Non-ionizing Radiation Safety

http://www.ee.cityu.edu.hk/

1. Introduction

2. What is ionizing and non-ionizing radiation?

3. The Propagation of Electromagnetic Waves.

4. Power Density and Field Strength.

5. Absorption of RF Energy into the Human Body.

6. Main Effects of RF Radiation.

7. Occupational and General Public Exposure.

8. Safety Limits.

9. IRPA Guideline

10. Example.

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Mr. W.S. Chan



1. Introduction

This paper was prepared for the workshop on laboratory safety with a view of making aware, the safety aspects of non-ionizing radiation. Non-ionizing radiation is more commonly known as RF radiation or microwave radiation above a frequency of 1 GHz, and is not to be confused with ionizing radiation such as X-rays. This is shown plotted in Figure 1, attached near the end of this paper.

To be able to determine what constitutes safe for non-ionizing radiation some mathematics [1] will need to be introduced, but will be minimized in consideration of the mixture of audience.

 


2. What is ionizing and non-ionizing radiation?

An oscillating electric charge generates electromagnetic waves which radiate away from the source. These are commonly called photons. The energy level of a photon that increases, with increase in frequency and is given by,

Where f is the frequency of oscillation and h is Plancks constant, equal to 6.63 x 10-34 Joule second.

At a frequency of approximately 2.42x1015Hz the photon energy level approaches 12.4eV, the energy that binds electrons to atoms. At this energy level water molecules can become ionized and it is this region that separates ionizing from non-ionizing radiation. This is shown plotted in Figure 1 below

Figure 1 Electromagnetic Spectrum

 


3. The Propagation of Electromagnetic Waves.wpe25.jpg (8985 bytes)

The propagation of an electromagnetic wave from an antenna is shown in Figure 2 and can be divided into three distinct regions, the reactive near field, the Fresnel region and the Fraunhofer region. The Fraunhofer region is the most stable with a plane wave condition, a free space impedance of 377 Ohm exists. In the near field region the impedance varies and has to be calculated according to E/H. In this region the energy is both stored and radiated, that is why at lower frequencies where the wavelength is large both the electric and the magnetic fields have to be measured. The far field begins at a distance d which can be calculated according to,






Figure 2

 







4. Power Density and Field Strength.

In the far field the free space impedance ZO is given by,

The power density S can be determined from either the electric or the magnetic field strength by,

Power densities can also be calculated from an antenna provided enough information is given. The gain of an antenna G is related to its effective aperture A by,

The far field power density S at a distance d can then be calculated using Friis free space formula,

Where PO is the power going into the antenna and A is the effective aperture of the antenna.

 


5. Absorption of RF Energy into the Human Body.

Absorption of RF energy in the human body depends on several factors. The incident RF energy can either be reflected, pass through or be absorbed. In general RF energy passes through fatty tissue and is dissipated in the muscle or brain tissue, with the depth of penetration inversely proportional to frequency. The body absorbs most energy when its size is approximately the same as the wavelength, this condition is known as the whole body human resonance. For the human population the whole body resonance lies between 30MHz and 300MHz, which is the range that spans the size of a newborn child to a very tall adult.

The frequency fr of whole body resonance can be approximated by,

Where h is the height of the human expressed in metres. At resonance, body absorption is the highest and occurs with unequal distribution of energy, below resonance little absorption occurs with equal distribution of energy. Above resonance, absorption is also less and confined to the irradiated area.

 


6. Main Effects of RF Radiation.

Thermal effects due to RF radiation poses a certain amount of stress on the body due to the thermoregulatory systems attempt to maintain equilibrium. If the thermoregulatory system of the human body cannot cope the temperature will rise and cell death will occur above 41.6oC. Special care needs to be taken for body parts with less blood circulation such as the eyes and the testes.

A thermal effects due to RF radiation are less well understood and there are suggestions that these do occur at levels below the thermal hazard levels. They may not necessarily constitute a health hazard because the effects disappear when the radiation is removed. For further reading see references [3] and [4].

 


7. Occupational and General Public Exposure.

It is recognized that exposure limits for the general public should be set lower thanwpe27.jpg (7188 bytes)forthe occupational population. The ration all being that the general public comprises individuals of all sizes and different health status exposed 24 hours a day. Whereas the occupational population consists of adults exposed under controlled conditions, and who are trained to be aware of potential risks and to take appropriate precautions.

