What do SPF numbers mean?

SPF stands for Sun Protection Factor. The SPF is a laboratory derived ratio which measures the increased amount of radiation which causes redness in skin when the sunscreen tested is applied, compared to when it is not used at all. For example if it takes 10 minutes for unprotected skin to show redness, then an SPF15 sunscreen correctly applied, will take 15 times as long or 150 minutes to burn.

In reality, it is rare for a person to achieve this exact level of protection as factors like how much you apply, the weather and even your skin type will affect your level of protection. In fact, many Australians apply too little sunscreen. This results in sunscreen users achieving an SPF of between 50-80% less than that specified on the product label.