An electromagnetic field (EMF) is generated when charged particles such as electrons are accelerated. Charged particles in motion produce magnetic fields. Electric and magnetic fields are present around any electrical circuit, whether it carries alternating current (AC) or direct current (DC) electricity. Since DC is static and AC varies in direction, fields from DC and AC sources have significant differences. Static fields, for example, do not induce currents in stationary objects, while AC fields do. Static magnetic fields do not vary over time, and thus do not have a frequency (0 hertz [Hz]).
The most familiar magnetic effects occur in ferromagnetic materials, which are strongly attracted by magnetic fields and can be magnetized to become permanent magnets, producing magnetic fields themselves. Only few substances are ferromagnetic; the most common ones are iron, nickel, cobalt, and their alloys.
The intensity of a magnetic field is usually measured in tesla (T or mT) or gauss (G). Household magnets have strengths on the order of several tens of millitesla (1 mT = 10–3 T), while the field strength of magnetic resonance imaging (MRI) equipment ranges from 1.5 T to 10 T.
Static Electric Fields
An electric field is the force field created by the attraction and repulsion of electric charges, and it is measured in volts per meter (V/m). A static electric field (also referred to as electrostatic field) is created by charges that are fixed in space. The strength of the natural static electric field in the atmosphere varies from about 100 V/m in fair weather to several thousand V/m under thunderclouds. Other source of static electric fields is the charge separation as a result of friction or static electric currents from varied technologies. In the home, charge potentials of several kilovolts can be accumulated while walking on non-conducting carpets generating local fields. High-voltage DC power lines can produce static electric fields of up to 20 kV/m and more.
Sources with field strength greater than 5 to 7 kV/m can produce a wide range of safety hazards such as startle reactions associated with spark discharges and contact currents from ungrounded conductors within the field.
Static Magnetic Fields
A magnetic field is a force field created by a magnet or charges that move in a steady flow as in direct current (DC). Static magnetic fields exert an attracting force on metallic objects containing for example, iron, nickel or cobalt. The quantity of ferrite (a form of iron) or martensitic steel (specific type of stainless steel alloy) in an object will affect its magnetic ability: the greater the quantity of these components, the greater the ferromagnetism. All types of 400 series stainless steel are magnetic. Austenitic steel is not magnetic. Most, but not all, series 300 stainless steel are austenitic and not magnetic.
Sources of static magnetic fields found at Berkeley Lab include nuclear magnetic resonance (NMR) equipment, MRI systems, spectroscopy systems, ion pumps, quadrupoles and sextupoles, bend magnets, superconducting magnets, and cryostats.
Static magnetic fields can also erase data stored on magnetic media or on the strips of credit or debit cards and badges.
Time-Varying Magnetic Fields
Time-varying magnetic fields are magnetic fields that reverse their direction at a regular frequency. They can induce an electric current in a conductor present in this field as well as in a human body. Time-varying magnetic fields are produced by devices using AC such as cellular telephone antennas, microwaves, etc. A general rule of thumb is that 1 T/sec can induce about 1 microampere per square centimeter (μA/cm2) in the body.
Induced currents in the body can cause local heating and possible burns, which is the major effect from time-varying fields. The cause is the high radiofrequency time-varying field. Low-frequency fields usually do not contribute greatly to this effect.
Sources of Electromagnetic Radiation
Static magnetic fields are created by magnets or by the flow of DC electricity. They can be produced by many natural sources also. Natural sources of static electric fields include the earth’s atmosphere during stormy conditions, the charge produced by shuffling across a carpet, and the “static cling” of clothing. The earth has an electric field of about 130 V/m near the surface due to separation of charges between the earth and the ionosphere. It is directed vertically. The earth and the ionosphere together form a spherical capacitor, with the two conducting surfaces being the earth and the upper atmosphere. This difference in potential is maintained by lightning, which brings negative charges to the earth.
The earth itself has a natural static magnetic field, which is used for compass navigation. Currents running deep within the earth’s core produce natural static magnetic fields on the earth’s surface. The earth has static magnetic flux density averaging 0.5 G with the field strengths lowest at the Equator and highest at the magnetic poles.
Common sources of static magnetic fields include permanent magnets (which are found in appliances, toys, and medical devices), battery-powered appliances, MRI scanners, some electrified railway systems, and certain industrial processes.
