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Electromagnetic radiation is everywhere, from radio waves to microwaves.
But what happens when these waves come into contact with living cells?
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Classification of the frequency regions of electromagnetic fields in one sense, is totally arbitrary.
Biophysical peculiarities like the typical binding energies of electrons and the orbital from which electrons are excited help define the different bands of photons reasonably.
This is shown in figure 4.26
The relativistic Doppler effect explains that frequency, wavelength, and amplitude of traveling photons can change caused by the relative motion of the source and the observer.
Skin Effect
In this frequency, becomes important.
This effect also determines the depth of penetration of HF electromagnetic fields in a homogeneous body.
It is characterized by skin depth where
Fig. 30
It shows that in the frequency range of 20-300 MHz, high absorbtion can occur in the whole body
At higher frequencies, the field does not penetrate deeply. It will be mainly absorbed at the surface.
Frequency of 1GHz
The distribution and penetration pf high frequency EM fields in the living body is an important factor in therapeutic applications, as well as in safety considerations.
Affects only the skin in humans
This equation gives rough estimate of penetration
There are two main dosimetric considerations that must be taken to consideration:
1. Dielectric parameters
2. Resonance phenomenon of the part or whole of body
Unlike the previous graph of skin depth, here conductivities themselves are functions of frequency
The dielectric parameters of different tissue vary to a large extent.
To establish realistic dosimetry of high-frequency field exposition, finite-difference time-domain (FDTD) is used.
FDTD, or Yee's Method, is a numerical analysis technique used for modeling computational electrodynamics
Dosimetric calculations are based on the intensity of the field characterized by the plane power density emitted by an external source.
Plane power density is simply the power per unit area.
When the region of exposure extend farther than about 2 wavelengths away from the source is called the far field.
At this condition, plane waves are to be expected.
Taking to account the resonance phenomenon, the human body, if not grounded, has a resonant absorption frequency close to 70 MHz.
According to Grothus-Draper principle, not the energy penetrating the organism , but only that part which is absorbed by in the system can be effective.
Therefore, a specific absorption rate (SAR) is defined to characterize the dose of high frequency exposition.
SAR is the energy absorbed per unit time in a unit of mass or volume of a biological body.
SAR considers the abosrbed penetrating the tissue or body. However, as we mentioned before, at high frequencies, surface absorption dominates and few field strength penetrates the body.
Thus, at high frequencies SAR is proofed not as good measure of assessing absorbed energy.
This is the reason why at microwave frequencies up to UV-light the incident power density (W/m^-2) is more appropriate dosimetric quantity.
In frequencies upto 100KHz, thermal affects do not dominate over excitation effects of muscles and nerves.
In frequency region greater than 10^5 Hz, there may exist any one of the following effects:
- Diathermal Heating
- Nonthermal Effects
Diathermy means the inhomogeneous heating of the body corresponding to the inhomogenity of field absorption due to nature of tissues and living body.
-Therapeutic muscle relaxation
-Create higher tissue temperatures to destroy neoplasms (cancer and tumors)
-In surgery diathermy is used to cauterize blood vessels to prevent excessive bleeding
Example of well known nonthermal effect according to the biophysical definition.
In the emperical definiton, a situation is said to be "nonthermal" when instensity is so low that changes in temperature of the exposed body are not measureable.
In the biophysical definiton, a mechanism is said to be nonthermal if the interaction of the electrical or magnetic vector of the EMF with charges or dipoles of molecules leads to effects other than heating.
Dielctrophoreses is the electrorotation of cells.
In these cases the electrical field induces dipoles which directly lead to cell movement or deformation
At extremely large field strength, heating must occur, thus nonthermal effects are not biophysically possible.
For this reason, exposure limits reccomendation are based exclusively on thermal effects of HF radiation.
Considering the system illustrated in Fig. 4., thermoreceptors can be activated as the result of low-intensity field interaction without a measureable increase in the body temperature.
From section 4.1 - Fig 4.2
This leads to local effects like the modification of blood circulation or EEG.
The absorbtion of HF fields in biological systems is determined by its water content.
Dipoles of free water oscillate in a frequency of 18.7 GHz.
The broad dispersion of dielectric properties of the biological material makes it impossible to expect frequency windows of HF-field interactions.
In contrast, dipoles of bound water can be shifted to a tenth of a MHz
At strong high frequency pulses a microwave auditory effect, or RF-hearing occurs
The Microwave Auditory Effect, Rf-hearing, or the Frey Effect is caused by abrupt heating of tissue water in the head.
RF-hearing has been reported at frequencies ranging from 2.4 MHz to 10 GHz.
This effect depends on the energy of a single pulse, and not the average power density.
To analyze possible biophysical mechanisms of high-frequency effects of weak fields on biological systems, hypothesis had been proposed:
- The idea that high-frequency electromagnetic fields could exhibit classical resonance phenomenon, absorbing exces energy.
References :
Glaser, R. (2012). Biophysics: An introduction (2nd ed.). Springer.
Dunaief, L. (2018, September 6). The Frey Effect. TBR Newsmedia.
Broad, W. J. (2018, September 1). Microwave Weapons Are Prime Suspect in Ills of U.S. Embassy Workers. The New York Times.
Lin, J. C. (2022). Microwave auditory effects among U.S. government personnel reporting directional audible and sensory phenomena in havana. IEEE Access, 10, 44577-44582. https://doi.org/10.1109/ACCESS.2022.3168656
Hurley, D. (2019, May 15). Was It an Invisible Attack on U.S. Diplomats, or Something Stranger? The New York Times Magazine.
Kiefer, D. (2014). Relativistic electron mirrors: From high intensity laser-nanofoil interactions (2015th ed.). Springer International Publishing AG.
- The vibratory motion by biological fluids is damped by water properties.
- The absence of reliable biophysical theory for possible low-intensity, nonthermal effects of high-frequency EMFs correspond to the unsuccessful experiments
-Therefore, only thermal, or possible microthermal effects are to be proposed below electrorotation or dielectrophoreses as nonthermal efects at strong field intensities.