Radiofrequency (RF) radiation is nonionizing electromagnetic energy characterized by relatively long wavelength, low frequency, and low photon energy. RF radiation is usually described by its frequency, expressed in hertz. The RF part of the electromagnetic spectrum extends from 30 kilohertz (kHz) to 300 gigahertz (GHz). Different RF bands are assigned to the RF and sub-RF portions of the spectrum for such uses as aeronautical radio, navigation, broadcasting, and personal wireless communication services. Specific frequencies are designated for industrial, scientific, and medical uses. Microwave (MW) radiation is typically considered a subset of RF radiation, with frequencies from 300 megahertz (MHz) to 300 GHz. The table below shows the bands of RF and sub-RF fields.
Bands of Radiofrequency and Sub-radiofrequency Fields and Radiation
|Frequency Range||Wavelength Range||Radiation Type|
|>0–300 Hz||≥1,000 km||Extremely low frequency (ELF)|
|300 Hz–3 kHz||1,000–100 km||Voice frequency|
|3–30 kHz||100–10 km||Very low frequency (VLF)|
|30–300 kHz||10–1 km||Low frequency (LF)|
|300 kHz–3 MHz||1 km–100 m||Medium frequency (MF)|
|3–30 MHz||100–10 m||High frequency (HF)|
|30–300 MHz||10–1 m||Very high frequency (VHF)|
|300 MHz–3 GHz||1 m–10 cm||Ultra high frequency (UHF)|
|>3–30 GHz||10–1 cm||Super high frequency (SHF)|
|30–300 GHz||1 cm–1 mm||Extremely high frequency (EHF)|
|>300 GHz||<1 mm||Infrared, visible light, ultraviolet light, ionizing radiation|
RF Generation and Applications
RF energy is generated by the acceleration of charge in circuits. Naturally occurring background sources of RF include terrestrial, extraterrestrial, and atmospheric electrical discharges (lightning), and even the human body. An antenna is used to transfer RF energy from a source to free space.
Solid-state devices such as Gunn-diode oscillators, tunnel diodes, and metal-oxide-semiconductor field-effect transistors (MOSFETs) can generate MW radiation. These have a number of low-powered applications, including automatic door-opening devices, police radar and radar detectors, hand-held radios, and intrusion alarm systems.
Many of the devices used for generating high power at MW frequencies use high-energy relativistic electron streams in vacuum tubes to generate RF energy. RF vacuum tubes include triode, tetrode, and pentode configurations. These gridded tubes are used as oscillators and amplifiers in low-frequency applications such as communications, broadcasting, radar, and industrial (dielectric and induction) heating. MW vacuum tubes include klystrons, magnetrons, traveling-wave tubes, and backward-wave oscillators that are used in MW heating, high-frequency (HF) radar, and MW communications applications.
Typical applications of RF energy are listed in the table below. Many of these applications are important sources of occupational RF exposure.
|Visual display terminals and televisions||
The nature and degree of the health effects of overexposure to RF fields depend on the frequency and intensity of the fields, the duration of exposure, the distance from the source, any shielding that may be used, and other factors. The main effect of exposure to RF fields is the heating of body tissues (thermal effects) as energy from the fields is absorbed by the body. Prolonged exposure to strong RF fields may increase body temperature, producing symptoms similar to those of physical activity. In extreme cases, or when the body is exposed to other sources of heat at the same time, its cooling system may be unable to cope with the heat load and may experience heat exhaustion and heat stroke as a result.
Localized heating, or “hot spots,” may lead to heat damage and burns to internal tissues. Hot spots can be caused by several circumstances: non-uniform fields, reflection and refraction of RF fields inside the body, and interaction of the fields with metallic implants (e.g., cardiac pacemakers and aneurism clips). Parts of the body that have poor thermoregulation, such as the lens of the eye and the testes, are at higher risk for heat damage.
Other health effects include contact shocks and RF burns. These can result when a person comes in contact with electric currents that flow from a conducting object at the same time the person is exposed to RF fields.
Heating of the body tissues depends on the power density level, which is expressed in milliwatts per square centimeters (mW/cm2). With MW radiation, the primary effect is tissue heating at levels above 10 mW/cm2.
The thermal effects depending of the power density can be divided as:
|Power Density||Thermal Effects|
|Low||<1 mW/cm2||Improbable, or at least not predominant|
|Medium||1–10 mW/cm2||Weak but noticeable|
The health effects of RF exposure are generally as follows:
|RF Exposure Level||
|150 MHz–1 GHz||Heat can be absorbed in deep body tissue|
|>10 GHz||Heating is mainly in the outer skin|
Although we are constantly exposed to weak RF fields from radio and television broadcasting, no health risks have been identified from this low-level exposure.
