How does a Metal Detector work?
Metal Detectors use a low frequency electromagnetic field to search for concealed metal objects like weapons, guns etc. Many people believe that the Metal Detectors uses harmful Xray. Which is entirely false, Xray are only used in the luggage scanners that are used to scan your bags.
What do the doctors say?
Most of the doctor advise that it's safe to walk through a metal detector at airports and other places.
Here is an good article by a Wendy Wilcox ob-gyn on the subject of safety of metal detectors. If you are still uncomfortable in walking though one you can always opt for the physical "pat down" search by a lady officer.