Exposure Limits for Radio Frequency EMR
Using a large amount of established scientific knowledge, exposure limits have been set to protect the general public from adverse effects associated with RF EMR. These limits apply to the levels of electric fields and magnetic fields measured at the body surface (10).
Medium frequency (MF) transmission is used in AM radio transmission, navigation systems for ships and aircraft, coast guards and emergency distress signals. MF is also reflected by the Earth’s ionosphere, making it suitable for long-distance communication.
Frequency is the number of waves that pass a point in a given amount of time. It is usually expressed in Hertz (Hz) – named for 19th-century physicist Heinrich Rudolf Hertz who proved the existence of electromagnetic waves.
The m radio frequency range is used by AM broadcasting stations, navigation systems for ships and aircraft, and emergency distress signals. It also gets reflected by the earth’s ionosphere, making it an ideal band for long-distance communication.
Generally, there are no established adverse health effects from exposure to RF EMFs at levels below those specified in Safety Code 6. However, as technology continues to advance rapidly, some unusual and unique exposure scenarios may arise. In these situations, interpretation of the specific restrictions in this code may be required. The amount of RF energy absorbed by a volume of tissue is known as the Specific Absorption Rate (SAR). The SAR limits in this code are based on a review of available scientific literature regarding the interaction between RF EMFs and the human body.
Amplitude is the amount of energy that a wave has. A wave with high amplitude has more energy, while a wave with low amplitude has less.
Amplitude is most often measured in decibels, which provide a logarithmic scale that closely matches our perception of loudness. The term amplitude also has other meanings, including:
In physics, amplitude is the measure of the distance between the peak of a wave and its trough. It is also called the height of a wave or the magnitude of a wave.
For AM radio, amplitude is important because it is the means by which information is encoded on a carrier wave. The carrier signal is a simple unmodulated sine wave, and the modulated signal contains the letters or words you want to hear. The amplitude of the modulated signal will vary based on the information sent, so that the letter ‘A’ will have a lower amplitude than the letter ‘Z’.
The wavelength of a wave is the distance between the starting point and the ending point of one complete wave cycle. It is measured in meters (m), centimeters (cm) and micrometers (mm).
For example, a bass note has such a long wavelength that you can hear it through the wall of your apartment next door. High frequencies, on the other hand, have short wavelengths and are easily blocked by walls and doors.
When determining whole body absorption of a radiofrequency field, the resonance range of the person influences how much he or she is exposed to the field. For children, the resonance range is influenced by body height as well as posture. This is particularly important because the size of a child’s head matches half of the wavelength for low-frequency radio waves. For adults, the resonance range is mainly influenced by body weight. The frequency-dependent resonance of the human body is also called the antenna effect.
Power is the quantity of energy that a radio wave has, which depends on its frequency and wavelength. The higher the frequency and shorter the wavelength, the more energy a radio wave has. This also affects the range of a radio signal, with lower frequencies traveling farther than higher frequencies.
An RF electromagnetic field has both electric and magnetic components, so it is often convenient to express the strength of a particular source at a given location in terms of units specific to each component. For example, volts per meter (V/m) is commonly used to describe the strength of an electric field, and amperes per meter (A/m) is used to describe the strength of a magnetic field.
Another common way to express the intensity of an RF field is to use its power density. This is the rate of energy flow per unit area and is usually expressed in watts per square meter (W/m2), milliwatts per cubic centimeter (mW/cm2), or microwatts per cubic centimeter (uW/cm2). Power density is most appropriate when the measurement point is far enough away from an antenna that the induced electric and magnetic fields do not have a substantially plane-wave character.