People who have metallic implants (e.g., cardiac pacemakers, cochlear implants, defibrillators, drug delivery systems, and other medical devices) may experience the device’s malfunctioning if subjected to strong RF fields. Conductive objects within the body tend to localize the RF field and enhance the absorption rate, causing interference with the operation of the implanted device.
RF Exposure and Limits
Specific absorption rate
Biological effects are related to the specific absorption rate (SAR) and may be expressed in terms of a whole-body average value or a spatial-peak SAR (averaged over a specific volume) for partial body exposure. These effects are measured in watts per kilogram (W/kg) in tissue. The SAR has been shown to be the most reliable quantity for establishing thresholds for possible biological effects, and it is used to derive power density and field strength limits for maximum permissible exposure. The SAR for occupational exposure and for individuals in controlled environment is 0.4 W/kg whole body and 8 W/kg partial body (spatial peak). The ACGIH exposure guides reflect the electric and magnetic field strengths that will maintain the SAR below harmful levels. If the measured value of field strength or power density does not exceed the applicable exposure limits, the SAR will not be exceeded.
In the case of whole-body exposure, a standing human adult can absorb RF energy at a maximum rate when the frequency of the RF radiation is in the range of 80–100 MHz; that is, the whole-body SAR is at a maximum under these conditions (resonance). Because of this resonance phenomenon, RF safety standards are generally most restrictive for these frequencies. For RF radiation near the whole-body grounded-resonance frequencies (10–40 MHz), the SAR can be reduced by separating the body from the ground plate by a small distance and using expanded polystyrene and hydrocarbon resin foam that can provide an air gap between the subject and the ground.
Contact and induced currents
An RF field at low frequencies (30 kHz–100 MHz) can produce alternating electric potential on ungrounded conducting objects. When the wavelength of the incident wave is greater than about 2.5 times the body length, a person touching a conducting object will be subjected to an RF current flowing to ground. This effect is known as contact current. The body can also be that conducting object in which a current is induced by the field. This effect is known as induced body current.
Threshold limit values
Occupational exposure to RF and MW radiation is evaluated using threshold limit values (TLVs), which are adopted from the American Conference of Governmental Industrial Hygienists’ (ACGIH’s) 2016 TLVs and biological exposure indices (BEIs). TLVs and BEIs are guidelines designed for use by industrial hygienists in making decisions regarding safe levels of exposure to various chemical substances and physical agents found in the workplace. Exposure at or below the level of the TLV or BEI is considered not to create a risk of disease or injury.
Consult the EHS Division’s RF point of contact for an evaluation and a full list of TLVs and their applicability.
Engineering controls should be implemented whenever feasible to prevent overexposure to RF and MW fields. Engineering controls are strongly recommended when potential exposure can exceed the applicable exposure limit by a factor of 10. Administrative controls, which imply user knowledge and require interpretation or action, should be used when engineering controls are either not feasible or not adequate.
Careful configuration of equipment at a site can minimize potential RF radiation exposure in many circumstances. Installing physical barriers (e.g., locked doors, Faraday cages, fences, walls) can restrict access to certain spaces where RF radiation may exceed applicable exposure limits. Shielding, grounding, remote operation, and use of safety interlocks and waveguides are effective controls that are discussed briefly below.
Sources of RF radiation should be properly shielded to minimize stray radiation. Shielding mechanisms include reflection, absorption (attenuation), and internal reflection.
- Reflection is the primary shielding technique for electric fields and plane waves where the characteristic impedance is in excess of or equal to 377 ohms.
- Absorption losses result from the exponential decrease of the field amplitude as an electromagnetic wave is transmitted into the shield. Absorption increases with increasing shield thickness and is of primary importance in shielding low-frequency, low-impedance magnetic fields.
- Losses resulting from internal reflection are due to multiple reflections within the material.
Shielding materials for electric fields include conductive polymers and metals such as silver, copper, gold, aluminum, brass, bronze, tin, and lead. Meshes, other woven textiles, and perforated materials may also be used as shields.
A shielded enclosure (e.g., microwave oven with a screen built into the window) reduces leakage and penetration of RF fields. An enclosure must use specific shield materials and protect against leakage from seams, panels, flanges, cover plates, doors, ventilation openings, and cable penetrations.
A Faraday cage or Faraday shield is an enclosure used to block electromagnetic fields. A Faraday shield may be formed by a continuous covering of conductive material or in the case of a Faraday cage, by a mesh of such materials. A Faraday cage operates because an external electrical field causes the electric charges within the cage’s conducting material to be distributed such that they cancel the field’s effect in the cage’s interior. This phenomenon is used to protect sensitive electronic equipment from external radio frequency interference (RFI). Faraday cages are also used to enclose devices that produce RFI, such as radio transmitters, to prevent their radio waves from interfering with other nearby equipment. They are also used to protect people and equipment against actual electric currents such as lightning strikes and electrostatic discharges, since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.
