How good a sunscreen is at protecting you from UV radiation is measured by its sun protection factor or SPF.
SPF measures HOW LONG it will take for UVB rays to redden the skin when using the sunscreen, compared to how long it will take without sunscreen.
Ministry of Skin is a RACGP accredited clinic.
How long it protects you for will therefore be very different for each individual and could vary daily dependent on the suns intensity. For example, in some conditions it may take you 20 minutes to redden, whereas on another day it could take you only 10 minutes.
The number of the SPF gives you this time scale. For example, SPF 15 means its will take 15 times longer to redden your skin than it would take without sunscreen.
SPF 30, means it will take 30 times longer to redden your skin than it would without sunscreen.
Therefore if it usually take you 10 minutes to go red without sunscreen, with SPF 15 it will take you 150 minutes to go red (15×10). So reapply sunscreen at least every 150 minutes.
However if it takes your friend 5 minutes to go red without sunscreen , with SPF 15 it will now take 75 minutes to go red (15×5). Your friend will need to reapply the sunscreen more often than you.
An SPF 15 will screen 93% of the suns UVB, whilst an SPF 30 will screen against 97% and an SPF 50 will screen against 98%. For this reason current recommendations are to use a broad spectrum SPF 30.
UVB and UVA
Sunscreens currently only have a UVB protection rating via the SPF system.
However, you need to protect your skin against both UVA and UVB rays as both are known to be harmful. To ensure you are getting this, a product should also contain a UVA screening ingredient such as – stabilised avobenzone, ecamsule, oxybenzone, titanium dioxide and zinc oxide. This will often be labeled on the packaging as Broad Spectrum or say UVA Protection. Currently there is no rating system for UVA protection.
The protective ingredients found in sunscreen are either chemical ie they form a film on the skin which absorbs UV light before it penetrates the sun, or physical, which reflect the UV rays away from the skin. Most sunscreens contain a mixture of both ingredients.