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state, the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields. Superconducting magnets are used in MRI scanners in hospitals and in scientific equipment such as nuclear magnetic resonance (NMR) spectrometers, mass spectrometers, and particle accelerators.
Superconducting magnets, such as NMR and MRI equipment, pose unique safety concerns. These concerns include cryogen safety, strong magnetic fields, and the potential for creation of oxygen deficient atmospheres. The highest potential for the most serious of these hazards exists during the magnet startup, cryogen filling, and maintenance activities. Once magnets are operational and magnetic fields have been established, the hazards are minimal as long as operators, maintenance personnel, patients, and/or visitors understand the proximity limits and procedures to follow when working near the magnet.
Nuclear Magnetic Resonance
The NMR system uses a static magnetic field and a radiofrequency pulse to make nuclear spins align in the magnetic field to maximize the NMR signal strength. NMR spectroscopy is a research technique that exploits the magnetic properties of certain atomic nuclei and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.
NMR are superconducting magnets and commonly produce core fieldsfrom 0.15 T to 20 T. These fields decrease in intensity as the distance from the core increases. Research NMRs are more powerful than medical devices, but their fields are of smaller volume, focused, and fall off quickly, making it easier to provide personnel protection.
Magnetic Resonance Imaging
MRI technique is used in radiology to generate images of organs in the body for diagnostic imaging. MRI scanning is based on the science of NMR using strong magnetic fields, radio waves, and field gradients to generate images of the organs in the body. An MRI scanner consists of a large, powerful magnet that a patient lies in. A radiowave antenna is used to send signals to the body and then receive signals back. These returning signals are converted into images by a computer attached to the scanner. Imaging of almost any part of the body can be obtained in any plane.
Most clinical magnets are superconducting magnets, which require liquid helium. The MRI magnetic field’s strength ranges from 0.15 T to 4 T. Superconducting magnets at 1.5 T and above allow functional brain imaging and MR spectroscopy with improved time and spatial resolution. Such magnets have additional challenges from radiofrequency (RF) heating of the subject.
An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump capable of reaching pressures as low as 10−11 millibars (mbar) under ideal conditions. An ion pump ionizes gas within the vessel it is attached to and employs a strong electrical potential, typically 3–7 kV, which allows the ions to accelerate into and be captured by a solid electrode and its residue.
The three main types of ion pumps are the conventional or standard diode pump, the noble diode pump, and the triode pump.
The basic structure consists of two electrodes (anode and cathode) and a magnet. Ion pumps are commonly used in ultra-high vacuum (UHV) systems, as they can attain ultimate pressures less than 10−11 mbar. In contrast to other common UHV pumps, such as turbomolecular pumps and diffusion pumps, ion pumps have no moving parts and use no oil. They are therefore clean, need little maintenance, and produce no vibrations. These advantages make ion pumps well-suited for use in scanning probe microscopy and other high-precision apparatuses. In addition, they do not need baking and are designed to minimize stray magnetic field.
Most of the ion pumps installed on ALS beam lines have the 5 G line within 20–30 cm from the surface.
Physical and Biological Effects in Static Electric and Magnetic Fields
By far the most important effect is from the attraction of magnetic objects in or on the body by the magnetic field. Objects such as pacemakers, surgical clips and implants, clipboards, tools, jewelry, watches, mops, buckets, scissors, and screws have all been documented as being potential hazards. Even low mass items can become hazardous when moving at high speed. Much of this experience has come from medical MRI systems. Magnetic objects will try to align themselves with the magnetic field lines. If an implanted object tries to do this, the torque may cause serious injury.
Modern pacemakers are designed to be tested or reprogrammed with the use of a small magnetic field external to the body. Static fields can close reed switches and cause the pacemaker to enter test, reprogram, bypass, and other modes of operation with possible injury.
Based on data from MRI usage, static fields may cause a small, reversible effect on electrocardiogram data. The cause is the interaction of moving blood (a conductive medium) and the field in the heart. The effect is minimal (below about 2 T) and is not considered a concern.
Currently available information does not indicate any serious health effects resulting from the acute exposure to static magnetic fields up to 8 T, but it can lead to potentially unpleasant effects such as vertigo during head or body movements. The extent of these sensations is highly dependent on individual factors such as personal predisposition to motion sickness and the speed of movement in the field.
