I came out to this topic searching for some information on the dielectrical constant of salt water since that approaches the human body. To make a device detecting the approximity of human body mass.. instead of the air.. Presumeably it should be working nice, though I believe the dielectrical constant changes over the frequency domain. So I should be looking for the right electromagnetic frequency to have the biggest dielectrical constant having the biggest response, since air(relative dielectric 1) should have a rather flat dielectrical constant over the frequency domain.