if it's not a thick cream.

All this means that a sunscreen’s SPF is more an indication of how well you could be protected, rather than an iron-clad guarantee.

So, when it comes to a sunscreen's SPF, is bigger really better? 

Mathematically the answer is yes – all things being equal. A broad-spectrum SPF 50+ sunscreen filters 98% of UV radiation, slightly more than the 96% of UV that is filtered by SPF 30+ sunscreens. But in practice, the effects of applying the sunscreen too thinly, not reapplying after you have sweated or swam can reverse their relative protection, while seeking out shade or wearing sun protective clothing could also override differences in the SPF of sunscreen.

So what should you do? There’s no debate that adequately-applied broad spectrum SPF 50+ can provide the best sunscreen protection against UV radiation, but as we’ve noted, even that won’t block all the damaging radiation.

However, some protection is better than none. If you’re using a moisturiser that happens to be SPF 30+, that’s still giving you some protection as you walk between the car and your office, as long as you've applied enough.

Sunscreen should be just one part of your summer routine. You also need to slip on a shirt, slap on a hat, seek out shade and slide on some sunnies. Roll on summer!