RF Radiation

As biological research continued, it became apparent that three primary questions needed to be answered.

• How do various RF fields affect the body?
• At what levels does the body suffer adverse effects?
• At what levels are the effects permanent?

Early on, we knew that the primary concern was thermal—quite simply, the body heats up in the presence of significant RF energy. The first ANSI standard was a best guess (see History). But, as research continued, it became apparent that many factors impact how much the body heats up. The concept of Specific Absorption Rate, or SAR, evolved in the early 1970s. SAR defines the rate of absorption of heat into the body in units of Watts per kilogram and has become the basis of all the major RF exposure standards worldwide. Research determined that SAR follows basic antenna theory—the better an antenna you are, the more RF energy you absorb when exposed to a particular field strength. Most of the time, you are not grounded and your body is equivalent to a wide, lossy dipole. What is a wide, lossy dipole? A dipole is a straight, center-fed antenna that is normally a little less than a half-wavelength long. You are wide because even a very thin person is wide compared to a metal rod. You are “lossy” because the impedance of the human body is much higher than a metal rod. In other words, your body has more internal resistance to the flow of RF energy than a highly conductive metal road. Where the metal dipole might have an impedance of 2 to 3 Ohms, the typical impedance for a human body is about 360 Ohms. The higher impedance means that some of the RF energy is “lost.” It is actually converted into heat in the same way that the wires in a heating element found in a toaster or electric oven convert electrical energy to heat. If you are well grounded, which is surprisingly difficult to achieve, your body is equivalent to a quarter-wave antenna rather than a dipole. Under grounded conditions, you absorb the most energy at half the frequency that you would under normal, ungrounded conditions.

Researchers consider the “standard man” to be 1.75 meters tall, about 5 foot 9 inches. That makes him resonant at about 86 MHz. So the average adult makes a perfect antenna for Channel 6 television! The average woman, who is shorter, makes a good antenna for FM radio. The biology is certainly more complicated than that but height, grounding, and polarization are the most important factors in determining SAR level.

It was determined that the most heat the human body can deal handle without risking permanent damage is approximately 4 W/kg. To put things in perspective, the normal metabolic rate when someone is sleeping is about 1.0 W/kg. It increases to about 2.0 to 2.5 W/kg during moderate exercise. Much of this research was based on exercise levels rather than on actual exposure experiments. This is because exposure to significant levels of RF energy is very similar to the effects of overexertion.

Field levels are averaged over the entire body, since our circulatory system functions much like a radiator and evens things out. If only part of the body, such as an arm or leg, was exposed to an RF source, it would absorb much less energy than if the entire body was exposed. The major standards typically allow for exposure of an appendage to fields up to 20 times higher than are allowed for the whole body. The eyes and a male’s testes are particularly vulnerable, however, since the limited blood flow of these organs limits the benefits of the circulatory system. The whole-body limits apply for these organs.

Time is also a factor—most standards average exposure over time, which only makes sense since we are dealing with heat and the human body can only take very short-term exposure to extremes of heat or cold. Six minutes is the averaging period for most occupational exposure limits.

Moderate level exposures cause heat stress and behavioral changes. The effects are often mistaken for the flu because the symptoms are similar. As the level of exposure increases, the potential for harm increases. Human cells die at 107° Fahrenheit. This is the reason why doctors get concerned if someone’s temperature goes above 105°. The body is constantly replacing cells, so the amount of damage that is done depends on how many cells are killed and the type of cells that are killed. Kill off some cells, and the effects may pass in minutes or hours. Destroy a lot of liver cells for instance, and you will have liver damage. If the damage is not too severe, the body can repair itself. However, if the damage is extensive, the effects may be permanent! Perhaps the most vulnerable organs are the eyes. The eyes have virtually no blood flow that can provide cooling from other parts of the body, and their dimensions make them very good antennas at microwave frequencies.

A shock or an RF burn—electrostimulation—occurs when you come into contact with either an RF radiator or a re-radiator. RF radiators are usually some type of antenna. Many antenna designs cause RF current to flow in their metallic components, which in turn, is radiated into space. Touch one of these surfaces, and the energy will flow through your body to ground. Similarly, the same thing can happen if you touch a re-radiator. Any ungrounded, conductive (usually metal) object that is in the field of a strong RF source can be illuminated by the RF field and re-radiate the energy back into space. These metallic objects can have high RF voltages present on them unless they are well grounded. It is often very difficult to make a good RF ground, so objects that appear to be well grounded are often “floating.” For example, the U.S. Navy goes to great lengths to add copper straps to the railings that are welded to the hull of the ship. If the straps are broken due to damage or corrosion from salt, it is not uncommon for sailors to receive burns when they touch the railing. When you touch a re-radiator, you provide a path to ground through you. A surge of energy occurs at the point of contact. This results in a shock and, in many cases, an RF burn.

The primary factors that determine if you will receive a shock or burn should you contact a conductive object are

• the strength of the electric field,
• the frequency,
• how well grounded you are, and
• how much of your body touches the object.

Although the relationship between field strength and shock and burn potential is very complicated and difficult to calculate, it is clear that the shock potential increases with the strength of the electric field. If you are very well grounded, you have a much higher chance of receiving a shock or burn. If the frequency is close to where you make an ideal quarter-wave antenna, then the potential is much higher. This occurs at about 40 MHz for well-grounded adults. Many engineers who have received shocks and burns approach a conductive object with trepidation and reach out to touch it much like one would lightly touch a wall to see if the paint was still wet. This is absolutely the worse thing that you can do! When you touch an object with the tip of your finger, all the current flows through that very small area. When you grasp the object or touch it with the palm of your hand, the area that makes contact is about 100 times larger. And the current density is only about 1 percent of the previous scenario. This is the reason that the IEEE standard for contact current is based on a grasp. The committee members felt that limiting the field levels to a point where a point contact would not produce a shock or burn would result in very restrictive exposure limits. The standard is based on concern for shock rather than body heating.
The major worldwide standards are based on SAR for all but the lowest frequencies. At the lower frequencies, typically below 10 to 30 MHz, there very little body heating that can take place because the human body is only a very small fraction of a wavelength and thus, not a very good antenna. While the portions of the standards based on body heating provide adequate protection under virtually all conditions, the same cannot be said of the exposure limits at the lower frequencies where electrostimulation is the major concern. For example, extremely high RF voltages have been measured on cranes located at construction sites that are close to an AM radio station. These severe burn-hazard conditions exist where the RF field level may be less than 1 percent of the MPE limit!