Creating a sensor to measure when hand is close to face

ok, application might sound bit odd.  trying figure out kind of sensor might use used sense when person moves or hand close face.  need sensors small , unobtrusive wearer -- and, said, sensors have wearable.  new physical computing, don't know right direction explore. 

the precision of measurement not have exact -- and, type of data looking collect doing running count.  example, ideal sensor bracelets, perhaps embedded magnets.  then, sensor connected persons collar or necklace.  so, assume person has chronic nail biting habit.  each time persons hand went or mouth, log event.  of course, isn't precise measurement because indicates whether hand reached general area of mouth.  and, yes, have lot of false positives in event of sneezing, scratching, , eating, these errors in data collection managed point of view.  again, really  trying solve problem of identifying event when person's hand goes close or face. 

any suggestions sensors consider?

thanks in advance!

what necklace (around throat) detects rfid thingy, attached wrists?
http://playground.arduino.cc/learning/prfid

according en:wp can b read 10cm 10m...  xd
http://en.wikipedia.org/wiki/rfid

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