Physical and Biological Effects in Time-Varying and Induced Electric Fields
Effects of time-varying fields are similar to those of static fields. In such a field small currents not normally present in the body can be produced. Usually this is not a concern, but they can evoke vertigo and sensory perceptions such as nausea, metallic taste in the mouth and faint flickering visual sensations (magnetophosphenes). Pacemaker users could be at risk also. The induced currents may cause the pacemaker to incorrectly start pacing or even prevent pacing when it is actually needed. Induced currents can cause local heating, which is the dominant effect from time-varying fields.
The main interaction of low-frequency time-varying electric and magnetic fields with the human body is the induction of an electric field and currents according to Faraday’s law: E=πfrB, where E is the electric field, f is the frequency, r is the radius of a loop perpendicular to the magnetic field, and B is the magnetic flux density. The larger the radius r, the larger the electric field and current. For a person, the radius is greatest at the perimeter of the body.
At 50–500 mT (500–5,000 G) stimulation of nerve and muscle tissue have been reported. Above 500 mT (5,000 G) the induced currents can upset cardiac rhythm or cause ventricular fibrillation. All of these effects are from induced currents (IRPA, 1990).
Electromagnetic Exposure Limits and Assessment
The ACGIH TLVs refer to static magnetic field flux densities to which it is believed that nearly all workers may be exposed day after day repeatedly without adverse health effects.
The TLVs for a routine (8-hour) occupational exposure from static magnetic fields are listed in Table 1. Workers with implanted ferromagnetic or electronic medical devices should not be exposed to static magnetic fields exceeding 0.5 mT (5 G).
Table 1.TLVs for Static Magnetic Fields
|5 G||Highest allowed field for implanted cardiac pacemakers.|
|10 G||Watches, credit cards, magnetic tape, computer disks may be damaged.|
|30 G||Small ferrous objects present a kinetic energy hazard.|
|20,000 G (2T)||Whole-body ceiling limit (no exposure allowed above this limit).|
|80,000 G (8T)||Whole-body (special worker training and controlled workplace environment).|
|200,000 G (20T)||Extremity ceiling limit (no exposure allowed above this limit).|
Note: Time-weighted average (TWA) exposure time is normally only a concern for extremely high field exposures to the whole body.
1 gauss (G) = 0.1 millitesla (mT)
The full list of TLVs can be downloaded from the below listed link: Full List of Threshold Limit Values.
To evaluate the hazard and assess the exposure from EMF generating devices, a measurement of the EMF radiation should be performed and compared to the appropriate TLVs. The evaluation should be performed at the time of the installation of the EMF generating device, after a change in operating parameters that increases the hazard, or after repair that may change the operating parameters. Devices already installed but not evaluated should be evaluated at the first opportunity. If the results of the initial evaluations are well below the TLVs, further monitoring is not required unless the activity is modified to expect increased exposures. If results are found to exceed TLV levels or are very close to TLVs, periodic monitoring should be conducted at a frequency that is sufficient to ensure the adequacy of control measures (typically annually).
General Safety Considerations
The most immediate danger associated with magnet environment is the attraction between the magnet and ferromagnetic objects. Ferromagnetic metal objects can become airborne projectiles when in a strong magnetic field. Tools and compressed-gas cylinders can become uncontrollable and fly like missiles toward magnets in areas where strong static fields and strong field gradients (changes in field strength over distance) exist. Mechanical hazards depend on the field strength and the field gradient, and also on how rapidly the magnetic field strength changes with distance. The obvious safety action is to prevent any magnetic material from entering the work area.
Never place any part of your body between the magnet and loose metal objects. If a large object is attracted to the magnet and hits the magnet, leave the room since it may cause the magnet to quench. Notify your supervisor. If an injury has occurred, call 911 immediately.
Electronic and Metal Implants
Persons wearing metallic implants, such as bone or articular prostheses, surgical clips, nails or screws in broken bones, body piercing, or even dental fillings may feel painful sensations, if exposed to high magnetic fields. Persons fitted with pacemakers encounter a specific risk as static or pulsed magnetic fields may influence the working order of their implanted devices.
