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Sunblock calculation for face and neck

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Hi everyone! I've seen a wide range of recommendations over how much sunblock should be applied to the face and neck area. Since I want to know an accurate answer, I decided to do a bit of research and math. Oh joy! LOL!

First, I did a search on the internet to find a reputable site that gives the standard for recommended application amount. What I found is that in laboratory testing, the sun protection factor (SPF) of a sunscreen is based on a uniform application thickness of 2 mg/cm<SUP><SMALL>2. </SMALL></SUP>So, if you apply the sunscreen at this thickness level, you will achieve the stated SPF factor. Cool!

To figure out how many milligrams to use, I need to find out what is the average surface area of the face and neck. Fortunately (for me because of my rusty geometry skills), another web search gave me a great answer, but gave a converted layman's dosage based on the holding capacity of a "crown tin lid" aka metal beer bottle cap. (the article was written by a doctor from Mexico and catered to the average tourist on the beach). However, the site did provide the metrics so I can figure out how much I need in US Teaspoon language:

<TABLE width=430 bgColor=pink border=0><TBODY><TR><TD>Segment</TD><TD>Percentage</TD><TD>Surface (cm<SUP>2</SUP>)</TD><TD>Recommended

Dosage (mg)

</TD><TD></TD></TR><TR><TD>Face, neck</TD><TD>< 9 %</TD><TD>685</TD><TD>1,370</TD><TD></TD></TR><TR><TD>Upper limb (right)</TD><TD>9 %</TD><TD>1,557</TD><TD>3,114</TD><TD></TD></TR><TR><TD>Upper limb (left)</TD><TD>9 %</TD><TD>1,557</TD><TD>3,114</TD><TD></TD></TR><TR><TD>Back</TD><TD>18 %</TD><TD>3,114</TD><TD>6,228</TD><TD></TD></TR><TR><TD>Torso</TD><TD>18 %</TD><TD>3,114</TD><TD>6,228</TD><TD></TD></TR><TR><TD>Lower limb (right)</TD><TD>18 %</TD><TD>3,114</TD><TD>6,228</TD><TD></TD></TR><TR><TD>Lower limb (left)</TD><TD>18 %</TD><TD>3,114</TD><TD>6,228</TD></TR></TBODY></TABLE>

Note: 685 square centimeter surface area is based on an average person's face and neck with a full head of hair.

Once I got the recommended dosage, I did a conversion to work with US teaspoons: 1370 milligrams = 0.0483253 ounce = 0.2899518 US teaspoon.

Based on this approach the appropriate amount to wear on the face and neck would be a slightly rounded quarter teaspoon!



Resources for application standard (2 mg/cm2):

http://www.dermatology.org/skintherapy/stl0205.html#TABLE_1

http://www.lef.org/magazine/mag2004/jun2004_report_skin_02.htm

Resource for the average surface area:

<SMALL><SUP></SMALL></SUP>http://dermatology.cdlib.org/93/letters/sunscreen/castanedo.html

My conversion calculator:

http://www.onlineconversion.com/

 

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