1. Radiofrequency radiation (RFR)
RFR is divided into different frequency bands assigned to communicate information in industry, medicine and science. Some types of RFR sources may be classified as deliberate emitters which emit their energy freely into space either in:
Other sources emit the energy within an enclosed area, e.g. microwave ovens.
The main sources are :
In the interaction of radiation with biological matter, two parameters are specified :
SAR= s |E|2/ r =|J|2/ rs
Orientation polarization occurs in biological aqueous substances (bound water, peptides, proteins or side chains of larger molecules) due to energy absorption which would become maximum at frequencies between 1-20 GHz. The absorbed energy is completely transformed into heat.
Polarization of bounding charges occurring in structures with different electric properties.
The electromagnetic field produces a high frequency electric current in tissue, which flows around the cell . In this case, the cell acts like a large dipole due to charge displacement. This charge displacement results in a difference in electric potential between the internal and external space of the cell which superimposes the existing resting potential of the membrane.
Under given physiological conditions, in all of the above mentioned cases, the heat effects predominate.
A power density of 100W/m2 increases the average temperature from 0.5-2ºC (depending on frequency), taking heat transport and normal blood flow into consideration. At a temperature rise of about 0.5ºC, the normal thermoregulation in the human hypothalamus begins to respond.
In poorly circulated organs such as the eye lens, the temperature increase is greater; power density of >1KW/m2 may lead to formation of a cataract. Accordingly, the eyes must be protected during HF therapy in the head region
A completely different energy distribution may result if metallic implants are present in the body. This is because RF may produce burns on the contact surfaces of metal-tissue. Thus, such effects must be taken into consideration.
- During MRI examinations (using RFR and strong magnetic field), due to the fact that the patient is practically part of the transmitter coil under near conditions, then special circumstances must be applied
- Other biological effects were observed without increase in temperature at extremely low frequency- ELF- modulated high frequency fields or microwaves in the mm range including changes in:
- Chromosomes or DNA
- Ion exchange over cell membranes
- Electrophysiological properties of nerve cells
- Mobility of Ca ions, particularly in brain tissue
- Blood system and immune system with SAR values more than 0.4 W/Kg
- Nerve cells of the CNS and behavior were influenced by high power densities as well as by chronic exposure to low energy with SAR values of 2W/Kg.
- Occurrence of cancer or co-carcinogenesis
d- Risk assessment and exposure limits
-Risk assessment of the effect of high frequency electromagnetic fields are considered acute (non-stochastic) effects that occur only after a certain minimum thresholds. Late effects with chronic exposure are insufficient for setting limits.
-For low power devices with antenna operating near the body (mobile phone) high local SAR values were calculated or measured and the max. Permissible local SAR value is 0.1W/10g.
Limits are highly frequency dependent :
-For frequencies of > 10 KHz we use the SAR averaged over the whole body mass to assess biological effects .
- For frequencies ~ 100 KHz , the non-thermal effects depend on tissue field strength E or current density.
-For frequencies ~ few MHz , the body current density J(effective value) may be used for the same purpose.
-The threshold value for HF (100 KHz-100 MHz) burns from finger contact with conductive surfaces is about 200mA.
-Typical exposure values resulting from electromagnetic field sources are listed in tables 3, 4, & 5. Comparison with the limits of exposure shows that in many cases it should be checked by measurements whether protection measures are necessary or not.
Table (3): Exposure limits at working places .
Table (4) : Exposure limits for the general public.
Table (5): Exposure limits for medical application
2.Extremely low frequency fields (ELF)
- ELF fields with frequency less than 300 Hz, import energy to living tissues that is too weak to break chemical or molecular bonds.
- They are referred to as fields rather than radiation since the electric and magnetic fields are not coupled as with higher frequencies (NIOSH, 1996).
- The electric and magnetic fields that exist in the space around sources of electromagnetic fields must be considered separately, because the very long wavelength characteristic of ELF means that the measurements are made in the non-radiating near field region.
b- Sources of ELF fields
The electrical and magnetic fields of the earth consist of :
a) a static component, which is dominant, and
b) a time–varying component, which is smaller than the static component by several orders of magnitude.
-The natural occurring electric field strength at the power frequencies of 50 Hz is about 10-4V/m, which means that fields in the close vicinity of high voltage transmission lines are 108 times stronger and the fields introduced into homes by wiring or appliances are still about 103- 106 times stronger than the natural background.
- On the other hand, the natural occurring time varying magnetic fields have several origins, including varying fields of the order of 30nT (T=104G) associated with solar and lunar influences on ionospheric currents.
- Electric and magnetic fields in the home and public applications are shown in table 6:
a) Home instruments, e.g. can openers, hair dryers, florescent lamps, microwave ovens, TV devices, clothes and dish washers,…etc. produce an ELF range of 0.03 –30 mT, measured at a distance of 30cm from a device (table 4).
b) High voltage transmission lines and land based communication systems ( the field at ground level beneath a 765-KV, 60 Hz power line carrying 1 KA is 15mT while that associated with naval communication systems is 20mT ).
c) Occupational exposure from working near industrial equipments using high currents ( local fields near ladle furnaces ~8mT and 0.07T near induction heating devices).
Table (6): Magnetic Flux densities (mT) at 60 Hz near various appliance .
c-Biological effects of ELF
ELF electric fields induce an alternating surface charge and weak electric fields and currents within the body.
ELF magnetic fields induce circulating electric current that flows in the aqueous medium which surrounds living cells. Because the membranes of these cells form a dielectric barrier to the passage of current, in the ELF frequency range, only a small fraction of the induced current can penetrate the cell surface. Therefore, it is believed that the induced current would produce electrochemical alterations in components of the cell membrane surface that produce alterations in intracellular biochemical and physiological functions.
Human and animal data suggest that the function of CNS can be affected at current density as low as 10mA/m2.
- Several processes in the human (and animal) body are based on electrical activity. For example, brain and heart functions are always connected to electrical activity which is accompanied by an electromagnetic field.
- The bioelectric field is measured with the help of surface electrodes, and is known as electrocardiogram (ECG) or electroencephalogram (EEG).
- Biomagnetism deals with the measurement and analysis of the associated magnetic fields which is measured with the help of super-conducting quantum interference devices (SQUIDs) and it is known as magnetocardiogram (MCG) or magneto-encephalogram (MEG).
- Magnetic fields are also generated by ferromagnetic, fine-grained, magnetically interacting particles found in the heart tissue as well as in the spleen, brain and the liver.
d- Risk assessment and exposure limits
- Many organizations have developed exposure guidelines for power frequency which intended to prevent effects, such as induced current in cells or nerve stimulation.
- The following table (table 5) presents the exposure limit of ELF (50/60Hz) established by the Inter-national Commission on Non- Ionizing Radiation Protection (ICNIRP) 1996.
Table (7): exposure limits for ELF
Sunday, December 27, 2009
Radiation in the environment : Types of non-ionizing radiation
Publié par Dr.taki eddine à l'adresse 7:13 AM