Australia has a maximum Sun Protection Factor rating of 30+, but other countries have higher ratings, why ?
A quotation from the Cancer Council will start us off:
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.
So, an SPF of 15 = you may be able to stay in the sun for 150 minutes
and, an SPF of 30 = you may be able to stay in the sun for 300 minutes
and, an SPF of 50 = you may be able to stay in the sun for 500 minutes
Based on that theory, I can now see why Australia does not allow any SPF 50 labels on Suncream, it may give a false sense of security.
The cancer council go on to say: “SPF30+ allows transmission of 3.4% of UV radiation”
Then it says:
SPF testing protocols are, unfortunately, not universal. Therefore SPF ratings on UK/EU sunscreens cannot be compared directly with SPF ratings of Australian/NZ sunscreens. The SPF rating for Australian/NZ sunscreens is determined ‘post immersion’ using an Australian standard method which subjects the sunscreen to the SPF test after it is applied to the skin AND after immersion in turbulent water under controlled standard conditions.
On the other hand, UK/EU SPF test protocol is ‘pre immersion’, which means that the sunscreen is SPF tested, without the skin surface that has had the sunscreen applied to it, being immersed in turbulent water BEFORE the SPF test.
In other words the Australian SPF test protocol is more robust than the UK/EU procedure and more relevant to Australian conditions.
The Cancer Council website with this information is at: http://shop.cancercouncil.com.au/t-faq.aspx