Module 4 - Static Magnetic Field Safety

Under certain conditions, sources of static magnetic fields can present health hazards. Factors affecting potential hazards include:

• Magnetic flux intensity associated with the source
• Design of the magnetic field source
• Accessibility of the magnetic field
• Equipment/hazardous materials associated with the magnetic field source

Sources of large static magnetic fields may require appropriate controls to mitigate potential hazards. For sources intended to produce human exposure to the magnetic field (such as MRI devices), it is critical that safety precautions cover not only the user of the device but also the research subject.

There are no known adverse bioeffects for flux densities within the ACGIH (American Conference of Governmental Industrial Hygienists) exposure limits. Implanted medical devices present a potential hazard to individuals exposed to fields above the ACGIH limits (see following section on kinetic energy hazards).

Due to the large fields associated with NMR magnets, ferrous objects can be accelerated toward the magnet with sufficient energy to seriously injure persons and/or damage the magnet. As a precaution, even small metal objects (screws, tools, razor blades, paper clips, etc.) should be kept at least 1.5 meters from the magnet (or anywhere the field exceed 30 G). Large ferrous objects (equipment racks, tool dollies, compressed gas cylinders, etc.) should be moved with care whenever the field approaches 300 G. There are many recorded instances in which large objects have been drawn towards and even into the bore of the magnet.

The ACGIH and International Council on Non-Ionizing Radiation Protection (ICNIRP) have set guidelines for continuous exposure to static electromagnetic fields as indicated in the table below:

Note: 1 Gauss (G) = 0.1 millitesla (mT)

NMR magnets commonly produce core fields from 0.2 T to 20 T. These fields decrease in intensity as the distance from the core increases. A field strength map of the area surrounding the magnet should be developed and posted for use by staff. If the magnetic fields in your laboratory have not yet been evaluated, please call Radiation Safety at (510) 642-3073 to schedule a survey.

All access points to rooms containing magnets fields in excess of 5 G shall be marked with magnetic field hazard signs (available from Radiation Safety). The 5 G threshold line shall be clearly identified with floor tape or equivalent markings. The location of the 5 G line will vary with the operating frequency and resulting magnetic fields. As an example, one vendor indicates the following values for their NMR spectroscopy equipment:

Operating frequency of 200 MHz - 5 G threshold line @ 1.3 meters

Operating frequency of 500 MHz - 5 G threshold line @ 3.5 meters

Operating frequency of 800 MHz - 5 G threshold line @ 6.0 meters

Persons with cardiac pacemakers or other implanted medical devices shall be restricted to areas outside the 5 G threshold line. Security (locked doors) and proper door markings shall be maintained to prevent unauthorized access to the magnet area.

Types and Expansion Ratios - The cryogenic (liquefied) gases used with NMR magnets are Liquid Nitrogen (-320 deg. F) and Liquid Helium (-452 deg. F). If these liquids are raised to room temperature, the resulting gases expand to hundreds of times their liquid volumes, possibly displacing the air in the room (LN = 694/1, LH = 700/1).

Quench - Quench is the (normally unexpected) loss of superconductivity in an NMR magnet resulting in rapid heating through increased resistance to the high current. This can violently damage the magnet and cause rapid venting of large volumes of He/N gas into the room, quickly resulting in an oxygen-deficient atmosphere. To avoid a quench situation, use cryogen level sensors and always refill or de-energize the magnet if low cryogen levels are indicated on the sensors. NOTE: Quench conditions can result from ferrous objects being drawn into the magnet bore.

Personal Protective Equipment (PPE) - When handling cryogens, use insulated gloves and face shields (or other splash eye/face protection), closed-toed shoes, and lab coats.

Dewars - The containers used for transporting cryogens should be made of metal. Glass dewars can easily implode, causing serious injury. All dewars should have appropriate pressure vents. Unvented containers can rupture as the liquid warms and expands. All transfers of cryogens should be continuously attended to prevent spills or frozen valves.

