Metal detectors pass safety test on pregnant women

Hand held metal detectors (HHMDs), such as those used for security checks in airports, do not cause harmful heating or nerve stimulation in pregnant women, according to research published today (22 July 2003) in the Institute of Physics journal Physics in Medicine and Biology. The role of the Center for Devices and Radiological Health (CDRH), Maryland, is to ensure the safety of radiological products, so electromagnetic wave emitting HHMDs were an obvious choice for them to investigate, especially following the heightened security measures of recent years.

Dr Wolfgang Kainz and his colleagues at CDRH tested nine different HHMDs on a model of a pregnant woman to see if they caused nerve stimulation or a temperature rise in the mother or foetus, and found that none of them did. They also compared their measurements against two international safety standards*, neither of which give specific guidelines on safe exposure levels for pregnant women. All the metal detectors operated at well below the recommended levels, but this raises the question of whether there should be separate guidelines for pregnant women. Future research could tackle this possibility, as well as potential long term effects from the HHMDs, which were not explored in this study.

The team used measurements of the surface shape of a woman in her 34th week of pregnancy to create a computer model of a pregnant woman. They approximated the foetus to be spherical, 20cm wide, and positioned one centimetre below the surface of the mother’s skin.

Then, the team measured the magnetic field emitted by each of the nine HHMDs in horizontal planes one, two, five and 11cm away from each detector. With these measurements, they computed the “induced current density” and the “specific absorption rate” (SAR) in the pregnant woman. Induced current density is the amount of electrical current flowing in a unit of area generated by the metal detector, and can cause harmful nerve stimulation if it is too high. The SAR is a measurement of the heat absorbed by the tissue, which tells you if there is a temperature rise in the body as a result of using the HHMD. If a device’s electromagnetic emissions cause a temperature rise of more than one degree Centigrade, it is considered damaging to the body.

Their results were compared to two international standards that set recommended upper limits for the induced current density and the SAR to protect exposed people from immediate effects of radiation. They found that the levels of both quantities were at least three times lower than the limits, and in some instances levels were thousands of times lower. This is due to the variation between the range of metal detectors, and the fact that the standards use different thresholds for safety.

“This study shows that, based on existing exposure standards, these metal detectors do not cause harmful heating or nerve stimulation in pregnant women. We are delighted to publish this good news, as metal detectors are used in our everyday lives for security checks in buildings and airports,” said Jane Roscoe, senior publisher for Physics in Medicine and Biology.