Physical Mechanisms for RF Effects on Biological Systems
There is a long chain of events that must be followed to go from fundamental physical interactions between electromagnetic fields and molecules or ions to the production of changes in biologic systems that could lead to adverse human health effects. This chain of events has yet to be described in detail. This section will outline a small portion of those events, beginning with the forces of electric and magnetic fields on electrons, ions, atoms, and molecules, and changes in their energy, configuration, or orientation. These changes in turn can lead to further changes in chemical reaction rates and in the binding of molecules to membranes. This progression can then lead to changes in cell activity that in turn can affect the biology of the organism. A subset of these biologic changes can lead to adverse health effects. The body has many feedback processes so that many biologic deviations from the norm are corrected before they become adverse health effects. This section of the report will focus on the physics involved in the initial portion of this chain of events.
The depth of RF penetration is dependent on the conductivity and dielectric constant of the biological material; usually in the range of 3 to 4 cm at 433 MHz for typical biological materials and ranges up to 16 cm in bone. Thus one can expect a significant fraction of the field to penetrate into the body.
Maxwell’s equations, the Lorentz force law, and the electrical characteristics of the material in question describe the basic interaction between electromagnetic waves and that material. The electric field is defined so that the force on a charged particle is given by the product of the charge, q, and the electric field, E. Similarly the force exerted by the magnetic flux density, B, is given by the vector product of the velocity, v, of a charged particle and the magnetic flux density. The vector product means that this force is at right angles to both the velocity of the particle
and B or it is perpendicular to the plane formed by these vectors. Given the value of the fields E and B, one can calculate the forces, F, being exerted on an electron, an ion, or a charged molecule. These relations can be expressed by the following formula:
F= q(E + v x B).
From these forces and the equations of motion, one can calculate changes in motion and kinetic energy of the particles as a result of the application of the fields as a function of time and space. It should be noted that at the PAVE PAWS radar frequency of 420 MHz, the direction of the force reverses every half cycle or every 1.2 × 10−9 seconds. Therefore, the average displacement of these charged particles in a uniform field is zero. Thus the dominant result of a uniform field is heating. An average incident power density of 1 µW/cm2 on a standing man would lead to a specific absorption rate, SAR, of approximately 5 × 10−5 W/kg. This is about four orders of magnitude lower than the metabolic-energy generation rate of a resting man.
In a non-uniform field the gradient of these fields can induce a directional force in atoms and molecules. If for example, the electric field induces a dipole moment that oscillates with the field, then the gradient of the field will exert a force on the particle that is in a constant direction. Thus the gradient of the field can lead to a drift current density, J, that is given by the formula:
J = N µα V(E • ∇)E,
where α is the polarizability, N is the density of the particles, V is the volume of the ion or molecule, and µ is the mobility. For the fields under consideration, this induced drift current density will be very small and is expected to be very small with respect to the diffusion current and the drift currents associated with the fields that occur naturally around cells associated with biological activity that are on the order of microamps per centimeter squared.
The applied fields can also change the state or energy of the bound electrons in an atom or molecule. For weak fields these changes are associated with the absorption of a quantum of energy from the RF field. The amount of energy, W, associated with a single quantum of the RF field is given by the formula:
W = hf,
where h is Planck’s constant and f is the frequency. The quantum of energy associated with a photon of microwave energy is about 10−5 times smaller than the photon of energy associated with room temperature radiation. The thermal radiation has its maximum energy in the infrared. This thermal background radiation nearly equalizes the population of low energy states that are separated by ener-
gies corresponding to a microwave photon at thermal equilibrium so that many quantum effects are completely masked by the thermal background radiation.
One area in which this may not be true is associated with excited states where most of the energy is supplied by another process such as a chemical reaction or ultraviolet radiation. For these excited states, spin selection rules may control the rate at which they decay or react with other materials and the population of molecules in these states may be changed by RF radiation. Examples where these processes may be important are free radicals in which relatively low levels of RF power have been shown to change the absorption spectra. In the 1-80 MHz region of the spectra, and at field strengths of 0.1 to 0.5 mT, Stass and others (2000) have shown a magnetic field effect on the photochemical reaction of anthracene-d10 with 1,3-dicyano-benzene in a cyclohexanol/acetonitrile solution corresponding to changes in the free radical life times. These transitions are associated with hyperfine spectra of the molecules.
In general, the fields are attenuated as they propagate through the tissue. It is a relatively complicated problem to estimate how strong the fields will be after they go through the skin and other anatomy taking into account the geometry of the body and the differing electrical properties of the skin, bone, fat, and other tissues to find the field strengths at the site of interest for a given biological effect. Tables for the electrical properties of many tissues, are given by Gabriel, Lau, and Corthout as a function of frequency in three papers . Using this kind of data, numerical models have been used to calculate the field distributions in the head and the body in various positions (Hagmann and Gandhi 1979; Jensen and Samii 1995; Iskander and others 2000). For a review of computational methods for computing field distributions see the Handbook of Biological Effects of Electromagnetic Fields, Chapter 9 (Lin and Gandhi 1996). In general, the fields will be weaker the farther away the biological site of interest is from the surface facing the source. In brain tissue the attenuation coefficient is 31.1 m−1 and the depth of penetration is about 3.2 cm. The fields in membranes and other low water content material will be larger than in the high water content material by the ratio of the dielectric constants. This is about a factor of 20 at 420 MHz for membranes in a fluid.