Room Ventilation - Generally speaking, five complete room air changes per hour are considered adequate for managing small spills or releases of cryogens. In the event of a major release, the staff should immediately leave the room and the room doors should be left open. If the risk of a catastrophic release exists, auxiliary ventilation should be considered to prevent the formation of an oxygen-deficient atmosphere.

Bioeffects of Cryogen Exposure - Direct contact with the skin or eye tissues can cause severe damage through frostbite (tissue damage from freezing). If the frostbite is severe, the damaged tissues may need to be amputated. Inhalation of concentrated cryogen gases may cause loss of consciousness and (eventually) death through oxygen deprivation (asphyxiation).

Electrical Safety Issues

Power Supplies - Although the power supplies used for NMR magnets operate at relatively low voltages (about 10 V), the current used is very large (about 100A). High amperage is extremely dangerous if allowed to come into contact with tissues.

Cables, Wires, and Connectors - All cables, wires, and connectors should be properly insulated to prevent contact with the operating current. These should be inspected on a regular basis to assure the integrity of the insulation. In order to prevent arcing; never break connections without first turning off the power to the circuit being handled.

LOTO (Lock Out, Tag Out) - Cal-OSHA requires all workers to follow LOTO procedures when working on equipment that is activated by a hazardous energy source. Contact EH&S for information on LOTO requirements.

Qualification of Electrical Workers - Cal-OSHA requires that persons who work on electrical equipment be properly qualified. Contact EH&S for information on qualification requirements.

Home Built Equipment - Must be designed and maintained so as to meet safety standards. Enclosures with proper grounding and safety markings are required for all home-built electronics.

Bioeffects of Electrical Exposure - Current moving across a break in an electrical circuit may cause a high-temperature arc to occur. Depending on the current, this arc can exceed 10,000 deg. F, causing severe burns. Blast effects resulting from the vaporization of copper or other metals in the arc can throw people and equipment across rooms, causing severe trauma injuries. Even if an arc is not struck, the current flowing through tissues can result in burns, “blow out” injuries, and possible cardiac failure (depending on the line frequency). Every effort must be made to follow good electrical safety practices and avoid direct contact with live current.

RF Sources - The RF source being used for the NMR should be commercially produced or equivalent quality if assembled in the laboratory. Units that are lab built or modified should be checked to assure they are safe and do not leak radiation.

Waveguides and Coils - Should be carefully checked to assure there are no gaps or loose bolts that will allow leakage of the radiation. Care should be taken to avoid direct contact with coils to avoid RF burns.

RF Measurements - If the RF fields in your laboratory need evaluation, please call Radiation Safety (510) at 642-3073 to obtain a survey.

Bioeffects - Exposure to high-level RF fields can cause heating and damage to tissues. Skin burns can occur from direct contact with RF coils.

Tripping Hazards - Cables and wires lying about on the floor can present tripping hazards. Please make every effort to keep cables in trays or covered by bridges.

Fire Protection - A Class C fire extinguisher should be kept nearby to deal with electrical fires. The power must be shut down before attempting to fight an electrical fire. All staff should be trained in fire protection and evacuation procedures.

Earthquake Concerns - Magnet assemblies may weigh several tons and must be restrained so they will not move about or tip over during an earthquake. Their placement should take into account structural steel support members. Power supplies should also be secured to prevent movement in an earthquake.

Ergonomic Concerns - The prolonged use or improper ergonomic setup of VDT stations may cause eye or neck strain problems. Back injuries may result from the use of improper lifting procedures or lifting heavy objects without assistance.

Depending on the accessibility and level of NIR hazards, it may be necessary to mark rooms or other areas with appropriate warning signs (static magnetic fields, UV light, etc.). Please consult with Radiation Safety on the need for such signs prior to placing them. Please refer to Appendix E for the appropriate design of warning signs. Radiation Safety provides these and other custom NIR signs upon request. Please contact Radiation Safety for assistance with warning signs.

Warning labels should be placed on equipment to indicate the presence of specific NIR hazards (UV light, RF fields, etc.). Again, please consult with Radiation Safety on the need for such labels prior to placing them. Radiation Safety provides these labels upon request. Please contact Radiation Safety for assistance with warning labels.