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Lately we have quite a few alternative health doctors or scientists saying that EMF and wifi etc are quite harmful to the body
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This may be possible because if you zap during too long time it may alter the composition of your blood in some way or the functionning of your cells.
Personally i highly recommend to not use a zapper while sleeping because when you are unconscious you don't know how the resistance of the body changes.
I have had my heart beat altered because of using a zapper while sleeping (several times). Bad idea !