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What is SPF?
This stands for Sun Protection Factor
What does it protect from?
It mainly protects against ultraviolet (UV) radiation of the type that
causes sunburn - UVB. It offers little protection against UVA, the type of
UV that is mainly responsible for skin cancer. See the
page about Sunscreens for more information
about proposals to replace the SPF classification with something better.
What do the numbers mean?
They mean the number of minutes, per day, that you can expose yourself
to strong sun without coming to harm. The amount of time varies from
individual and depends on how rapidly they burn without protection. Fair
haired, especially red haired, people burn the fastest, say in 15 minutes
they start to turn a little red. If the SPF factor they are using is 20,
then they can be in the sun no longer than 15 minutes x20 = 300 minutes = 5
hours. This is a per day amount and CANNOT be increased by simply applying
more cream! However, if you go swimming and some of your original cream
comes off, then replace BUT, you do NOT get another 15x20 minutes. You have
left what remains from your original application.
In practice
Although you may prevent burning, as indicated above, you don't get the
same protection from the cancer causing UVA. Thus, KEEP WELL BELOW THE
CALCULATED TIME. When the new classification starts to come into effect in
2007, then the advice above will change.