The effect of the RF fields on the biological system may take place by changing chemical reaction rates or the binding of molecules to a membrane surface. This could occur in at least five ways (Barnes 1996). First, it may affect the transport of ions or charged molecules and thus the probability of the two particles coming close enough to each other to react. Second, it may affect the energy with which they collide. Third, it may affect the orientation or configuration of the colliding particles. Fourth, it may change the energy state of one of the molecules. Fifth, it may affect the average temperature of the environment. Of these effects, only those related to changes in the average temperature are well studied and are generally accepted by the scientific community at large as described in a review article by Adair (2003).
Effects that are currently being studied include changes in the molecular configuration of large biological molecules as a result of the application of RF fields, dielectrophoresis or the effects of the gradients of the fields on the transport of molecules in the vicinity of membranes, and the effects of RF fields on free radical lifetimes. For dielectrophoresis to be important the resulting current must be a significant fraction of the natural current of the same material. Initial results of the studies of these mechanisms indicate that to be important, the field strengths need to be sufficient so that the energy absorbed from the RF fields is a reasonable fraction of the thermal energy, kT.
Other physical changes that have been suggested include changes in the diffusion constants, (Seto and Hsieh 1976) and rectification of the electric field by membranes (CRC 1996). For these to occur, the signals must be large enough for non-linearities to become significant. Non-linearities are expected to be most important in biological systems with gain. At low frequencies, one such system is the cardiac pacemaker where cells were shown to have nonlinear effects on the oscillation frequency on the order of a hundred microvolts (CRC 1996). However, at frequencies above 10 MHz the membrane capacity shorts out the applied field and the field reverses direction so rapidly that ions cannot transit a membrane. Measurements of RF field range applied to cell membranes have not shown any rectification (Pickard and Barsoum 1981).
An important problem is determining the minimum signal that a biological system can detect in the presence of noise. For reliable communications signal-to-noise ratios of greater than one are usually required and typical values are one hundred-to-one and one thousand-to-one. Sources of noise include thermal noise, shot noise, 1/f noise, and the electrical signals generated by other parts of the biological system. At low frequencies, the electrical signals from muscle activity of the heart are usually the largest source of electrical noise. In biological systems, repetitive pulses that have a pattern that can be distinguished from the noise are usually required to initiate important changes or to put information into memory. These signals often have both space and time coherence. For signals that are coherent in space, the signal-to-noise ratio grows as the square root of the number of events in parallel with respect to random noise. For signals that are coherent in time one gets a similar increase in signal-to-noise ratio with the square root of the length of time for which the signal is applied (Weaver and Astumian 1990).
At 420 MHz, the thermal noise is expected to be the largest source of noise. The noise from this source leads to mean-squared average-voltage fluctuation across a membrane that is given by
<V2> = 4kTΔfR,
where k is Boltzmann’s constant, T is the absolute temperature, Δf is the bandwidth, and R is the resistance.
The value of the resistance and the effective bandwidth of cell membranes will depend on the geometry of the cell. Additionally, because cells are electrically coupled to each other, the effective values for the resistance and capacitance of a cell will depend on its environment and thus the noise voltage will also be dependent on the geometry of the cells. For an externally applied voltage, the voltage across a particular cell is dependent on the geometry of the cell and the structure of the surrounding tissue. Typical models for a cell membrane would consist of resistor and capacitor in parallel. Another resistor and capacitor in parallel would model the fluid portion of the cell and the applied field would extend across a large number of cells in series. At high frequencies, this would behave like a capacitive voltage divider so that the applied voltage to a given membrane would be roughly equal to the applied field divided by twice the number of cells per unit distance.
At low frequencies the currents through a cell membrane are nonlinear functions of the applied voltage. These nonlinearities are such that cell membranes in nerve cells can behave as poor rectifiers with a typical efficiency of about 0.1%. At radiofrequencies, this efficiency has typically not been measured and there is at least one proposal that is outstanding to make measurements of this kind.
A single molecular event can be amplified by a variety of means. For example, the binding of a single neural transmitter at a synaptic junction can lead to the release of thousands of calcium ions that in turn become a part of the signal that is used to excite the next synapse. It typically takes approximately 20 dendritic inputs to a summing junction to fire an axon. Repetitive stimulation can lead to persistent changes that either increase or decrease the threshold for firing. Stochastic resonance is another means of amplification that can lead to an increase in the signal-to-noise ratio. In these processes a small periodic signal in a nonlinear system can be amplified by extracting energy from the noise. Gains on the order of 100 and similar improvements in the signal-to-noise ratios have been shown for physical systems, and stochastic resonance has been shown to be one method for improving the sensitivity of biological systems to weak signals (Gammaitoni and others 1998).
There are a number of possible mechanisms and pathways by which electric and magnetic fields could lead to biological changes at high-power exposure levels. However, at this time, the committee does not know of a physical mechanism that has been shown to change biological processes at the field-strength levels associated with exposures to the PAVE PAWS radar.
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