An example of shielded enclosure is the door of a microwave oven with a screen built into the window. From the perspective of microwaves (with wavelengths of 12 cm) this screen finishes a Faraday cage formed by the oven’s metal housing.
Metallic structures producing contact shocks should be electrically grounded or insulated.
Devices that produce high levels of stray RF radiation (e.g., induction heaters) should be operated remotely whenever possible.
Devices that can cause acute thermal injuries (e.g., industrial microwave ovens) should have interlocked doors. Interlocks must not be tampered with; if they are defective, they may be unreliable and should be repaired or replaced.
A waveguide is a hollow metal tube constructed from conductive materials such as copper, aluminum and brass, used to confine and convey (guide) electromagnetic waves. Waveguides minimize transmission loss and increase attenuation. If the wavelength is twice the width of the waveguide, waves will not propagate in the waveguide.
Effective administrative controls include monitoring the duration of RF radiation exposure. Exposure times should be kept as short as reasonably possible and should not exceed the TLVs in the applicable averaging times. In addition, contact with external surfaces of radiating devices should be minimized. Exposure can be controlled by varying the distance from the source. For example, antennas that will routinely exceed the occupational standards should be placed in locations that are least likely to be encountered by common foot traffic (e.g., cell relays mounted on the exterior face of the upper floor of a building). RF users should be aware of the horizontal and vertical extent of the exclusion zone.
Other administrative controls include providing proper safety training to workers, wearing protective gear, and posting and heeding warning signs.
All individual workers who have access to an RF environment should, at a minimum, receive RF awareness safety training.
Personal protective clothing
Everyday footwear and socks (thick leather shoes with rubber soles and wool socks are preferable) can modify the absorption of electromagnetic energy at frequencies between 10 and 40 MHz by reducing the grounding effect.
RF-protective suits can be worn for RF protection. These are made of wool or polyamides (e.g., nylon) impregnated with highly conductive metal (e.g., silver) or woven with stainless steel thread. A mesh design, in which the fibers are in vertical and horizontal orientation, is the most effective. Note that RF-protective suits may not protect against leakage at access points and openings such as at the zipper and cuffs.
Hazard warning signs
Potentially hazardous RF devices should be appropriately labeled, and areas of excessive exposure around them clearly demarcated. Where required, notices with warnings and the necessary precautions must be conspicuously posted. “RF Warning” and “RF Caution” signs identify potential safety hazards, warn workers before they enter the environment, and restrict access to those who are not authorized to enter.
Variations of the RF hazard warning sign are shown below.
Any person suffering or suspecting an overexposure from a RF incident should receive medical treatment. Personnel should inform the supervisor, the division safety coordinator (DSC), and the nonionizing radiation subject matter expert (SME) of the suspected or actual RF overexposure as soon as practicable. The same advice applies to an incident of interference with a medical device. Possible indications of overexposure are symptoms such as pain, reddening of the skin, unusually elevated body temperature, and other evidence of tissue burning.
If you believe that you or another worker has been overexposed to RF radiation, follow these steps:
- Remove the worker from the exposure area to a cool environment and provide cool drinking water.
- Apply cold water or ice to burned areas.
- Seek immediate medical attention. Severe MW or RF overexposure may damage internal tissues without apparent skin injury.
- Notify your supervisor as soon as you can, no matter how minor the injury may seem.
- Report all unsafe working conditions to your supervisor or the nonionizing radiation SME in the EHS Division.
American Conference of Governmental Industrial Hygienists (ACGIH), 2016 TLVs and BEIs, incorporated by reference 10 CFR 851 Worker Safety and Health – Department of Energy, §851.27.
ACGIH, 2012 TLVs and BEIs.
Code of Federal Regulations, Title 10, Part 851 (10 CFR 851), Department of Energy Worker Safety and Health Program, §851.23 Safety and Health Standards.
Hitchcock, R.T., 2004. Radio-Frequency and Microwave Radiation, Third Edition. Nonionizing Radiation Guide Series, American Industrial Hygiene Association.
Institute of Electrical and Electronics Engineers (IEEE), IEEE Standard C95.1-2005, IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz.
IEEE, IEEE Standard C95.3.1-2010, IEEE Recommended Practice for Measurements and Computations of Electric, Magnetic, and Electromagnetic Fields with Respect to Human Exposure to Such Fields, 0 Hz to 100 kHz.
IEEE, IEEE Standard C95.7-2005, IEEE Recommended Practice for Radio Frequency Safety Programs, 3 kHz to 300 GHz.