Static Magnetic Fields (Zero Hz)

As part of the design process, the static magnetic field in the facility shall be mathematically modeled to identify where pacemaker hazards (> 5 G) and kinetic energy hazards (> 30 G) will exist. Places where excessive whole-body exposures (> 600 G) could occur shall also be identified. If it is determined that shielding is required, an experienced consulting firm should be hired to design the magnetic field shielding.


ACGIH - TLV/BEI 2007

ICNIRP “Guidelines on Limits of Exposure to Static Magnetic Fields”

 

Provisions shall be made to prevent access to places where whole body magnetic fields exceed 600 G. Areas such as hallways, stairways, and offices shall be located where fields are < 5 G to allow completely unrestricted access.

The ACGIH TLV for static magnetic fields is somewhat more restrictive than the ICNIRP standard. The campus needs to determine which exposure criterion to apply.

 

ACGIH - TLV/BEI 2007

ICNIRP “Guidelines on Limits of Exposure to Static Magnetic Fields”

 

Provisions shall be made to secure and restrict access to places where whole-body magnetic fields exceed 5 G. In some cases, as with MRI systems installed in vans or in isolated buildings, it might be necessary to construct a fence around the facility to ensure that magnetic-field exposures at the perimeter of the fence are below 5 G.

A variety of prosthetic devices, makeup, and personal articles can behave in a hazardous manner in stronger fields. A number of medical electronic implants, such as artificial cardiac pacemakers, can malfunction above 5 G.

 

ACGIH - TLV/BEI 2007

 

Appropriate ANSI Z535 specification warnings signs shall be provided to identify such areas. Signs should be mounted near the entryways such that they are visible before entering controlled, high-static, magnetic-field areas, and at some distance from them. Signs should not be mounted above doorways, as these can be overlooked.

 

Provisions should be made for persons to securely store their metal keys, wristwatches, and other ferrous-alloy tools and articles for safekeeping before entering areas where the magnetic field exceeds 30G.

Engineered access controls, such as locked doors, are preferred over stanchions and portable signs. Kinetic energy hazards from even small ferrous items, like razor blades and nails, can cause serious injuries. Larger items, like wrenches, could kill or cause major equipment damage. A field of 50 G has been associated with erasing credit cards and other magnetic media, but it is recommended that such items be kept out of areas in which the magnetic field exceeds 10 G.

 

ACGIH - TLV/BEI

ICNIRP “Guidelines on Limits of Exposure to Static Magnetic Fields”

 

Appropriate discharge shall be made to direct cryogenic gases from a quenched superconducting magnet to a safe, unoccupied location to avoid exposing people to an oxygen-deficient atmosphere. The issue of preventing oxygen deficiency during a quench condition shall be addressed in the design of locations for superconducting magnets. Doors to locations that may be subjected to gases during a quench shall open outwards to assure they can be opened should the laboratory become pressurized.

It is estimated that 80 liters of liquid helium (56,000 liters of gas at 1:700 expansion ratio) can be ejected from a magnet Dewar flask in 15 to 30 seconds.

 

29 CFR 1910.134

8 CCR 5144 and 5157