 




8. Safety Limits.

It is generally accepted that health problems start to occur if the radiated power absorbed is greater than 4W/kg of the human body weight. Allowing for a safety factor of ten, a specific absorption rate (SAR) of 0.4W/kg is normally used as a limit on the working level. For a person weighing 60kg, the total limit for uniformly absorbed power is 24W. This corresponds to a power flux density (PFD) of 1OW/m2 if the person has an effective aperture of 2.5m2. This limit would be applicable at whole body resonance, away from resonance this limit can be relaxed.

 


9. IRPA Guideline

The Guidelines on Limits of Exposure to Radio Frequency Electromagnetic Fields in the Frequency Range from l00kHz to 300GHz [5] published by the International Radiation Protection Association (IRPA) is currently the one used in Hong Kong. The limits of exposure are shown as equations in appendix I in tables 1 and 2. They are also shown graphically in appendix II in graphs 1 and 2.

Whole body human resonance is taken into account with an occupational SAR of 10W/m2 ( l mW/cm2) between the frequencies of l0MHz and 400MHz. Above resonance the limit is relaxed to 50W/m2(5mW/cm2) where the effective aperture of the human body resembles more of its cross sectional area. Below resonance the limit is relaxed further because absorption reduces rapidly with reduction in frequency, up to the point where the limits are then determined by athermal effects. For the general population the limits are set at approximately five times lower.

IRPA Guidelines on Limits of exposure to Radio Frequency Electromagnetic Fields in the Frequency Range from 100kHz to 300GHz [1].

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IRPA Guidelines on Limits of exposure to Radio Frequency Electromagnetic Fields in the Frequency Range from 100kHz to 300GHz [1].

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10. Example.

Example 1wpe2C.jpg (9281 bytes)

This example is a very common occurrence in a microwave laboratory where power levels are thought to be low.

A signal generator with an output power of 20dBm, operating at 10GHz in X-band is connected to an X-band waveguide which has the other end left open as shown in Figure 10.1. What is the power density at the open assuming that all the power is radiated as a plane wave?


This exceeds the occupational safety guideline by a factor of ten, so avoid temptation by looking into an energized open X-band waveguide. Ref. Figure 10.1


 


Example 2:

This example is also a very common occurrence usually to the occupational population working near transmitters. A transmitter operating at 280MHz is radiating l00W ERP, at what distance from the antenna will the safety guideline be exceeded for the occupational population?

Using 4.5,

 
  

FrequencyERP(W)Safe distaNce
170MHz25W>0.57m
280MHz100W>1.14m
450MHz5W>0.24m
800MHz100W>0.81m
900MHz100W>0.76m
1800MHz25W>0.27m

Table 10.1 Safe distances from radio transmitters

Therefor the safety level for the occupational population will be exceeded at distances less than 1.14m. Note that at distances less than 1.14m you might be in the near field region which will require a more rigorous analysis, which is well beyond the scope of this paper. Some other calculated examples are shown in Table 10.1

 


Example 3:

What is the SAR of a human head located close to a portable radio of output power 6.4W?

This example is extremely difficult to calculate analytically and the results were obtained from the measurements of a phantom.

It would seem that the SAR limit is exceeded under certain conditions, however the effect of time averaging has not been taken into account. If time averaging is taken into account the SAR values calculated above will reduce by a factor of 18. This would imply that the maximum SAR limit will not be exceeded.

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Table 10.2 SAR of a phantom head with a portable radio transmitter power of 6.4W located at a distance d from the front of the head.




11.Conclusion

The safety considerations for non-ionizing radiation has been presented. Equations have also been presented that will allow the calculation of power density, which will help assess the safety for certain environments. For other environments and for more detailed explanations, the reader should refer to the listed reference.

 


12.Reference

[1] "Non-ionizing Radiation General Information" Loral Microwave-Narda, 1993.

[2] "Health and Safety Considerations in the use of Radio Transmitting Apparatus" S. R. Wise and H. Wise, BME'92 Biomedical Engineering Symposium, April 10_11 1992, Hong Kong.

[3] "Safety Aspects of Transmitting Station Radiation" H. Wise, The Electromagnetic Environment in Hong Kong, 18th June 1994, pp4-13.

[4] "Biological Effects of RF Waves" K. Y. Cheung, The Electromagnetic Environment in Hong Kong, 18th June 1994, pp14-20.

[5] "Guidelines on Limits of Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from l00kHz to 300GHz" Health Physics Vol. 54, No.1

[6] "IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3kHz to 300GHz" IEEE Standard IEEE C95.1-1991.

 
 

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