Re: Sunscreen/Sunblock … how much SPF?
here is some good reading in order to get correct information 
http://www.gosunsmart.org/yourskin/yourskin_protection.shtml
SPF Math
SPF Number x Time to Burn Without Sun Protection = Time to Burn while wearing sunscreen*
*assuming that sunscreen is applied properly
Example: If your skin would burn in 10 minutes in the afternoon sun without any sun protection, and you applied a sunscreen with an SPF 15, you would have 15 x 10 = 150 minutes (2.5 hours) before you would burn.
Percentage of Protection from damaging UV rays:
SPF 15 = 92%
SPF 30 = 97%
SPF 40 = 97.5%
There is some controversy about very high SPF numbers and just how much more protection they provide. As you can see from the percentages of protection of the different SPF numbers, the difference in UV ray protection in SPF numbers becomes minimal as the number increases past about 30 or 40. The difference of protection between SPF 15 and SPF 30 is great (5%), but the difference between SPF 30 and SPF 40 is minimal (0.5%).
The Confusing Part About Sunscreen
Many people think that SPF values can be added. Actually, SPF values cannot be added – if you apply a sunscreen of 8 and then one of 12, you will not have the protection of an SPF 20. You will only be getting the protection of an SPF 12.
Reapplication only helps to ensure that you have the amount of protection and time before you burn that you calculated when you first applied sunscreen that day. Since no sunscreens can protect your skin from all UV rays, some rays will get through.
After your calculated “time to burn” has expired, your skin has absorbed all the UV rays that it can handle before it will start to burn. Even if you reapply sunscreen at this point, you will still burn because some rays will get through to your skin and your skin has already been saturated with UV. Therefore, you need to get out of the sun at this point and let your skin rest before exposing it to more rays.
Reapplication only ensures that you have your original time outside – sunscreen can wear off because of sweat, wind, and other factors you need to reapply in order to be protected.
So what’s the lesson to be learned about sunscreen? If you know that you will be in the sun for a long period of time, start with a higher-SPF sunscreen in order to have protection for the entire time that you are exposed.
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Sunscreen vs. Sunblock
Sunscreen contains chemicals that absorb UV rays before they damage the skin and sunblocks contain particles that physically block UV rays from reaching the skin.
Sunscreen needs to be applied about 30 minutes before sun exposure so that the chemicals have time to soak into your skin and form a layer of protection.
Sunblock does not need to be applied before going into the sun – it will begin to work as soon as you rub it on because it sits on top of the skin.
Environmental Factors Related to Sunscreen Performance
Certain conditions can actually increase the intensity of UV rays and put you at greater risk for skin damage. These conditions may also decrease the time that sunscreens keep you safe in the sun:
Higher altitudes
Low latitudes
Clear, sunny days
Mid-day sunshine (10am-3pm)
When any of these factors are present, you should be sure to wear a sunscreen with an SPF 15 or higher and re-apply your sunscreen every 2 hours – or better yet, minimize UV exposure by wearing cover-up clothing, a hat, sunglasses or goggles, and seeking shade when possible.
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Sunscreen Use
Always make sure that your sunscreen is SPF 15 or higher, “broad-spectrum,” and waterproof or sweatproof.
A generous amount of sunscreen should be applied to all exposed skin (do not forget your ears and the back of your neck.) In the summer, it is recommended to use one ounce of sunscreen – the amount to fill a shot glass – to cover your exposed skin.
Allow at least 30 minutes for your sunscreen to absorb before you head into the sun so that the chemicals can form a protective shield.
Be sure to reapply another layer of sunscreen if you will be in the sun for more than 2 hours throughout the day under intense UV conditions, and especially when it’s windy or you’re sweating.
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