Assuming the question is asking about sunscreen SPF, it stands for sun protection factor. This is put on sunscreen bottles to show the consumer how strong the sunscreen is.
500
No. SPF 30 lets in 1/30 of burning UV rays, while SPF 50 lets in 1/50. Adding the two together is pointless and would not result in high levels of skin protection.
A million SPF
It does. The higher the SPF, the higher UV protection it contains. Let's say you have a sunscreen with a 30 SPF and one with 60, the one with 60 offers more protection.
It is proven that people with light skins, are best protected when using a SPF 50 sunscreen and upwards. Sunscreen has SPF ranging from 8, all the way up to 70. Depending on your skin tone, it is better to be too careful by protecting with a high number SPF.
The highest SPF I have located is 110 for both Face and Body: Face: Neutrogena Age Shield Face - SPF 110 Body: Banana Boat Kids Max Protect & Play Continuous Spray Sunscreen SPF 110 Banana Boat Ultra Defense Sunscreen Lotion SPF 110 Banana Boat UltraMist Sport Performance Sunscreen SPF 110
"The sun protection factor of a sunscreen is a laboratory measure of the effectiveness of sunscreen --- the higher the SPF, the more protection a sunscreen offers against UV-B (the ultraviolet radiation that causes sunburn)."
"The Sun Protection Factor (SPF) displayed on the sunscreen label ranges from 2 to as high as 50 and refers to the product's ability to screen or block out the sun's harmful rays. For example, if you use a sunscreen with an SPF 15, you can be in the sun 15 times longer that you can without sunscreen before burning. Consumers need to be aware that SPF protection does not increase proportionally with an increased SPF number. While an SPF of 2 will absorb 50% of ultraviolet radiation, an SPF of 15 absorbs 93% and an SPF of 34 absorbs 97%." from http://www.melanomafoundation.org/prevention/facts.htm
it makes the skin blue
Use sunscreen. SPF 30 (but you will not get tan)
Sun Protection Factor