Cryogenic Gas Issues
Quench is the (normally unexpected) loss of superconductivity in an NMR magnet, resulting in rapid heating through increased resistance to the high current. The superconducting magnet contains both liquid helium and liquid nitrogen. A substantial volume of liquid helium will be converted to gas if the magnet quenches. In a magnet quench, the superconducting magnet loses the ability to superconduct and the stored energy is released as heat, which boils off the liquid helium. The helium gas is vented out of the magnet dewar and fills the room from the top down (helium is lighter than air), and forms a cloud near the ceiling. A quench is obvious: a big cloud of helium vapor will form above the magnet, accompanied by a loud whooshing sound that can create an oxygen deficient atmosphere. If a quench occurs, leave the room immediately, pull the fire alarm to evacuate the building, and call 911.
The quench can violently damage the magnet, and ferrous objects are drawn into the magnet bore.
Superconducting magnets using liquid helium and/or nitrogen present an additional safety concern with the handling of cryogenic liquids. 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).
In general, five complete room air changes per hour is considered adequate for managing small spills or releases of cryogens. In the event of a major release, personnel should immediately leave the room and keep the doors open. If the risk of a catastrophic release exists, auxiliary ventilation should be considered to prevent the formation of an oxygen deficient atmosphere.
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.
Personal protective equipment
When handling cryogens, use insulated gloves, face shields or other splash eye/face protection, closed-toe shoes, and lab coats.
Electrical Safety Issues
Although the power supplies used for NMR magnets operate at relatively low voltages (approx. 10 V), the current used is very high (about 100 A). High amperage is extremely dangerous if allowed to come into contact with human tissue.
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 ensure the integrity of the insulation. To prevent arcing, never break connections without first turning off the power to the circuit being handled.
Following lockout-tagout procedures is required when working on equipment that is activated by a hazardous energy source.
Other Safety Concerns
Keep a Class C fire extinguisher 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.
Magnet assemblies may weigh several tons and must be restrained so they will not move or tip over during an earthquake; their placement should take into account structural steel support. Power supplies should also be secured to prevent movement during an earthquake.
Switching of field gradients causes a change in the Lorentz force experienced by the gradient coils, producing minute expansions and contractions of the coil. As the switching typically is in the audible frequency range, the resulting vibration produces loud noises (clicking, banging, or beeping). This is most marked with high-field machines and rapid-imaging techniques in which sound pressure levels may reach 120 dB(A) (A-weighted decibels), which is equivalent to a jet engine at takeoff; therefore, appropriate ear protection is essential for anyone inside the MRI scanner room during the examination.
RF in itself does not cause audible noises (at least for human beings), since modern systems are using frequencies of 8.5 MHz (0.2 T system) or higher. The RF power that is capable of being produced matches that of many small radio stations (15–20 kW). As a result, heating effects are present from the RF. In most pulse sequences, the heating is insignificant and does not exceed U.S. Food and Drug Administration guidelines.
Potential for electrical shock exists with RF coils, so proper grounding and insulation of coils is necessary. Any damage to coils or their cables needs prompt attention. Looping of the cable to a coil can result in burns to anyone who touches them. It is best to avoid all contact with the RF coil cables.
Two approaches to controlling exposure are using engineering controls (e.g., shielding) and administrative controls (e.g., personal protective equipment).
Magnetic fields are controlled by using permeable alloy that confines the magnetic flux lines and diverts them. Magnetic shielding can be made using high nickel alloys called mu metal or soft iron. Forming mu metal into a complex shield is expensive and mu metal is easily damaged. Such shielding is best applied near the field source, whenever practical. Another approach is to use non-permeable metals such as copper or aluminum to produce eddy currents that cancel out the original magnetic field.
To avoid a quench situation, use a cryogen level sensor system to detect the quench and trigger a lowering of the current and stored magnetic energy to prevent burnout of the conductor. Always refill or de-energize the magnet if low cryogen levels are indicated on the sensors.
Examples of engineering controls for superconducting magnets are as follows:
- Installation of liquid helium purge vent to allow excess helium gas to escape through an exhaust vent extending out through the roof
- Internal sensors to indicate low levels of liquid helium
- Visual and audible alarms
- Positive access control such as locked doors and restricted access to authorized personnel only
Metallic structures producing contact shocks should be electrically grounded or insulated.
Areas where whole-body exposures to 60 Hz fields exceed 25 kV/m or 1 mT (10 G) must be restricted by positive means such as locked enclosures, interlocks, or safety chains.
