SPF LEVELS
We have already discussed the six different skin types and the impact of UV rays on each. There are other natural factors that can increase or decrease this impact:

• Geographic location (closer to equator means more sun) 
• Elevation 
• Time of year 
• Time of day 
• Amount of tanning already achieved 
• Length of time spent in the sun

This last factor brings us to a review of sun care products. The SPF classification, or number, indicates the level of protection against UVB rays (the cause of sunburn) and helps determine the amount of time a person can stay in the sun without getting a burn.

The higher the SPF number, the more rays blocked and the longer the user can stay out. The FDA classifies sunscreens as those products with SPFs of 2-14; sun blocks have Spa's of 15 and higher. The following table correlates SPF rating to the percent of UVB rays screened/blocked: 

For example, let's assume you're at sea level and have no prior tan. From personal experience, you know that after about 15 minutes of sun, you start to burn. 

If you plan to be in the sun for an hour, you need protection equivalent to the remaining 45 minutes, so you would use SPF 4 which screens out 75% of UVB rays or 45 minutes worth (75% x 60 minutes = 45 minutes of block).

The SPF to be used is determined by dividing the time a person will be in the sun by the amount of time it takes to start burning (60 minutes divided by 15 minutes = SPF 4).

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