03.SPF and UPF
SPF: Sun Protection Factor
This is the factor by which the sunscreen increases the time it takes you to get sunburnt. A properly applied coat of SPF 30 sunscreen will keep you burn-free for 30 times longer than normal, so if you’d normally get mild sunburn in 10 minutes, the SPF 30 sunscreen will in theory keep you burn-free for 300 minutes, or five hours. (Not that it’s wise to spend that much time in the sun even with sunscreen on, especially if your skin is really so fair it would normally start to burn in 10 minutes. It just gives you a sense of the safety margin sunscreen can give you.)
In any case it’s a good idea to reapply sunscreen every two hours, because water and sweat wash away the coating on your skin.
The highest claimable SPF rating in Australia is 30+. This isn’t because SPF 30 is ‘good enough’ (although it does block 97% of the sun’s rays), but rather because the SPF measurement method isn’t completely reliable once the SPF exceeds 30. Some products labelled SPF 30+ may be much higher.
SPF multiplies, so if you’ve covered yourself with SPF 30 sunscreen, then put on a light shirt (the equivalent of say, SPF 5), you’ve got SPF 150 protection.
UPF: Ultraviolet Protection Factor
This rating is for clothing and shadecloth: a shirt with a UPF of 20 allows only one twentieth of the UVR through to your skin. UPF ratings go up to 50, and a fabric with a rating higher than 50 is labelled as UPF 50+.
Different fabrics have different UVR-absorbing properties: generally, the thicker the fabric, the tighter the weave and the darker the colour, the more UVR is blocked. Clothes and swimsuits are available in high-UPF fabrics; look for a label showing they pass the Australian standard AS/NZS 4399.