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Chapter 4: Physical Factors of the Environment

4.7 Radio- and Microwave EMFs

Electromagnetic radiation is everywhere, from radio waves to microwaves.

But what happens when these waves come into contact with living cells?

Anfal Aljishi 2200002444 Areej Mutiloq 2200005454

Bana Anabosi 2200006459 Reem AlNasser 2190004631

Physical Background and Dosimetry

Classification of the frequency regions of electromagnetic fields in one sense, is totally arbitrary.

Physical Background

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

From a relativistic approach, Doppler Shift exhibits that there can not be a truly fundamental difference between waves in the spectrum.

The Doppler Effect affects light too!

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.

In High Frequency (HF) region a spliting into the magnetic and elecric field vector is impossible.

Skin Effect

In this frequency, becomes important.

The skin effect

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.

Topic

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

Dielectric and Anatomic Peculiarities

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

Dosimetric Calculations

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

Parameters of Dosimetric Calculations

Plane power Density (W / m-2)

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.

Specific Absorption Rate (SAR)

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.

Specific Absorption Rate (SAR)

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.

Effects of HF EMFs

In frequencies upto 100KHz, thermal affects do not dominate over excitation effects of muscles and nerves.

Thermal Effects of HF Radiation

In frequency region greater than 10^5 Hz, there may exist any one of the following effects:

- Diathermal Heating

- Nonthermal Effects

Diathermy

Diathermy means the inhomogeneous heating of the body corresponding to the inhomogenity of field absorption due to nature of tissues and living body.

Uses of Diathermy

-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

Nonthermal effects

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.

Nonthermal Effects

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.

Low-intensity field effects

Thermoregulation

of the

living body

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.

HF Fields effect

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.

HF fields effect

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

Microwave Auditory Effect

Microwave Auditory Effect

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.

Conclusion

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.

Conclusion

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.

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