As part of the design process, the static magnetic field in the facility must be identified by measurement or calculations where pacemaker hazards (>5 G) and kinetic energy hazards (>30 G) will exist. Places where excessive whole-body exposures (>600 G) could occur must also be identified.
Tools and magnetizable objects must be kept out of places where elevated static magnetic fields are present.
If it is determined that shielding is required, an experienced consulting firm should be hired to design the magnetic field shielding.
Provisions must be made to secure and restrict access by pacemaker users to places where whole-body magnetic fields exceed 5 G. The 5 G line is a demarcation between uncontrolled and controlled areas and must be clearly identified. For fields with exposure less than 5 G, no controls or posting are required.
In addition to the warning signs posted at the doorways, some other method to indicate the 5 G line around the magnet is required. For example, a painted line or tape placed on the floor around the magnet where the field is 5 G could be used. Another example is a chain, rope, or fence indicating the 5 G line around the magnet.
Whatever method is used, egress from the area in the event of an emergency must not be blocked or prevented.
Hazard warning signs
A warning sign is required to be posted at the entrance to labs or spaces where magnetic fields exceed any of the limits listed above. Areas where potential mechanical hazards exist must be conspicuously demarcated. Tools, compressed-gas cylinders, and other articles made of magnetically permeable material must be kept out of such areas.
Caution signs must be posted in areas where magnetic field strengths could exceed 0.5 mT (5 G), and/or areas where 60 Hz electric fields exceed 1 kV/m, as demonstrated by measurement or calculation, warning individuals with pacemakers or other medical electronic implants to keep out.
Caution signs must be posted where electrical fields exceed 5 kV/m warning individuals that irritating sparks are possible.
People with pacemakers must be kept out of areas where 60 Hz magnetic fields exceed 0.1 mT (1 G), as demonstrated by measurement or calculation.
Areas where whole-body exposures to 60 Hz fields exceed 25 kV/m or 1 mT (10 G) must be limited by positive means such as locked enclosures, interlocks, or safety chains.
Areas where magnetic fields exceed 3 mT must be surveyed to determine where potential mechanical hazards exist. People with metallic medical implants must be kept out of areas where field strengths exceed 3 mT (30 G).
Examples of hazard warning signs are shown below.
Equipment that could create 60 Hz electric fields above 2.5 kV/m or magnetic fields above 0.1 mT (1 G) must be labeled or a warning sign must be posted.
Examples of labels are shown below.
Illuminated warning light
Some electromagnets are designated by a red flashing warning light that turns on when the magnet is energized. Magnets that create strong static magnetic field are typically de-energized when personnel exposures could occur (i.e., during lengthy downtimes associated with accelerator operations).
Personal protective clothing
When handling cryogens, wear insulated gloves and face shields or other splash eye/face protection, closed-toed shoes, and lab coats.
Insulating garments and equipment should be used in areas where 60-Hz electric fields exceed 5 kV/m, as demonstrated by measurement or calculation. Insulating gloves or, preferably, engineered controls (e.g., enclosure or shielding of a field source) must be used to avoid contact with objects that could expose personnel to sparks associated with field strengths greater than or equal to 5 kV/m.
- 10 CFR 851 Worker Safety and Health – Department of Energy, § 851.23 Safety and Health Standards.
- American Conference of Governmental Industrial Hygienists (ACGIH) TLVs and BEIs – 2016incorporated by reference 10 CFR 851 Worker Safety and Health – Department of Energy, §851.27.
- ACGIH TLVs and BEIs – 2012.
- ICNIRP Guidelines on Limits of Exposure to Static Magnetic Fields. Health Physics, Vol. 96(4):504-514. 2009.
- ICNIRP Guidelines For Limiting Exposure To Electric Fields Induced by Movement of the Human Body in a Static Magnetic Field and by Time-Varying Magnetic Fields Below 1 Hz. Health Physics, Vol. 106(3):418-425. 2014.
- Plogg, H., and Miller, G. Fundamentals of Industrial Hygiene. Fourth Edition, Chapter 11: Nonionizing Radiation. 2001.
- IPRA Interim Guidelines on Limits of Exposure to 50/60 Hz Electric and Magnetic Fields. Health Physics, Vol. 58(1): 113-122. 1990.