View previous topic | View next topic

SPF -10

Page 1 of 1

NinOfEden
903770.  Mon Apr 23, 2012 11:34 am Reply with quote

Today, I've been thinking about sun lotion.
I bought a new bottle today as it was going cheap, + although I have a bit left it's a few years old; I've looked at it's sell-by date, it actually expired in 2007. I read a magazine article once that said sun lotion's SPF falls by about 7 per year, which means (if it's true) my old one now has an SPF of -10!
This set me off thinking, what IS SPF? What substance do they put in sun/anti-ageing cream that stops the sun damaging your skin, + how does it work? How is the factor number measured, + is it very accurate? Does it really become less effective over time, + if so, how?
How do Avon get away with charging 16 a bottle when most supermarkets do it for less than half that? I paid 3.50 for factor 30 today.

 
Spike
903787.  Mon Apr 23, 2012 12:05 pm Reply with quote

I believe suntan lotions have about a 3 year shelf life. Heat degrades the UV absorbing chemicals, so the SPF drops.

This Wiki article has details on how Sun protection works.

 
suze
903825.  Mon Apr 23, 2012 1:57 pm Reply with quote

NinOfEden wrote:
This set me off thinking, what IS SPF? How is the factor number measured, + is it very accurate?


SPF stands for Sun Protection Factor. The formula for calculating it is rather complicated and includes integrals, but the higher the number, the longer one can stay in the sun without burning. The formula was devised by an Austrian chemist called Franz Greiter, who more or less invented sunscreen in the 30s and later started the Piz Buin company.

There's a common belief that the number indicates how much longer one can stay in the same. So for instance, if you would burn after one minute in the sun, then SPF 30 allows you thirty minutes, SPF 50 allows you fifty minutes, and so on. That's not actually how the numbers are worked out at all, but people have drawn complicated graphs to show that it's not a million miles away. (Unless you go swimming, because the water washes some of the sunscreen away.)

In the USA, the SPF level of a new sunscreen product is actually determined by applying the stuff to a person and then seeing how long it takes him to burn! In Europe it's done using lab machinery.

Nin wrote:
What substance do they put in sun/anti-ageing cream that stops the sun damaging your skin, + how does it work?


There are several chemicals used for the purpose. Several of the leading brands use either titanium dioxide or zinc oxide, which reflect light and so stop the rays reaching your skin. Other brands use "designer" chemicals, especially developed in labs at huge expense to have even better sun reflection or absortion properties.

Nin wrote:
How do Avon get away with charging 16 a bottle when most supermarkets do it for less than half that? I paid 3.50 for factor 30 today.


For much the same reasons that Perrier can put fizzy water into a fancy green bottle and charge ten times the price that Tesco does for a bottle of fizzy water.

 
NinOfEden
904024.  Tue Apr 24, 2012 12:59 pm Reply with quote

Cool, thanks! :-D

 

Page 1 of 1

All times are GMT - 5 Hours


Display posts from previous:   

Search Search Forums

Powered by phpBB © 2001, 2002 phpBB Group