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Occupational Magnetic Fields
Guidelines for DC Fields
 

The biological effects of AC fields are detectable at much lower levels than for DC fields. It is therefore inappropriate to use the AC field Guidelines for DC fields.

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) Guidelines for Occupational Static Magnetic Fields are 200mT for continuous exposure, 2000mT for short-term whole-body and 5000mT for exposure to arms & legs. These field levels are high and indicate the lack of evidence for biological effects from DC fields. The ICNIRP level for persons with pacemakers and other implanted devices is set at the much lower level 0.5mT (5gauss). This level is apparently not related to biological effects but rather to possible effects on electrical or electronic devices (particularly reed-relays) or metal prosthesis.

Adoption of the 0.5mT (5gauss) level and installing shielding or access restrictions avoids potential problems with operators or visitors having implanted devices and also avoids malfunction of most electronic control & test equipment used in the vicinity of magnets. Setting DC Field exposure levels at less than 0.5mT seems unnecessary except for special, magnetically sensitive processes. (Some electron beam equipment such as display devices or electron lithography systems require lower fields.)

The US Food and Drug Administration (USFDA) in "Guidance for the Submission of Premarket Notifications for Magnetic Resonance Devices" requires iso-field contours at 0.5mT (5gauss), 1mT, 10mT, 20mT, 40mT, and 200mT (see Section 4, Site Planning) in planes parallel and perpendicular to the magnetic field. Entry into regions where the magnetic field is in excess of 0.5mT is only allowed for authorized personnel.

The earth's geomagnetic field is about 0.05mT (50µT or 0.5gauss). Inside buildings there can be variations due to steel in the building construction. GMW has measured DC fields to about 0.1mT in the open space of buildings and to 0.5mT at the corners of large steel components such as machine tools.

In measuring the actual fringing magnetic field it is necessary to remember that the magnetic field (magnetic flux density, B) is a vector quantity with three components. The typical Hall effect Gaussmeter or Teslameter only measures one component of the field. To measure the total field it is necessary to measure the three components Bx, By & Bz and then calculate the vector total (square root of the sum of the squares: Formula The three component measurement can be difficult and time consuming with a standard Hall gaussmeter or teslameter. The Metrolab THM 7025 Three Axis Hall Effect Teslameter measures the three components simultaneously & provides the vector total with a resolution down to 0.01mT (0.1gauss). Senis Three Axis Magnetic Field Transducers provide three analog components with a frequency response from dc to approximately 1kHz and can be used with a data logger to map or monitor magnetic fields in the range 0.01mT to 2T. For high resolution measurement or mapping of magnetic fields of less than 1mT, the Bartington Mag-03 range provides very high resolution with frequency response from dc to 3kHz. Acquisition and spectral analysis in a portable instrument is provided by the Bartington Spectramag-6.

For more detail please refer to:
Occupational Safety and Health Administration (OSHA) ELF Radiation
University of Michigan's Radiation and Health Physics Page
The World Health Organization (WHO) International EMF project
The US Food and Drug Administration
The Joint Commission for Health Care and Certification in the USA publishes information realting to MRI, see example Sentinel Event Alert
Diagnostic Imaging Online: "Regulators pose potential threat to high-field MR"
EMF Portal with links to research papers on EMF health related effects.
Brookhaven National Laboratory THM 7025 3 Axis Hall Magnetometer. Instrument Operation
Brookhaven National Laboratory Static Magnetic Field Measurement Principles: Area Surveys

Ian J. Walker, August 2000, revised April 2008.


 
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